deadlock-tester-1.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/file.h>
#include <unistd.h>

int main() {
    FILE * f1 = fopen("output1.txt", "a");
    FILE * f2 = fopen("output2.txt", "a");
    char buf[4] = {0};

    flock(fileno(f1), LOCK_EX);
    printf("File 1 has been locked by us.\n");

    sleep(10);

    flock(fileno(f2), LOCK_EX);
    printf("File 2 has been locked by us.\n");

    for (int i = 0; i < 10;) {
        sprintf(buf, "a%d\n", i++);
        fwrite(buf, sizeof(buf), 1, f1); fflush(f1);
        fwrite(buf, sizeof(buf), 1, f2); fflush(f2);
        if (i > 9) i = 0;
        sleep(1);
    }

    flock(fileno(f1), LOCK_UN);
    flock(fileno(f2), LOCK_UN);
    fclose(f1);
    fclose(f2);
}

deadlock-tester-2.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/file.h>
#include <unistd.h>

int main() {
    FILE * f2 = fopen("output2.txt", "a");
    FILE * f1 = fopen("output1.txt", "a");
    char buf[4] = {0};

    flock(fileno(f2), LOCK_EX);
    printf("File 2 has been locked by us.\n");

    sleep(10);

    flock(fileno(f1), LOCK_EX);
    printf("File 1 has been locked by us.\n");

    for (int i = 0; i < 10;) {
        sprintf(buf, "b%d\n", i++);
        fwrite(buf, sizeof(buf), 1, f2); fflush(f2);
        fwrite(buf, sizeof(buf), 1, f1); fflush(f1);
        if (i > 9) i = 0;
        sleep(1);
    }

    flock(fileno(f2), LOCK_UN);
    flock(fileno(f1), LOCK_UN);
    fclose(f2);
    fclose(f1);
}