FCNTL (2)

SYNOPSIS

#include <unistd.h>
#include <fcntl.h>
  int fcntl(int  fd , int  cmd );  
  int fcntl(int  fd , int  cmd , long  arg );  
  int fcntl(int  fd , int  cmd , struct flock *  lock );  

DESCRIPTION

fcntl performs one of various miscellaneous operations on fd . The operation in question is determined by cmd :

F_DUPFD

Find the lowest numbered availiable file descriptor greater than or equal to arg and make it be a copy of fd . This is different form dup2 (2) which uses exactly the descriptor specified. The old and new descriptors may be used interchangeably. They share locks, file position pointers and flags; for example, if the file position is modified by using lseek on one of the descriptors, the position is also changed for the other. The two descriptors do not share the close-on-exec flag, however. The close-on-exec flag of the copy is off, meaning that it will not be closed on exec. On success, the new descriptor is returned.

F_GETFD

Read the close-on-exec flag. If the FD_CLOEXEC bit is 0, the file will remain open across exec , otherwise it will be closed.

F_SETFD

Set the close-on-exec flag to the value specified by the FD_CLOEXEC bit of arg .

F_GETFL

Read the descriptor's flags (all flags (as set by open (2)) are returned).

F_SETFL

Set the descriptor's flags to the value specified by arg . Only O_APPEND , O_NONBLOCK and O_ASYNC may be set; the other flags are unaffected. The flags are shared between copies (made with dup (2), fork (2), etc.) of the same file descriptor. The flags and their semantics are described in open (2). F_GETLK , F_SETLK and F_SETLKW are used to manage discretionary file locks. The third argument lock is a pointer to a struct flock (that may be overwritten by this call).

F_GETLK

Return the flock structure that prevents us from obtaining the lock, or set the l_type field of the lock to F_UNLCK if there is no obstruction.

F_SETLK

The lock is set (when l_type is F_RDLCK or F_WRLCK ) or cleared (when it is F_UNLCK ). If the lock is held by someone else, this call returns -1 and sets errno to EACCES or EAGAIN .

F_SETLKW

Like F_SETLK , but instead of returning an error we wait for the lock to be released. If a signal that is to be caught is received while fcntl is waiting, it is interrupted and (after the signal handler has returned) returns immediately (with return value -1 and errno set to EINTR ). F_GETOWN , F_SETOWN , F_GETSIG and F_SETSIG are used to manage I/O availability signals:

F_GETOWN

Get the process ID or process group currently receiving SIGIO and SIGURG signals for events on file descriptor fd . Process groups are returned as negative values.

F_SETOWN

Set the process ID or process group that will receive SIGIO and SIGURG signals for events on file descriptor fd . Process groups are specified using negative values. ( F_SETSIG can be used to specify a different signal instead of SIGIO).