C - Create/Write/Read named pipe

Question

I want to create a named pipe and then write to it and after that I want read it. Here is my code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#define FIFO "fifo0001"
int main(intargc, char *argv[]){
    char input[256];
    FILE *fp;
    char str[50];
    printf("Please write some text:\n");
    scanf("%s", input);
    unlink(FIFO); /* Because it already exists, unlink it before */
    umask(0);
    if(mkfifo(FIFO, 0666) == -1){
        printf("Something went wrong");
        return EXIT_FAILURE;
    }
    if((fp = fopen(FIFO, "a")) == NULL){
        printf("Something went wrong");
        return EXIT_FAILURE;
    }
    fprintf(fp, "%s", input);
    if(fgets(str, 50, fp) != NULL){
        puts(str);
    }
    fclose(fp);
    return EXIT_SUCCESS;
}

After I write a text, nothing happen anymore. And there is no message. I have to quit the program with STRG C. Someone know what is wrong? I have to use the functions mkfifo, fopen, fprintf, fgets and fclose. So would be nice if I could keep them into the code.

FIFOs don't work this way. You need two programs, one (only) writes to the FIFO and the other (only) reads. — n. m. could be an AI, May 28 '16 at 18:56
Yes I know thats my goal, but i wrote in one program to test it. But when I remove the reading part there is the same problem — Nono, May 28 '16 at 19:01
Your program will block until another program opens the other end of the FIFO for reading. — n. m. could be an AI, May 28 '16 at 19:06
If you are going to test in a single program, then you need `popen` and `dup2` to write to the pipe and read from that same pipe via a second descriptor. — David C. Rankin, May 28 '16 at 19:06
@DavidC.Rankin no, `popen` does something entirely different. — n. m. could be an AI, May 28 '16 at 19:08
Possible duplicate of [Why does my program hang when opening a mkfifo-ed pipe?](http://stackoverflow.com/questions/8507810/why-does-my-program-hang-when-opening-a-mkfifo-ed-pipe) — n. m. could be an AI, May 28 '16 at 19:08
And `mkfifo(FIFO, 0666)` doesn't necessarily make a fifo with `0666` permissions. [Read about `umask`.](https://en.wikipedia.org/wiki/Umask) — Andrew Henle, May 28 '16 at 19:11
@n.m., yes popen does something different, I was referring to testing within a single file of the write to and read from a pipe in an analogous manner without riding the ragged edge of deadlock using a fifo that will probably only work on Linux. (see *man 7 fifo*). — David C. Rankin, May 28 '16 at 19:24
@PSkocik Yes, he is. Thank the gods! But way too many coders miss the umask. — Andrew Henle, May 29 '16 at 03:25

Petr Skocik · Answer 1 · 2020-05-19T07:36:51.040

FIFO's don't work nice with just one thread. You'll get blocked on a reading open until a writing open is performed and vice versa, so you'll need to open in RDWR (non-portable) mode or with RDONLY in one thread and WRONLY in another or you'll get blocked.

E.g.:

fp = fopen(FIFO, "r+");

then you'll need to write no more than the size of the FIFO buffer (which is ulimit -p * 512 ?) (or else you get blocked). After that, you'll need to read no more than what you've written.

All in all, this should work (although it's not the usual way to use FIFOs):

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#define FIFO "fifo0001"

int main(int argc, char *argv[]){
    char input[256] = "hw";
    FILE *fp;
    char str[50];
    printf("Please write some text:\n");
    scanf("%s", input); //!!!

    size_t input_len = strlen(input);

    unlink(FIFO); /* Because it already exists, unlink it before */
    umask(0);
    if(mkfifo(FIFO, 0666) == -1){
        printf("Something went wrong");
        return EXIT_FAILURE;
    }
    if((fp = fopen(FIFO, "r+")) == NULL){
        printf("Something went wrong");
        return EXIT_FAILURE;
    }
    fprintf(fp, "%s", input);
    if(fgets(str, input_len+1, fp) != NULL){
        puts(str);
    }
    fclose(fp);
    return EXIT_SUCCESS;
}

C - Create/Write/Read named pipe

1 Answers1