fcntl()

The operating characteristics of sockets can be controlled with the fcntl() call.

Note: COMMAND values that are supported by the UNIX Systems Services fcntl() callable service are also supported.

#include <manifest.h>
#include <socket.h>
#include <bsdtypes.h>
#include <fcntl.h>
int fcntl(int s, int cmd, int arg)

Parameter: Description
s: Socket descriptor
cmd: Command to perform
arg: Data associated with cmd

The operations to be controlled are determined by cmd. The arg parameter is a variable, the meaning of which depends on the value of the cmd parameter. The following commands are valid fcntl() commands:

Command: Description
F_SETFL: Sets the status flags of socket descriptor s. (One flag, FNDELAY, can be set.)
F_GETFL: Returns the status flags of socket descriptor s. (One flag, FNDELAY, can be queried.)
The FNDELAY flag marks s as being in nonblocking mode. If data is not present on calls that can block [read(), readv(), and recv()] the call returns with -1, and errno is set to EWOULDBLOCK.

Note: This function does not reject other values that might be rejected downstream.

Return values

For the F_GETFL command, the return value is the flags, set as a bit mask. For the F_SETFL command, the value 0 indicates success; the value -1 indicates an error. Errno identifies the specific error.

Errno: Description
EBADF: The s parameter is not a valid socket descriptor.
EINVAL: The arg parameter is not a valid flag, or the command is not a valid command.

Example

int s;
int rc;
int flags;
⋮
/* Place the socket into nonblocking mode */
rc = fcntl(s, F_SETFL, FNDELAY);
/* See if asynchronous notification is set */
flags = fcntl(s, F_GETFL, 0);
if (flags & FNDELAY)
   /* it is set */
else
   /* it is not */

Related calls

ioctl(), getsockopt(), setsockopt(), socket()