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Fix FREESCALE_EDMA_NBYTES_MLOFFYES_MLOFF() so it works with negative offsets.
author vae
date Wed, 29 Apr 2015 23:31:48 +0000
parents eb9fd8c04db3
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
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1 File System Support Infrastructure
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2 ==================================
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3
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4 Nick Garnett
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5 v0.2
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7
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8 This document describes the filesystem infrastructure provided in
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9 eCos. This is implemented by the FILEIO package and provides POSIX
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10 compliant file and IO operations together with the BSD socket
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11 API. These APIs are described in the relevant standards and original
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12 documentation and will not be described here. This document is,
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13 instead, concerned with the interfaces presented to client
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14 filesystems and network protocol stacks.
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15
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16 The FILEIO infrastructure consist mainly of a set of tables containing
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17 pointers to the primary interface functions of a file system. This
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18 approach avoids problems of namespace pollution (several filesystems
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19 can have a function called read(),so long as they are static). The
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20 system is also structured to eliminate the need for dynamic memory
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21 allocation.
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22
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23 New filesystems can be written directly to the interfaces described
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24 here. Existing filesystems can be ported very easily by the
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25 introduction of a thin veneer porting layer that translates FILEIO
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26 calls into native filesystem calls.
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27
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28 The term filesystem should be read fairly loosely in this
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29 document. Object accessed through these interfaces could equally be
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30 network protocol sockets, device drivers, fifos, message queues or any
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31 other object that can present a file-like interface.
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32
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33
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34 File System Table
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35 -----------------
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36
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37 The filesystem table is an array of entries that describe each
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38 filesystem implementation that is part of the system image. Each
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39 resident filesystem should export an entry to this table using the
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40 FSTAB_ENTRY() macro.
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41
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42 The table entries are described by the following structure:
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43
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44 struct cyg_fstab_entry
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45 {
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46 const char *name; // filesystem name
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47 CYG_ADDRWORD data; // private data value
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48 cyg_uint32 syncmode; // synchronization mode
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49
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50 int (*mount) ( cyg_fstab_entry *fste, cyg_mtab_entry *mte );
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51 int (*umount) ( cyg_mtab_entry *mte );
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52 int (*open) ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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53 int mode, cyg_file *fte );
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54 int (*unlink) ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
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55 int (*mkdir) ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
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56 int (*rmdir) ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
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57 int (*rename) ( cyg_mtab_entry *mte, cyg_dir dir1, const char *name1,
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58 cyg_dir dir2, const char *name2 );
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59 int (*link) ( cyg_mtab_entry *mte, cyg_dir dir1, const char *name1,
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60 cyg_dir dir2, const char *name2, int type );
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61 int (*opendir) ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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62 cyg_file *fte );
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63 int (*chdir) ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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64 cyg_dir *dir_out );
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65 int (*stat) ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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66 struct stat *buf);
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67 int (*getinfo) ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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68 int key, char *buf, int len );
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69 int (*setinfo) ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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70 int key, char *buf, int len );
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71 };
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72
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73 The _name_ field points to a string that identifies this filesystem
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74 implementation. Typical values might be "romfs", "msdos", "ext2" etc.
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75
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76 The _data_ field contains any private data that the filesystem needs,
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77 perhaps the root of its data structures.
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78
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79 The _syncmode_ field contains a description of the locking protocol to
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80 be used when accessing this filesystem. It will be described in more
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81 detail in the "Synchronization" section.
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82
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83 The remaining fields are pointers to functions that implement
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84 filesystem operations that apply to files and directories as whole
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85 objects. The operation implemented by each function should be obvious
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86 from the names, with a few exceptions.
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87
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88 The _opendir_ function opens a directory for reading. See the section
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89 on Directories later for details.
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90
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91 The _getinfo_ and _setinfo_ functions provide support for various
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92 minor control and information functions such as pathconf() and
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93 access().
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94
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95 With the exception of the _mount_ and _umount_ functions, all of these
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96 functions take three standard arguments, a pointer to a mount table
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97 entry (see later) a directory pointer (also see later) and a file name
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98 relative to the directory. These should be used by the filesystem to
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99 locate the object of interest.
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100
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101 Mount Table
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102 -----------
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103
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104 The mount table records the filesystems that are actually active.
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105 These can be seen as being analogous to mount points in Unix systems.
