why my program fork function didn't run as expected

Question

#include <stdio.h>
#include <unistd.h>

int main(void)
{
    FILE    *fp;
    int     pid;
    char    msg1[] = "Test 1 2 3 ..\n";
    char    msg2[] = "Hello, hello\n";

    if ((fp = fopen("testfile2", "w")) == NULL)    // create the file
        return 0;
    fprintf(fp, "%d: %s", getpid(), msg1);         // parent print the message1

    if ((pid = fork()) == -1)                      // I fork a child process
        return 0;

    fprintf(fp, "%d: %s", getpid(), msg2);         // both parent and child print
    fclose(fp);                                    // and close

    return 1;
}

Here's the content of the "testfile2":

6969: Test 1 2 3 ..
6969: Hello, hello
6969: Test 1 2 3 ..
6970: Hello, hello

What *do* you expect it to do? Please add that information to your question. — Jost, Apr 18 '15 at 10:28
The program runs exactly as expected, it's just that your expectation is wrong. What's really strange is that your program reports that it was successful only when `fopen` or `fork` fails, and reports failure in other cases. (You should return `EXIT_FAILURE` or 1 when your program fails, and `EXIT_SUCCESS` or 0 when your program succeeds.) — William Pursell, Apr 18 '15 at 10:55

score 2 · Accepted Answer · edited May 23 '17 at 11:43

Let me guess: you would expect the output to be

6969: Test 1 2 3 ..
6969: Hello, hello
6970: Hello, hello

Right? So why isn't it like this? Well, I guess that the output of

fprintf(fp, "%d: %s", getpid(), msg1);

has not been flushed to the file at the point of time, when fork() is executed. So the output buffer is copied as well and that contains still

6969: Test 1 2 3 ..

So finally you what you got ;) I think this could be different, if you call fflush(fp); after the first printff(...).

Check this nice SO post as well ...

score 1 · Answer 2 · edited Jun 20 '20 at 09:12

You are not using fork() correctly and your if tags are not enclosed.

example

#include <sys/types.h> /* pid_t        */#include <sys/wait.h>  /* waitpid */#include <stdio.h>     /* printf, perror */#include <stdlib.h>    /* exit */#include <unistd.h>    /* _exit, fork */
int main(void)
{
   pid_t pid = fork();
 
   if (pid == -1) {
      // When fork() returns -1, an error happened.
      perror("fork failed");
      exit(EXIT_FAILURE);
   }
   else if (pid == 0) {
      // When fork() returns 0, we are in the child process.
      printf("Hello from the child process!\n");
      _exit(EXIT_SUCCESS);  // exit() is unreliable here, so _exit must be used
   }
   else {
      // When fork() returns a positive number, we are in the parent process
      // and the return value is the PID of the newly created child process.
      int status;
      (void)waitpid(pid, &status, 0);
   }
   return EXIT_SUCCESS;
}

Example from : http://en.m.wikipedia.org/wiki/Fork_(system_call)#Example_in_C

why my program fork function didn't run as expected

2 Answers2

example