filelock/region_locking.c

This is filelock/region_locking.c (Listing 55-3, page 1134), an example from the book, The Linux Programming Interface.

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Function list (Bold in this list means a function is not static)

lockReg()
lockRegion()
lockRegionWait()
regionIsLocked()

/* region_locking.c

   Some useful functions for file region (fcntl()) locking.
*/
#include <fcntl.h>
#include "region_locking.h"             /* Declares functions defined here */

/* Lock a file region (private; public interfaces below) */

static int
lockReg(int fd, int cmd, int type, int whence, int start, off_t len)
{
    struct flock fl;

    fl.l_type = type;
    fl.l_whence = whence;
    fl.l_start = start;
    fl.l_len = len;

    return fcntl(fd, cmd, &fl);
}

int                     /* Lock a file region using nonblocking F_SETLK */
lockRegion(int fd, int type, int whence, int start, int len)
{
    return lockReg(fd, F_SETLK, type, whence, start, len);
}

int                     /* Lock a file region using blocking F_SETLKW */
lockRegionWait(int fd, int type, int whence, int start, int len)
{
    return lockReg(fd, F_SETLKW, type, whence, start, len);
}

/* Test if a file region is lockable. Return 0 if lockable, or
   PID of process holding incompatible lock, or -1 on error. */

pid_t
regionIsLocked(int fd, int type, int whence, int start, int len)
{
    struct flock fl;

    fl.l_type = type;
    fl.l_whence = whence;
    fl.l_start = start;
    fl.l_len = len;

    if (fcntl(fd, F_GETLK, &fl) == -1)
        return -1;

    return (fl.l_type == F_UNLCK) ? 0 : fl.l_pid;
}

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