How to set O_NONBLOCKING flag in Unix - c

Question

I am writing a sender/reader IPC C program for class, and I am having trouble setting the O_NONBLOCK flag to 0 so that my reader will block when the buffer it is attempting to read from is empty. Here are the functions I am using:
int set_nonblock_flag(int desc, int value)
{
int oldflags = fcntl(desc, F_GETFL, 0);
if (oldflags == -1)
return -1;
if (value != 0)
oldflags |= O_NONBLOCK;
else
oldflags &= ~O_NONBLOCK;
return fcntl(desc, F_SETFL, oldflags);
}
main()
main ()
{
int fd[2], nbytes;
char readbuff[26];
int r_pid = 0;
int s_pid = 0;
/* THIS IS ALL UPDATED!*/
fd[0] = open("fd.txt",O_RDONLY);
fd[1] = open("fd.txt",O_WRONLY);
set_nonblock_flag(fd[0], 0);
set_nonblock_flag(fd[1], 0);
/* END UPDATES */
pipe(fd);
r_pid = fork();
if (r_pid < 0) /* error */
{
fprintf( stderr, "Failed to fork receiver\n" );
exit( -1 );
}
else if (r_pid == 0) /* this is the receiver */
{
fprintf( stdout, "I, %d am the receiver!\n", getpid() );
close( fd[1] ); /* close write end */
nbytes = read( fd[0], readbuff, 1 );
printf ("nonblocking flag = %d\n", fcntl(fd, F_GETFL, 0));
printf ("Nbytes read: %d\n", nbytes );
}
... /* rest of function removed */
The line printf ("nonblocking flag = %d\n", fcntl(fd, F_GETFL, 0));
is just returning -1 as the flag status. Shouldn't it be 0 if it is cleared?

Answer 1

You are calling set_nonblock_flag with first argument as an array of ints.
Here's a snippet from fcntl manpage. The first argument should be a file descriptor.
SYNOPSIS
#include <fcntl.h>
int fcntl(int fildes, int cmd, ...);
DESCRIPTION
The fcntl() function shall perform the operations described below on open files. The fildes argument is a file
descriptor.
I think you want to first call pipe and then call set_nonblock_flag. So, I think what you really want is the following:
int fd[2];
...
pipe(fd);
set_nonblock_flag(fd[0], 0);