timed_write.c

[詳解]

/*++
/* NAME
/*  timed_write 3
/* SUMMARY
/*  write operation with pre-write timeout
/* SYNOPSIS
/*  #include <iostuff.h>
/*
/*  ssize_t timed_write(fd, buf, len, timeout, context)
/*  int fd;
/*  const void *buf;
/*  size_t  len;
/*  int timeout;
/*  void    *context;
/* DESCRIPTION
/*  timed_write() performs a write() operation when the specified
/*  descriptor becomes writable within a user-specified deadline.
/*
/*  Arguments:
/* .IP fd
/*  File descriptor in the range 0..FD_SETSIZE.
/* .IP buf
/*  Write buffer pointer.
/* .IP len
/*  Write buffer size.
/* .IP timeout
/*  The deadline in seconds. If this is <= 0, the deadline feature
/*  is disabled.
/* .IP context
/*  Application context. This parameter is unused. It exists only
/*  for the sake of VSTREAM compatibility.
/* DIAGNOSTICS
/*  When the operation does not complete within the deadline, the
/*  result value is -1, and errno is set to ETIMEDOUT.
/*  All other returns are identical to those of a write(2) operation.
/* LICENSE
/* .ad
/* .fi
/*  The Secure Mailer license must be distributed with this software.
/* AUTHOR(S)
/*  Wietse Venema
/*  IBM T.J. Watson Research
/*  P.O. Box 704
/*  Yorktown Heights, NY 10598, USA
/*--*/

/* System library. */

#include <sys_defs.h>
#include <unistd.h>
#include <errno.h>

/* Utility library. */

#include <msg.h>
#include <iostuff.h>

/* timed_write - write with deadline */

ssize_t timed_write(int fd, const void *buf, size_t len,
                    int timeout, void *unused_context)
{
    ssize_t ret;

    /*
     * Wait for a limited amount of time for something to happen. If nothing
     * happens, report an ETIMEDOUT error.
     * 
     * XXX Solaris 8 read() fails with EAGAIN after read-select() returns
     * success. The code below exists just in case their write implementation
     * is equally broken.
     * 
     * This condition may also be found on systems where select() returns
     * success on pipes with less than PIPE_BUF bytes of space, and with
     * badly designed software where multiple writers are fighting for access
     * to the same resource.
     */
    for (;;) {
    if (timeout > 0 && write_wait(fd, timeout) < 0)
        return (-1);
    if ((ret = write(fd, buf, len)) < 0 && timeout > 0 && errno == EAGAIN) {
        msg_warn("write() returns EAGAIN on a writable file descriptor!");
        msg_warn("pausing to avoid going into a tight select/write loop!");
        sleep(1);
        continue;
    } else if (ret < 0 && errno == EINTR) {
        continue;
    } else {
        return (ret);
    }
    }
}