FCNTL(2)

NAME
fcntl - manipulate file descriptor

SYNOPSIS
#include <unistd.h>
#include <fcntl.h>

int fcntl(int fd, int cmd);
int fcntl(int fd, int cmd, long arg);

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

F_DUPFD
    Makes arg be a copy of fd, closing fd first if necessary.

    The same functionality can be more easily achieved by using dup2(2).

    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 be closed on exec.

    On success, the new descriptor is returned.

F_GETFD
    Read the close-on-exec flag. If the low-order 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 arg (only the least significant bit is used).

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 and O_NONBLOCK may be set.

    The flags are shared between copies (made with dup etc.) of the same file descriptor.

    The flags and their semantics are described in open(2).

F_GETLK, F_SETLK and F_SETLKW
    Manage discretionary file locks. The third argument arg 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.

F_GETOWN
    Get the process ID (or process group) of the owner of a socket.

    Process groups are returned as negative values.

F_SETOWN
    Set the process or process group that owns a socket.

    For these commands, ownership means receiving SIGIO or SIGURG signals.

    Process groups are specified using negative values.

RETURN VALUE
The return value depends on the operation:

F_DUPFD
    The new descriptor.

F_GETFD
    Value of flag.

F_GETFL
    Value of flags.

F_GETOWN
    Value of descriptor owner.

On error, -1 is returned, and errno is set appropriately.

ERRORS

EACCES
    Operation is prohibited by locks held by other processes.

EAGAIN
    Operation is prohibited because the file has been memory-mapped by another process.

EDEADLK
    Specified write lock operation would cause a deadlock.

EBADF
    fd is not an open file descriptor.

EINVAL
    For F_DUPFD, arg is negative or is greater than the maximum allowable value.

EMFILE
    For F_DUPFD, the process already has the maximum number of file descriptors open.

ENOLCK
    Too many segment locks open, lock table is full.

NOTES
The errors returned by dup2 are different from those returned by F_DUPFD.

CONFORMING TO
SVr4, SVID, POSIX, X/OPEN, BSD 4.3. Only the operations F_DUPFD, F_GETFD, F_SETFD, F_GETFL, F_SETFL, F_GETLK, F_SETLK and F_SETLKW are specified in POSIX.1; F_GETOWN and F_SETOWN are BSDisms not supported in SVr4. The flags legal for F_GETFL/F_SETFL are those supported by open(2) and vary between these systems; O_APPEND, O_NONBLOCK, O_RDONLY, and O_RDWR are specified in POSIX.1. SVr4 supports several other options and flags not documented here.

POSIX.1 documents an additional EINTR condition. SVr4 documents additional EFAULT, EINTR, EIO, ENOLINK and EOVERFLOW error conditions.

SEE ALSO
dup2(2), open(2), socket(2), flock(2)