fcntl.h

C POSIX library

fcntl.h is the header in the C POSIX library for the C programming language that contains constructs for file control operations, e.g. opening a file, retrieving and changing the permissions of file, or locking a file for edit.

Contents

Member functions

int fcntl(int filedes, int cmd, ...)

Performs the file operation specified by cmd on the file descriptor filedes, with optional other arguments. cmd can be any of the F_* constants listed below. The return value depends on the command issued.

int open(const char *path, int oflag, ... )

Opens a file at path, where oflag is one of the O_* constants listed below. Returns a new file descriptor.

int creat(const char *path, mode_t mode)

Creates a file at path with mode mode. Equivalent to open(path, O_WRONLY|O_CREAT|O_TRUNC, mode). As with open, returns the new file descriptor.

Member constants

The cmd argument to fcntl may be any of the following:

Constant Description
F_DUPFD duplicate file descriptor.
F_GETFD get file descriptor flags.
F_SETFD set file descriptor flags.
F_GETFL get file status flags and file access modes.
F_SETFL set file status flags.
F_GETLK get record locking information.
F_SETLK set record locking information.
F_SETLKW set record locking information; wait if blocked.
F_GETOWN get process or process group ID to receive SIGURG signals.
F_SETOWN set process or process group ID to receive SIGURG signals.

The oflag argument to open must specify exactly one of the following:

Constant Description
O_RDONLY open for reading only.
O_WRONLY ppen for writing only.
O_RDWR open for reading and writing.

And any combination of:

Constant Description
O_CREAT create file if it does not exist.
O_EXCL exclusive use flag.
O_NOCTTY do not assign controlling terminal.
O_TRUNC truncate flag.
O_APPEND set append mode.
O_NONBLOCK non-blocking mode.
O_RSYNC synchronized read I/O operations.
O_SYNC write according to synchronized I/O file integrity completion.

Member types

fcntl.h defines the struct flock, which describes a file lock. It contains the following members:

short l_type    /* type of lock, see below */
short l_whence  /* starting offset */
off_t l_start   /* relative offset in bytes */
off_t l_len     /* size */
pid_t l_pid     /* process of ID of lock holder */

l_type is one of:

Constant Description
F_RDLCK shared or read lock.
F_UNLCK unlock.
F_WRLCK exclusive or write lock.

Example

A short example of fcntl.h usage is:

//C program using fcntl.h to get the size of a file.
 
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
 
int main(int argc, char** argv) { 
 
  int fp;
  long int file_size;
 
  if (argc < 2) {
    fprintf(stderr, "Correct usage: %s <filename>\n", argv[0]); 
    return 1; 
  }
 
  if ((fp = open(argv[1], O_RDONLY)) == -1) {
    fprintf(stderr, "Error opening the file \n"); 
    return 1;
  }
 
  file_size = filelength(fp); 
  printf("The file size in is %ld bytes.\n", file_size); 
  close(fp); 
 
  return 0; 
}

References