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106
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107 There are two sources of mount table entries. Filesystems (or other
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108 components) may export static entries to the table using the
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109 MTAB_ENTRY() macro. Alternatively, new entries may be installed at run
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110 time using the mount() function. Both types of entry may be unmounted
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111 with the umount() function.
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112
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113 A mount table entry has the following structure:
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114
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115 struct cyg_mtab_entry
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116 {
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117 const char *name; // name of mount point
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118 const char *fsname; // name of implementing filesystem
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119 const char *devname; // name of hardware device
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120 CYG_ADDRWORD data; // private data value
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121 cyg_bool valid; // Valid entry?
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122 cyg_fstab_entry *fs; // pointer to fstab entry
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123 cyg_dir root; // root directory pointer
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124 };
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125
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126 The _name_ field identifies the mount point. This is used to translate
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127 rooted filenames (filenames that begin with "/") into the correct
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128 filesystem. When a file name that begins with "/" is submitted, it is
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129 matched against the _name_ fields of all valid mount table
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130 entries. The entry that yields the longest match terminating before a
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131 "/", or end of string, wins and the appropriate function from the
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132 filesystem table entry is then passed the remainder of the file name
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133 together with a pointer to the table entry and the value of the _root_
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134 field as the directory pointer.
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135
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136 For example, consider a mount table that contains the following
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137 entries:
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138
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139 { "/", "msdos", "/dev/hd0", ... }
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140 { "/fd", "msdos", "/dev/fd0", ... }
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141 { "/rom", "romfs", "", ... }
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142 { "/tmp", "ramfs", "", ... }
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143 { "/dev", "devfs", "", ... }
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144
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145 An attempt to open "/tmp/foo" would be directed to the RAM filesystem
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146 while an open of "/bar/bundy" would be directed to the hard disc MSDOS
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147 filesystem. Opening "/dev/tty0" would be directed to the device
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148 management filesystem for lookup in the device table.
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149
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150 Unrooted file names (those that do not begin with a '/') are passed
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151 straight to the current directory. The current directory is
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152 represented by a pair consisting of a mount table entry and a
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153 directory pointer.
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154
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155 The _fsname_ field points to a string that should match the _name_
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156 field of the implementing filesystem. During initialization the mount
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157 table is scanned and the _fsname_ entries looked up in the
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158 filesystem table. For each match, the filesystem's _mount_ function
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159 is called and if successful the mount table entry is marked as valid
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160 and the _fs_ pointer installed.
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161
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162 The _devname_ field contains the name of the device that this
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163 filesystem is to use. This may match an entry in the device table (see
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164 later) or may be a string that is specific to the filesystem if it has
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165 its own internal device drivers.
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166
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167 The _data_ field is a private data value. This may be installed either
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168 statically when the table entry is defined, or may be installed during
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169 the _mount_ operation.
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170
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171 The _valid_ field indicates whether this mount point has actually been
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172 mounted successfully. Entries with a false _valid_ field are ignored
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173 when searching for a name match.
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174
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175 The _fs_ field is installed after a successful mount operation to
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176 point to the implementing filesystem.
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177
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178 The _root_ field contains a directory pointer value that the
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179 filesystem can interpret as the root of its directory tree. This is
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180 passed as the _dir_ argument of filesystem functions that operate on
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181 rooted filenames. This field must be initialized by the filesystem's
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182 _mount_ function.
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183
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184
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185
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186 File Table
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187 ----------
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188
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189 Once a file has been opened it is represented by an open file
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190 object. These are allocated from an array of available file
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191 objects. User code accesses these open file objects via a second array
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192 of pointers which is indexed by small integer offsets. This gives the
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193 usual Unix file descriptor functionality, complete with the various
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194 duplication mechanisms.
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195
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196 A file table entry has the following structure:
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197
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198 struct CYG_FILE_TAG
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199 {
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200 cyg_uint32 f_flag; /* file state */
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201 cyg_uint16 f_ucount; /* use count */
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202 cyg_uint16 f_type; /* descriptor type */
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203 cyg_uint32 f_syncmode; /* synchronization protocol */
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204 struct CYG_FILEOPS_TAG *f_ops; /* file operations */
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205 off_t f_offset; /* current offset */
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206 CYG_ADDRWORD f_data; /* file or socket */
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207 CYG_ADDRWORD f_xops; /* extra type specific ops */
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208 cyg_mtab_entry *f_mte; /* mount table entry */
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209 };
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210
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211 The _f_flag_ field contains some FILEIO control bits and some of the
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212 bits from the open call (defined by CYG_FILE_MODE_MASK).
