Coded FIFO issue

MDBlueIce · 07-15-2005, 11:10 AM

The behavior of the C program below is confusing me. It creates a FIFO, with permissions for the owner to read, write, and execute. It then opens the FIFO with an access mode flag of read only. It then attempts to read from the FIFO. Because no process has the pipe open for writing, I expect at this point for my program to return 0 on the read. This does not happen. The program hangs forever, waiting for info to read from the pipe. Is my expectation wrong or my code? The code appears below.

/* ====================Start Code====================*/
#include <fcntl.h>
#include <sys/stat.h>
#include <errno.h>

#define FIFO_PERMS (S_IRWXU)
#define FIFO_FLAG (O_RDONLY)

int main ()
{
char t [10];
int fd;

if ((mkfifo("fifolsky", FIFO_PERMS) == -1) && (errno != EEXIST))
{
printf ("failed to create fifo\n");
return 1;
}

if ((fd = open ("fifolsky", FIFO_FLAG)) == -1)
{
printf ("failed to open fifo\n");
return 1;
}

if (read(fd, t, 10) == 0)
{
printf("could not read anything from fifo\n");
return 1;
}

}

/*===================End Code=======================*/

Looking_Lost · 07-15-2005, 01:16 PM

It's more java I know but dabbling with C, anyway, read operates in a blocking fashion as far as I can tell, had a quick look at "man 2 read"

Edit: deleted nonsense bit of code
(waiting for the C people to slaughter me now

)

MDBlueIce · 07-15-2005, 01:44 PM

I learned the FIFOs, or more generally pipes, are special in that they block on reads unless there are no processes running which have them open for writing. If no processes have them open for writing, the read attempt returns 0, indicating end of file conditions.

Matir · 07-15-2005, 02:03 PM

I do realize that your question is about linux and the following information comes from a Solaris page, but since both of them are supposed to behave POSIX-ly with file handling, the information is likely still relevant. It also describes the situation you describe exactly.

Quote:

From Basic IO Functions: Sun Developer's Guide:

O_NONBLOCK or O_NDELAY

These flags affect future reads or writes to the file. If the file to be opened is a FIFO, then setting either O_NONBLOCK or O_NDELAY will cause a read-only open to return without delay; a write-only open will fail if no process currently has the FIFO open for reading. If both O_NONBLOCK and O_NDELAY are clear, then a read-only open will block until a process opens the FIFO for writing. Similarly, a write-only open of a FIFO will block until a process opens it for reading.