Mercurial > flash_v2
diff packages/net/tcpip/current/doc/select.html @ 97:ced4577552cd ecos-sw-2000-06-06
Merge from eCos master repository on 2000-06-06-08:44:00-BST
| author | jlarmour |
|---|---|
| date | Tue, 06 Jun 2000 08:39:36 +0000 |
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| children |
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new file mode 100644 --- /dev/null +++ b/packages/net/tcpip/current/doc/select.html @@ -0,0 +1,108 @@ +<html> +<body> +<pre> +NAME + select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous + I/O multiplexing + +SYNOPSIS + #include <network.h> + + int select(int n, fd_set *readfds, fd_set *writefds, + fd_set *exceptfds, struct timeval *timeout); + + FD_CLR(int fd, fd_set *set); + FD_ISSET(int fd, fd_set *set); + FD_SET(int fd, fd_set *set); + FD_ZERO(fd_set *set); + +DESCRIPTION + select waits for a number of file descriptors to change + status. + + Three independent sets of descriptors are watched. Those + listed in readfds will be watched to see if characters + become available for reading, those in writefds will be + watched to see if it is ok to immediately write on them, + and those in exceptfds will be watched for exceptions. On + exit, the sets are modified in place to indicate which + descriptors actually changed status. + + Four macros are provided to manipulate the sets. FD_ZERO + will clear a set. FD_SET and FD_CLR add or remove a given + descriptor from a set. FD_ISSET tests to see if a + descriptor is part of the set; this is useful after select + returns. + + n is the highest-numbered descriptor in any of the three + sets, plus 1. + + timeout is an upper bound on the amount of time elapsed + before select returns. It may be zero, causing select to + return immediately. If timeout is NULL (no timeout), + select can block indefinitely. + +RETURN VALUE + On success, select returns the number of descriptors con- + tained in the descriptor sets, which may be zero if the + timeout expires before anything interesting happens. On + error, -1 is returned, and errno is set appropriately; the + sets and timeout become undefined, so do not rely on their + contents after an error. + +ERRORS + EBADF An invalid file descriptor was given in one of the + sets. + + EINTR A non blocked signal was caught. + + EINVAL n is negative. + + ENOMEM select was unable to allocate memory for internal + tables. + +NOTES + Some code calls select with all three sets empty, n zero, + and a non-null timeout as a fairly portable way to sleep + with subsecond precision. + +EXAMPLE + #include <stdio.h> + #include <sys/time.h> + #include <sys/types.h> + #include <unistd.h> + + int + main(void) + { + fd_set rfds; + struct timeval tv; + int retval; + + /* Watch stdin (fd 0) to see when it has input. */ + FD_ZERO(&rfds); + FD_SET(0, &rfds); + /* Wait up to five seconds. */ + tv.tv_sec = 5; + tv.tv_usec = 0; + + retval = select(1, &rfds, NULL, NULL, &tv); + /* Don't rely on the value of tv now! */ + + if (retval) + printf("Data is available now.\n"); + /* FD_ISSET(0, &rfds) will be true. */ + else + printf("No data within five seconds.\n"); + + exit(0); + } + + Generally portable to/from non-BSD systems supporting + clones of the BSD socket layer (including System V vari- + ants). However, note that the System V variant typically + sets the timeout variable before exit, but the BSD variant + does not. +</pre> +</body> +</html>