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213
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214 The _f_ucount_ field contains a use count that controls when a file
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215 will be closed. Each duplicate in the file descriptor array counts for
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216 one reference here and it is also incremented around each I/O
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217 operation.
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218
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219 The _f_type_ field indicates the type of the underlying file
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220 object. Some of the possible values here are CYG_FILE_TYPE_FILE,
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221 CYG_FILE_TYPE_SOCKET or CYG_FILE_TYPE_DEVICE.
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222
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223 The _f_syncmode_ field is copied from the _syncmode_ field of the
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224 implementing filesystem. Its use is described in the "Synchronization"
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225 section later.
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226
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227 The _f_offset_ field records the current file position. It is the
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228 responsibility of the file operation functions to keep this field up
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229 to date.
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230
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231 The _f_data_ field contains private data placed here by the underlying
130
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232 filesystem. Normally this will be a pointer to or handle on the
115
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233 filesystem object that implements this file.
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234
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235 The _f_xops_ field contains a pointer to any extra type specific
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236 operation functions. For example, the socket I/O system installs a
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237 pointer to a table of functions that implement the standard socket
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238 operations.
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239
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240 The _f_mte_ field contains a pointer to the parent mount table entry
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241 for this file. It is used mainly to implement the synchronization
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242 protocol. This may contain a pointer to some other data structure in
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243 file objects not derived from a filesystem.
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244
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245 The _f_ops_ field contains a pointer to a table of file I/O
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246 operations. This has the following structure:
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247
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248 struct CYG_FILEOPS_TAG
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249 {
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250 int (*fo_read) (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
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251 int (*fo_write) (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
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252 int (*fo_lseek) (struct CYG_FILE_TAG *fp, off_t *pos, int whence );
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253 int (*fo_ioctl) (struct CYG_FILE_TAG *fp, CYG_ADDRWORD com,
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254 CYG_ADDRWORD data);
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255 int (*fo_select) (struct CYG_FILE_TAG *fp, int which, CYG_ADDRWORD info);
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256 int (*fo_fsync) (struct CYG_FILE_TAG *fp, int mode );
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257 int (*fo_close) (struct CYG_FILE_TAG *fp);
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258 int (*fo_fstat) (struct CYG_FILE_TAG *fp, struct stat *buf );
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259 int (*fo_getinfo) (struct CYG_FILE_TAG *fp, int key, char *buf, int len );
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260 int (*fo_setinfo) (struct CYG_FILE_TAG *fp, int key, char *buf, int len );
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261 };
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262
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263 It should be obvious from the names of most of these functions what
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264 their responsibilities are. The _fo_getinfo_ and _fo_setinfo_
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265 function, like their counterparts in the filesystem structure,
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266 implement minor control and info functions such as fpathconf().
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267
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268 The second argument to _fo_read_ and _fo_write_ is a pointer to a UIO
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269 structure:
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270
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271 struct CYG_UIO_TAG
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272 {
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273 struct CYG_IOVEC_TAG *uio_iov; /* pointer to array of iovecs */
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274 int uio_iovcnt; /* number of iovecs in array */
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275 off_t uio_offset; /* offset into file this uio corresponds to */
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276 ssize_t uio_resid; /* residual i/o count */
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277 enum cyg_uio_seg uio_segflg; /* see above */
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278 enum cyg_uio_rw uio_rw; /* see above */
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279 };
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280
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281 struct CYG_IOVEC_TAG
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282 {
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283 void *iov_base; /* Base address. */
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284 ssize_t iov_len; /* Length. */
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285 };
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286
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287 This structure encapsulates the parameters of any data transfer
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288 operation. It provides support for scatter/gather operations and
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289 records the progress of any data transfer. It is also compatible with
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290 the I/O operations of any BSD-derived network stacks and filesystems.
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291
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292
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293 When a file is opened (or a file object created by some other means,
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294 such as socket() or accept()) it is the responsibility of the
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295 filesystem open operation to initialize all the fields of the object
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296 except the _f_ucount_, _f_syncmode_ and _f_mte_ fields. Since the
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297 _f_flag_ field will already contain bits belonging to the FILEIO
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298 infrastructure, any changes to it must be made with the appropriate
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299 logical operations.
