FCNTL(2) — System Interface Manual — System Calls

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 execve(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, see fcntl(5) for their definitions.

F_SETFL Set descriptor status flags, see fcntl(5) for their definitions.

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 SIGIO facilities are enabled by setting the FASYNC flag with F_SETFL.

RETURN VALUE

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

F_DUPFDA new file descriptor.
F_GETFDValue of flag (only the low-order bit is defined).
F_GETFLValue of flags.
F_GETOWN Value of file descriptor owner.
otherValue 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 the maximum allowable number (see getdtablesize(2)).

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