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_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 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. 

4.2 Berkeley Distribution  —  Nov 30, 1994