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300
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301
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302 Directories
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303 -----------
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304
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305 Filesystem operations all take a directory pointer as one of their
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306 arguments. A directory pointer is an opaque handle managed by the
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307 filesystem. It should encapsulate a reference to a specific directory
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308 within the filesystem. For example, it may be a pointer to the data
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309 structure that represents that directory, or a pointer to a pathname
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310 for the directory.
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311
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312 The _chdir_ filesystem function has two modes of use. When passed a
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313 pointer in the _dir_out_ argument, it should locate the named
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314 directory and place a directory pointer there. If the _dir_out_
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315 argument is NULL then the _dir_ argument is a previously generated
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316 directory pointer that can now be disposed of. When the infrastructure
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317 is implementing the chdir() function it makes two calls to filesystem
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318 _chdir_ functions. The first is to get a directory pointer for the new
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319 current directory. If this succeeds the second is to dispose of the
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320 old current directory pointer.
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321
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322 The _opendir_ function is used to open a directory for reading. This
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323 results in an open file object that can be read to return a sequence
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324 of _struct dirent_ objects. The only operation that are allowed on
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325 this file are _read_, _lseek_ and _close_. Each read operation on this
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326 file should return a single _struct dirent_ object. When the end of
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327 the directory is reached, zero should be returned. The only seek
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328 operation allowed is a rewind to the start of the directory, by
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329 supplying an offset of zero and a _whence_ specifier of _SEEK_SET_.
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330
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331 Most of these considerations are invisible to clients of a filesystem
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332 since they will access directories via the POSIX
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333 opendir()/readdir()/closedir() functions.
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334
130
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335 Support for the _getcwd()_ function is provided by three mechanisms.
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336 The first is to use the _FS_INFO_GETCWD_ getinfo key on the filesystem
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337 to use any internal support that it has for this. If that fails it
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338 falls back on one of the two other mechanisms. If
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339 _CYGPKG_IO_FILEIO_TRACK_CWD_ is set then the current directory is
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340 tracked textually in chdir() and the result of that is reported in
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341 getcwd(). Otherwise an attempt is made to traverse the directory tree
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342 to its root using ".." entries.
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343
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344 This last option is complicated and expensive, and relies on the
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345 filesystem supporting "." and ".." entries. This is not always the
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346 case, particularly if the filesystem has been ported from a
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347 non-UNIX-compatible source. Tracking the pathname textually will
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348 usually work, but might not produce optimum results when symbolic
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349 links are being used.
115
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350
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351
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352 Synchronization
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353 ---------------
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354
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355 The FILEIO infrastructure provides a synchronization mechanism for
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356 controlling concurrent access to filesystems. This allows existing
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357 filesystems to be ported to eCos, even if they do not have their own
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358 synchronization mechanisms. It also allows new filesystems to be
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359 implemented easily without having to consider the synchronization
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360 issues.
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361
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362 The infrastructure maintains a mutex for each entry in each of
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363 the main tables: filesystem table, mount table and file table. For
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364 each class of operation each of these mutexes may be locked before the
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365 corresponding filesystem operation is invoked.
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366
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367 The synchronization protocol implemented by a filesystem is described
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368 by the _syncmode_ field of the filesystem table entry. This is a
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369 combination of the following flags:
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370
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371 CYG_SYNCMODE_FILE_FILESYSTEM Lock the filesystem table entry mutex
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372 during all filesystem level operations.
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373
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374 CYG_SYNCMODE_FILE_MOUNTPOINT Lock the mount table entry mutex
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375 during all filesystem level operations.
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376
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377 CYG_SYNCMODE_IO_FILE Lock the file table entry mutex during all
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378 I/O operations.
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379
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380 CYG_SYNCMODE_IO_FILESYSTEM Lock the filesystem table entry mutex
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381 during all I/O operations.
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382
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383 CYG_SYNCMODE_IO_MOUNTPOINT Lock the mount table entry mutex during
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384 all I/O operations.
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385
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386 CYG_SYNCMODE_SOCK_FILE Lock the file table entry mutex during
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387 all socket operations.
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388
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389 CYG_SYNCMODE_SOCK_NETSTACK Lock the network stack table entry mutex
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390 during all socket operations.
