NAME

fcntl − file control

SYNOPSIS

#include <fcntl.h>

int fcntl (fildes, cmd, arg)
int fildes, cmd, arg;

DESCRIPTION

Fcntl provides for control over open files.  Fildes is an open file descriptor obtained from a creat, open, dup, fcntl, or pipe system call. 

The cmds available are:

F_DUPFD
Return a new file descriptor as follows:

Lowest numbered available file descriptor greater than or equal to arg.

Same open file (or pipe) as the original file. 

Same file pointer as the original file (i.e., both file descriptors share one file pointer). 

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 exec(2) system calls.

F_GETFD
Get the close-on-exec flag associated with the file descriptor fildes. 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 fildes to the low-order bit of arg (0 or 1 as above). 

F_GETFL
Get file status flags. 

F_SETFL Set file status flags to arg. Only certain flags can be set; see fcntl(7).

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

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

Cmd is F_DUPFD and 20 file descriptors are currently open.  ­[EMFILE]

Cmd is F_DUPFD and arg is negative or greater than 20.  ­[EINVAL]

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_SETFD Value other than −1. 

F_GETFL
Value of file flags.

F_SETFL Value other than −1. 

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

SEE ALSO

close(2), exec(2), open(2), fcntl(7). 

May 16, 1980