FCNTL(2) Linux Programmer's Manual 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 inter-
changeably. 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.
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
are shared between copies (made with dup etc.) of
the same file descriptor.
The flags and their semantics are described in
open(2).
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FCNTL(2) Linux Programmer's Manual FCNTL(2)
F_GETLK, F_SETLK and F_SETLKW
Manage discretionary file locks. The third argu-
ment 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 val-
ues.
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
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.
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FCNTL(2) Linux Programmer's Manual FCNTL(2)
NOTES
The errors returned by dup2 are different from those
returned by F_DUPFD.
CONFORMING TO
SVID, AT&T, POSIX, X/OPEN, BSD 4.3.
SEE ALSO
dup2(2), open(2), socket(2).
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