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391
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392 CYG_SYNCMODE_NONE Perform no locking at all during any
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393 operations.
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394
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395
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396 The value of the _syncmode_ in the filesystem table entry will be
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397 copied by the infrastructure to the open file object after a
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398 successful open() operation.
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399
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400
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401 Initialization and Mounting
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402 ---------------------------
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403
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404 As mentioned previously, mount table entries can be sourced from two
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405 places. Static entries may be defined by using the MTAB_ENTRY()
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406 macro. Such entries will be automatically mounted on system startup.
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407 For each entry in the mount table that has a non-null _name_ field the
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408 filesystem table is searched for a match with the _fsname_ field. If a
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409 match is found the filesystem's _mount_ entry is called and if
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410 successful the mount table entry marked valid and the _fs_ field
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411 initialized. The _mount_ function is responsible for initializing the
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412 _root_ field.
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413
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414 The size of the mount table is defined by the configuration value
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415 CYGNUM_FILEIO_MTAB_MAX. Any entries that have not been statically
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416 defined are available for use by dynamic mounts.
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417
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418 A filesystem may be mounted dynamically by calling mount(). This
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419 function has the following prototype:
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420
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421 int mount( const char *devname,
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422 const char *dir,
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423 const char *fsname);
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424
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425 The _devname_ argument identifies a device that will be used by this
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426 filesystem and will be assigned to the _devname_ field of the mount
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427 table entry.
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428
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429 The _dir_ argument is the mount point name, it will be assigned to the
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430 _name_ field of the mount table entry.
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431
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432 The _fsname_ argument is the name of the implementing filesystem, it
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433 will be assigned to the _fsname_ entry of the mount table entry.
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434
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435 The process of mounting a filesystem dynamically is as follows. First
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436 a search is made of the mount table for an entry with a NULL _name_
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437 field to be used for the new mount point. The filesystem table is then
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438 searched for an entry whose name matches _fsname_. If this is
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439 successful then the mount table entry is initialized and the
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440 filesystem's _mount_ operation called. If this is successful, the
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441 mount table entry is marked valid and the _fs_ field initialized.
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442
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443 Unmounting a filesystem is done by the umount() function. This can
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444 unmount filesystems whether they were mounted statically or
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445 dynamically.
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446
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447 The umount() function has the following prototype:
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448
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449 int umount( const char *name );
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450
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451 The mount table is searched for a match between the _name_ argument
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452 and the entry _name_ field. When a match is found the filesystem's
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453 _umount_ operation is called and if successful, the mount table entry
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454 is invalidated by setting its _valid_ field false and the _name_ field
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455 to NULL.
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456
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457 Sockets
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458 -------
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459
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460 If a network stack is present, then the FILEIO infrastructure also
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461 provides access to the standard BSD socket calls.
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462
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463 The netstack table contains entries which describe the network
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464 protocol stacks that are in the system image. Each resident stack
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465 should export an entry to this table using the NSTAB_ENTRY() macro.
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466
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467 Each table entry has the following structure:
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468
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469 struct cyg_nstab_entry
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470 {
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471 cyg_bool valid; // true if stack initialized
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472 cyg_uint32 syncmode; // synchronization protocol
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473 char *name; // stack name
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474 char *devname; // hardware device name
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475 CYG_ADDRWORD data; // private data value
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476
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477 int (*init)( cyg_nstab_entry *nste );
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478 int (*socket)( cyg_nstab_entry *nste, int domain, int type,
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479 int protocol, cyg_file *file );
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480 };
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481
130
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482 This table is analogous to a combination of the filesystem and mount
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483 tables.
115
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484
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485 The _valid_ field is set true if the stack's _init_ function returned
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486 successfully and the _syncmode_ field contains the CYG_SYNCMODE_SOCK_*
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487 bits described above.
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488
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489 The _name_ field contains the name of the protocol stack.
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490
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491 The _devname_ field names the device that the stack is using. This may
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492 reference a device under "/dev", or may be a name that is only
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493 meaningful to the stack itself.
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494
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495 The _init_ function is called during system initialization to start
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496 the protocol stack running. If it returns non-zero the _valid_ field
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497 is set false and the stack will be ignored subsequently.
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498
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499 The _socket_ function is called to attempt to create a socket in the
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500 stack. When the socket() API function is called the netstack table is
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501 scanned and for each valid entry the _socket_ function is called. If
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502 this returns non-zero then the scan continues to the next valid stack,
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503 or terminates with an error if the end of the table is reached.
