Messing with nuances of stdin/stdout timing in C

delayed-receiver.c

#define _GNU_SOURCE
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <errno.h>

int set_lock(short type) {
    struct flock fl;
    fl.l_whence = SEEK_SET;
    fl.l_start = 0;
    fl.l_len = 0;

    fl.l_type = type;
    return fcntl(STDIN_FILENO, F_SETLK, &fl);
}

int lock() {
    if (set_lock(F_RDLCK) == -1) {
        perror("receiver fcntl: couldn't read-lock stdin\n");
        return -1;
    }
    return 0;
}

int unlock() {
    if (set_lock(F_UNLCK) == -1) {
        perror("receiver fcntl: couldn't unlock stdin\n");
        return -1;
    }
    return 0;
}

int main() {
    int bytes_read;
    int pipe_capacity = fcntl(STDIN_FILENO, F_GETPIPE_SZ);
    char *buffer;

    fprintf(stderr, "receiving pipe size: %d\n", pipe_capacity);

    buffer = malloc(pipe_capacity);
    setvbuf(stdin, NULL, _IONBF, 0);
    if (lock() == -1) {
        return 1;
    }
    while ((bytes_read = read(STDIN_FILENO, buffer, 11)) > 0) {
        if (unlock() == -1) {
            break;
        }
        fprintf(stderr, "received %dB at %d: ", bytes_read, (int) time(NULL));
        fputs(buffer, stderr);
        if (lock() == -1) {
            break;
        }
        sleep(1);
    }
    free(buffer);
    return 0;
}

Makefile

CFLAGS = -Wall -pedantic

all: print-time delayed-receiver
	./print-time | ./delayed-receiver

clean:
	$(RM) print-time delayed-receiver *.o

print-time: print-time.o

delayed-receiver: delayed-receiver.o

print-time.c

#define _GNU_SOURCE
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <errno.h>

short get_lock(short type) {
    struct flock fl;
    fl.l_whence = SEEK_SET;
    fl.l_start = 0;
    fl.l_len = 0;

    fl.l_type = type;
    if (fcntl(STDOUT_FILENO, F_GETLK, &fl) == -1) {
        perror("print fcntl: couldn't check existing locks on stdout\n");
        return -1;
    }
    return fl.l_type;
}

int main() {
    int reps = 0;
    int pipe_capacity = fcntl(STDOUT_FILENO, F_SETPIPE_SZ, 16);
    char *buffer;
    
    fprintf(stderr, "tried to set pipe size to 16B, got %d\n", pipe_capacity);

    buffer = malloc(pipe_capacity);
    setvbuf(stdout, NULL, _IONBF, 0);

    while (reps++ < 10) {
        int count;
        while (get_lock(F_WRLCK) == F_UNLCK) {
            /* wait */
        }
        count = snprintf(buffer, pipe_capacity, "%d\n", (int) time(NULL));
        write(STDOUT_FILENO, buffer, count);
        while (get_lock(F_WRLCK) != F_UNLCK) {
            /* wait */
        }
    }
    free(buffer);
    return 0;
}