Advanced Programming in the UNIX® Environment

ptg10805159

Section 3.14 fcntlFunction 85

operation of the program, when invoked frombash(the Bourne-again shell). Results

will vary,depending on which shell you use.

$./a.out 0 < /dev/tty

read only

$./a.out 1 > temp.foo

$cat temp.foo

write only

$./a.out 2 2>>temp.foo

write only, append

$./a.out 5 5<>temp.foo

read write

The clause5<>temp.fooopens the filetemp.foofor reading and writing on file

descriptor 5.

Example

When we modify either the file descriptor flags or the file status flags, we must be

careful to fetch the existing flag value, modify it as desired, and then set the new flag

value. Wecan’t simply issue anF_SETFDor anF_SETFLcommand, as this could turn

offflag bits that werepreviously set.

Figure3.12 shows a function that sets one or more of the file status flags for a

descriptor.

#include "apue.h"

#include <fcntl.h>

void

set_fl(int fd, int flags) /* flags are file status flags to turn on */

{

int val;

if ((val = fcntl(fd, F_GETFL, 0)) < 0)

err_sys("fcntl F_GETFL error");

val |= flags; /* turn on flags */

if (fcntl(fd, F_SETFL, val) < 0)

err_sys("fcntl F_SETFL error");

}

Figure 3.12 Turn on one or more of the file status flags for a descriptor

If we change the middle statement to

val &= ̃flags; /* turn flags off */

we have a function namedclr_fl,which we’ll use in some later examples. This

statement logically ANDs the one’s complement offlagswith the currentval.