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504
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505 The result of a successful socket call is an initialized file object
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506 with the _f_xops_ field pointing to the following structure:
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507
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508 struct cyg_sock_ops
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509 {
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510 int (*bind) ( cyg_file *fp, const sockaddr *sa, socklen_t len );
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511 int (*connect) ( cyg_file *fp, const sockaddr *sa, socklen_t len );
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512 int (*accept) ( cyg_file *fp, cyg_file *new_fp,
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513 struct sockaddr *name, socklen_t *anamelen );
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514 int (*listen) ( cyg_file *fp, int len );
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515 int (*getname) ( cyg_file *fp, sockaddr *sa, socklen_t *len, int peer );
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516 int (*shutdown) ( cyg_file *fp, int flags );
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517 int (*getsockopt)( cyg_file *fp, int level, int optname,
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518 void *optval, socklen_t *optlen);
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519 int (*setsockopt)( cyg_file *fp, int level, int optname,
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520 const void *optval, socklen_t optlen);
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521 int (*sendmsg) ( cyg_file *fp, const struct msghdr *m,
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522 int flags, ssize_t *retsize );
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523 int (*recvmsg) ( cyg_file *fp, struct msghdr *m,
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524 socklen_t *namelen, ssize_t *retsize );
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525 };
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526
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527 It should be obvious from the names of these functions which API calls
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528 they provide support for. The _getname_ function provides support for
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529 both getsockname() and getpeername() while the _sendmsg_ and _recvmsg_
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530 functions provide support for send(), sendto(), sendmsg(), recv(),
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531 recvfrom() and recvmsg() as appropriate.
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532
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533
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534
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535 Select
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536 ------
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537
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538 The infrastructure provides support for implementing a select
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539 mechanism. This is modeled on the mechanism in the BSD kernel, but has
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540 been modified to make it implementation independent.
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541
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542 The main part of the mechanism is the select() API call. This
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543 processes its arguments and calls the _fo_select_ function on all file
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544 objects referenced by the file descriptor sets passed to it. If the
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545 same descriptor appears in more than one descriptor set, the
130
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546 _fo_select_ function will be called separately for each appearance.
115
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547
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548 The _which_ argument of the _fo_select_ function will either be
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549 CYG_FREAD to test for read conditions, CYG_FWRITE to test for write
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550 conditions or zero to test for exceptions. For each of these options
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551 the function should test whether the condition is satisfied and if so
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552 return true. If it is not satisfied then it should call
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553 cyg_selrecord() with the _info_ argument that was passed to the
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554 function and a pointer to a cyg_selinfo structure.
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555
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556 The cyg_selinfo structure is used to record information about current
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557 select operations. Any object that needs to support select must
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558 contain an instance of this structure. Separate cyg_selinfo
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559 structures should be kept for each of the options that the object can
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560 select on - read, write or exception.
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561
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562 If none of the file objects report that the select condition is
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563 satisfied, then the select() API function puts the calling thread to
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564 sleep waiting either for a condition to become satisfied, or for the
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565 optional timeout to expire.
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566
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567 A selectable object must have some asynchronous activity that may
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568 cause a select condition to become true - either via interrupts or the
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569 activities of other threads. Whenever a selectable condition is
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570 satisfied, the object should call cyg_selwakeup() with a pointer to
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571 the appropriate cyg_selinfo structure. If the thread is still waiting,
130
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572 this will cause it to wake up and repeat its poll of the file
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573 descriptors. This time around, the object that caused the wakeup
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574 should indicate that the select condition is satisfied, and the
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575 _select()_ API call will return.
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576
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577 Note that _select()_ does not exhibit real time behaviour: the
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578 iterative poll of the descriptors, and the wakeup mechanism mitigate
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579 against this. If real time response to device or socket I/O is
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580 required then separate threads should be devoted to each device of
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581 interest.
115
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582
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583
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584 Devices
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585 -------
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586
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587 Devices are accessed by means of a pseudo-filesystem, "devfs", that is
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588 mounted on "/dev". Open operations are translated into calls to
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589 cyg_io_lookup() and if successful result in a file object whose
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590 _f_ops_ functions translate filesystem API functions into calls into
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591 the device API.
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592
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593 // EOF fileio.txt