如何在Unix中设置O_NONBLOCKING标志

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);