FCNTL(2)

NAME
fcntl - file control

SYNOPSIS
#include <fcntl.h>

res = fcntl(fd, cmd, arg)
int res;
int fd, cmd, arg;

DESCRIPTION
Fcntl provides for control over descriptors. The argument fd is a descriptor to be operated on by cmd as follows:

F_DUPFD
    Return a new descriptor as follows:

    Lowest numbered available descriptor greater than or equal to arg.

    Same object references as the original descriptor.

    New descriptor shares the same file pointer if the object was a file.

    Same access mode (read, write or read/write).

    Same file status flags (i.e., both file descriptors share the same file status flags).

    The close-on-exec flag associated with the new file descriptor is set to remain open across execv(2) system calls.

F_GETFD
    Get the close-on-exec flag associated with the file descriptor fd. If the low-order bit is 0, the file will remain open across exec, otherwise the file will be closed upon execution of exec.

F_SETFD
    Set the close-on-exec flag associated with fd to the low order bit of arg (0 or 1 as above).

F_GETFL
    Get descriptor status flags, as described below.

F_SETFL
    Set descriptor status flags.

F_GETOWN
    Get the process ID or process group currently receiving SIGIO and SIGURG signals; process groups are returned as negative values.

F_SETOWN
    Set the process or process group to receive SIGIO and SIGURG signals; process groups are specified by supplying arg as negative, otherwise arg is interpreted as a process ID.

The flags for the F_GETFL and F_SETFL flags are as follows:

O_NONBLOCK
    Non-blocking I/O; if no data is available to a read call, or if a write operation would block, the call returns -1 with the error EWOULDBLOCK.

O_APPEND
    Force each write to append at the end of file; corresponds to the O_APPEND flag of open(2).

O_ASYNC
    Enable the SIGIO signal to be sent to the process group when I/O is possible, e.g., upon availability of data to be read.

RETURN VALUE
Upon successful completion, the value returned depends on cmd as follows:

	F_DUPFD	A new file descriptor.
	F_GETFD	Value of flag (only the low-order bit is defined).
	F_GETFL	Value of flags.
	F_GETOWN	Value of file descriptor owner.
	other	Value other than -1.

Otherwise, a value of -1 is returned and errno is set to indicate the error.

ERRORS
Fcntl will fail if one or more of the following are true:

[EBADF]
    Fildes is not a valid open file descriptor.

[EMFILE]
    Cmd is F_DUPFD and the maximum allowed number of file descriptors are currently open.

[EINVAL]
    Cmd is F_DUPFD and arg is negative or greater than the maximum allowable number (see getdtablesize(2)).

[ESRCH]
    Cmd is F_SETOWN and the process ID given as argument is not in use.

SEE ALSO
close(2), execve(2), getdtablesize(2), open(2), sigvec(2)

BUGS
The asynchronous I/O facilities of O_NONBLOCK and O_ASYNC are currently available only for tty and socket operations.