How to use a pipe properly between processes

Question

I had a similar question before. My parent process should send a string which contains characters. The child should receive the string and convert all characters into big characters. My problem is that my pipes aren't working at all. It doesn't receive any messages nor I am not sure if the messages are sent properly. Any advices how to use pipe between processes properly?

Thank you in advance

Here is my code:

#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>

#define BUF 64

void recv_and_send(int fd1, int fd2){
    char *buffer = malloc(sizeof(char)*BUF);
    read(fd2, buffer, strlen(buffer));
    for(int i = 0; i < strlen(buffer); i++){
        if(buffer[i] < 123 && buffer[i] > 96) {
            buffer[i] -= 32;
        }
    }
    write(fd1, buffer, strlen(buffer));
    free(buffer);
}

int main(void) {
    pid_t pid;
    int fd1[2], fd2[2];
    pipe(fd1);
    pipe(fd2);
    char *buffer = malloc(sizeof(char)*BUF);

    switch(pid = fork()) {
        case -1:
            perror("Error in fork()\n");
            break;
        case 0: //child
            close(fd1[0]); //fd1[1] zum schreiben
            close(fd2[1]); //fd2[0] zum lesen
            recv_and_send(fd1[1], fd2[0]);
            close(fd1[1]);
            close(fd2[0]);
            break;
        default: //parent
            close(fd1[1]); //fd1[0] zum lesen
            close(fd2[0]); //fd2[1] zum schreiben
            char string1[] = "a b c d";
            strcpy(buffer, string1);
            write(fd2[1], buffer, strlen(buffer));
            memset(buffer, 0, BUF);
            read(fd1[0], buffer, strlen(buffer));
            printf("Die Großbuchstaben sind: %s\n", buffer);
            break;
    }
}

It would be really helpful if anyone has a working code example that shows how to use pipes between processes.

You are using `strlen()` inappropriately. It expects its parameter to be a null-terminated string. — Ian Abbott, Apr 12 '23 at 16:37
There's no need to use `malloc()` for a string that's only used within a single function. Just use `char buffer[BUF];` — Barmar, Apr 12 '23 at 16:38
@ChanyoungWoo You're using it correctly in `main()` when you call `write()`. But you should use `sizeof()` in `recv_and_send()` when you call `read()`. — Barmar, Apr 12 '23 at 16:40
And `read()` returns the number of bytes read. You should use that in the `for` loop, because `read()` doesn't add a null terminator. — Barmar, Apr 12 '23 at 16:41
you mean I shouldn't use read() like that and instead read all the characters one by one in a for loop? — Chanyoung Woo, Apr 12 '23 at 16:45
For a working pipe solution, see my answer: [fd leak, custom Shell](https://stackoverflow.com/a/52825582/5382650) — Craig Estey, Apr 12 '23 at 17:02
No. I mean you should do `int n = read(...)` and then `for (int i = 0; i < n; i++)` — Barmar, Apr 12 '23 at 17:22

How to use a pipe properly between processes

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