IPC FIFO Producer-Consumer Deadlock - c

Question

This is the producer.
// speak.c
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#define FIFO_NAME "american_maid"
int main(void)
{
char s[300];
int num, fd;
mknod(FIFO_NAME, S_IFIFO | 0666, 0);
printf("waiting for readers...\n");
fd = open(FIFO_NAME, O_WRONLY);
printf("got a reader--type some stuff\n");
while (gets(s), !feof(stdin)) {
if ((num = write(fd, s, strlen(s))) == -1)
perror("write");
else
printf("speak: wrote %d bytes\n", num);
}
return 0;
}
And this is the consumer.
//tick.c
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#define FIFO_NAME "american_maid"
int main(void)
{
char s[300];
int num, fd;
mknod(FIFO_NAME, S_IFIFO | 0666, 0);
printf("waiting for writers...\n");
fd = open(FIFO_NAME, O_RDONLY);
printf("got a writer\n");
do {
if ((num = read(fd, s, 300)) == -1)
perror("read");
else {
s[num] = '\0';
printf("tick: read %d bytes: \"%s\"\n", num, s);
}
} while (num > 0);
return 0;
}
When I run them, Producer outputs,
waiting for readers...
And consumer outputs,
waiting for writers...
speak doesn't find the reader, tick. As from the theory here I got that, open() (speak.c) will be keep blocked until open() (tick.c) is opened. And the vice versa. So I guess there a deadlock or something happening. I need a solution of this.

Answer 1

It looks like you have a race condition between the reader and the writer.
To fix this, you need a method of not launching the reader until the writer is "active". For this, I'd suggest making a pipe and writing to it when the writer is ready. Then, when reading from the read end of the fork succeeds, the fifo is prepared and the reader should work.
You need to use forks here because coordinating mutexes between a parent and a child process is non-trivial and properly done pipes is easier.
Also, you called mknod() twice. Granted, it'll return -1 with errno == EEXIST, but be more careful. To avoid this, make the reader and writer a function that takes a path as an argument.
Rewrite your writer as int speak(const char *fifo, int pipefd) and your reader as int tick(const char *fifo).
Then make a wrapper like this:
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
const char fifo_name[] /* = ... */;
int speak(const char *fifo, int pipefd);
int tick(const char *fifo);
int main() {
int pipefd[2];
pipe(pipefd);
mknod(fifo_name, S_IFIFO | 0666, 0);
if (fork() == 0) {
close(pipefd[0]);
return speak(fifo_name, pipefd[1]);
} else {
close(pipefd[1]);
char foo;
read(pipefd[0], &foo, 1);
return tick(fifo_name);
}
}
Modify your writer to print a byte (of anything) to the passed fd after the fifo is created (i.e. right after the call to open(..., O_WRONLY)).
Don't use my code verbatim, as I've omitted error checking for the sake of brevity.

Answer 2

it runs ok in my env. and if reader and writer is ready, open will return. because open is blocked, so in my opinion, mknod function is success. May be you excute these two process at different path.