Locking and Unlocking a File
The following example demonstrates how to place a lock on bytes
100 to 109 of a file and then later remove it. F_SETLK is used to
perform a non-blocking lock request so that the process does not
have to wait if an incompatible lock is held by another process;
instead the process can take some other action.
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
int fd;
struct flock fl;
fd = open("testfile", O_RDWR);
if (fd == -1)
/* Handle error */;
/* Make a non-blocking request to place a write lock
on bytes 100-109 of testfile */
fl.l_type = F_WRLCK;
fl.l_whence = SEEK_SET;
fl.l_start = 100;
fl.l_len = 10;
if (fcntl(fd, F_SETLK, &fl) == -1) {
if (errno == EACCES || errno == EAGAIN) {
printf("Already locked by another process\n");
/* We cannot get the lock at the moment */
} else {
/* Handle unexpected error */;
}
} else { /* Lock was granted... */
/* Perform I/O on bytes 100 to 109 of file */
/* Unlock the locked bytes */
fl.l_type = F_UNLCK;
fl.l_whence = SEEK_SET;
fl.l_start = 100;
fl.l_len = 10;
if (fcntl(fd, F_SETLK, &fl) == -1)
/* Handle error */;
}
exit(EXIT_SUCCESS);
} /* main */
Setting the Close-on-Exec Flag
The following example demonstrates how to set the close-on-exec
flag for the file descriptor fd.
#include <unistd.h>
#include <fcntl.h>
...
int flags;
flags = fcntl(fd, F_GETFD);
if (flags == -1)
/* Handle error */;
flags |= FD_CLOEXEC;
if (fcntl(fd, F_SETFD, flags) == -1)
/* Handle error */;"
