mkfifo don't make a FIFO special file - c

Question

when i compile my program it does not create a fifo type file, yet i change the permission, but still nothing
I also changed files but still nothing, the code compiles without problem, however when I run then I check if the fifo file is generated, but it is not so no file is generated
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <limits.h>
#include <sys/types.h>
#include <sys/stat.h>
#define FIFO_NAME "/tmp/my_fifo"
#define BUFFER_SIZE PIPE_BUF
#define TEN_MEG (1024 * 1024 * 10)
int main()
{
int pipe_fd;
int res;
int open_mode = O_WRONLY;
int bytes_sent = 0;
char buffer[BUFFER_SIZE + 1];
res = mkfifo(FIFO_NAME, 0777);
if (access(FIFO_NAME, F_OK) == -1) {
res = mkfifo(FIFO_NAME, 0777);
if (res != 0) {
fprintf(stderr, "Could not create fifo %s\n", FIFO_NAME);
exit(EXIT_FAILURE);
}
}
printf("Process %d opening FIFO O_WRONLY\n", getpid());
pipe_fd = open(FIFO_NAME, open_mode);
printf("Process %d result %d\n", getpid(), pipe_fd);
if (pipe_fd != -1) {
while(bytes_sent < TEN_MEG) {
res = write(pipe_fd, buffer, BUFFER_SIZE);
if (res == -1) {
fprintf(stderr, "Write error on pipe\n");
exit(EXIT_FAILURE);
}
bytes_sent += res;
}
(void)close(pipe_fd);
}
else {
exit(EXIT_FAILURE);
}
printf("Process %d finished\n", getpid());
exit(EXIT_SUCCESS);
}
here is the code of the second entity with which the first communicates
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <limits.h>
#include <sys/types.h>
#include <sys/stat.h>
#define FIFO_NAME "/tmp/my_fifo"
#define BUFFER_SIZE PIPE_BUF
int main()
{
int pipe_fd;
int res;
int open_mode = O_RDONLY;
char buffer[BUFFER_SIZE + 1];
int bytes_read = 0;
memset(buffer, '\0', sizeof(buffer));
printf("Process %d opening FIFO O_RDONLY\n", getpid());
pipe_fd = open(FIFO_NAME, open_mode);
printf("Process %d result %d\n", getpid(), pipe_fd);
if (pipe_fd != -1) {
do {
res = read(pipe_fd, buffer, BUFFER_SIZE);
bytes_read += res;
} while (res > 0);
(void)close(pipe_fd);
}
else {
exit(EXIT_FAILURE);
}
printf("Process %d finished, %d bytes read\n", getpid(), bytes_read);
exit(EXIT_SUCCESS);
}

Answer 1

I think the problem is blocking. The open will block if no process is reading.
For example, you might read from the fifo using:
cat /tmp/my_fifo
Here is a modified version of your code, to demonstrate:
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <limits.h>
#include <sys/types.h>
#include <sys/stat.h>
#define FIFO_NAME "/tmp/my_fifo"
#define BUFFER_SIZE PIPE_BUF
#define TEN_MEG (1024 * 1024 * 10)
int main()
{
int pipe_fd;
int res;
int open_mode = O_WRONLY;
int bytes_sent = 0;
char buffer[BUFFER_SIZE + 1];
res = mkfifo(FIFO_NAME, 0666);
if (access(FIFO_NAME, F_OK) == -1) {
res = mkfifo(FIFO_NAME, 0666);
if (res != 0) {
fprintf(stderr, "Could not create fifo %s\n", FIFO_NAME);
exit(EXIT_FAILURE);
}
}
printf("Process %d opening FIFO O_WRONLY\n", getpid());
pipe_fd = -1;
while(pipe_fd == -1) {
printf("Sleeping in loop \n");
sleep(1);
//Either of these versions work, but one will block, while the
//other will allow the loop to run
// pipe_fd = open(FIFO_NAME, open_mode ); // blocking version
pipe_fd = open(FIFO_NAME, open_mode | O_NONBLOCK); // non-blocking
}
printf("Process %d result %d\n", getpid(), pipe_fd);
char greeting[] = "Hi there\n";
write(pipe_fd, greeting, strlen(greeting));
printf("Process %d finished\n", getpid());
exit(EXIT_SUCCESS);
}
You can use the open() line with O_NONBLOCK or the one without. In either case the process will wait until someone reads from the fifo.