C Research vowel and consonant with fork()

Question

As the title says I'm tryng to create a program in C in which 2 son process research a vowel(son2) and a consonant(son2). To perform the research i use 2 function, the first to research a vowel and the second to research a consonant. I give to the program 3 more file from command line:

-1° is the file in which is stored a combination of vowel and consonant

-2° is the file in which will be stored all the vowel founded

-3° is the file in which will be stored all the consonant founded

The program is compiled without error/warning but it doesn't complete the research correctly, here is an example:

-first file: qwerty

-second file: ey

-third file: qr

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <ctype.h>

int test_vowel(char ch) 
{
if(ch == 'a' || ch == 'e' || ch == 'i' || ch == 'o' || ch == 'u' || 
ch == 'y')
    return(1);
else 
    return(0);
}


int main(int argc, char *argv[])
{
char c, d;
int s;
pid_t pid1 = 10, pid2 = 10;

if((pid1 = fork()) < 0)
{
  printf("Error in the creation of the first fork\n");
  _exit(0);
}
if(pid1 > 0)
{
 if((pid2 = fork()) < 0)
    {
      printf("Error in the creation of the second fork\n");
      _exit(0);
    }
}

int input = open(argv[1],O_RDONLY);
int output1 = open(argv[2],O_RDWR | O_CREAT | O_TRUNC, 0666);
int output2 = open(argv[3],O_RDWR | O_CREAT | O_TRUNC, 0666);

if(pid2 == 0)
    {
     while((s = read(input, &c, 1)) != 0)
        {
         if(test_vowel(tolower(c)) == 1)
            {
             printf("I've read a vowel = %d\n", d);
             write(output1, &c, 1);
            }
        }
    }

if(pid1 == 0)
    {
     while((s = read(input, &d, 1)) != 0)
        {
         if(test_vowel(tolower(d)) == 0)
            {
             printf("I've read a consonant = %d\n", d);
             write(output2, &d,1);
            }
        }
    }

}

I use these command to compile it

gcc c.c
./a.out c.txt v.txt b.txt

Thanks in advance for the help!

EDIT(final) Code (now is working) simplified following all of the tips of David C. Rankin

Answer 1

You don't initialize pid1 or pid2 to any particular value. And if pid1 is zero, you don't set pid2 to any value at all. But your first if checks the value of pid2. That can't be right.

Answer 2

Here's one probable source of trouble:

if(pid1 = fork() < 0)

In C, < has higher precedence than =, so the order of operations is something like this:

1. Call fork().
2. Evaluate fork() < 0. If fork() is negative, this evaluates to 1, otherwise to 0.
3. Set pid1 to the value of fork() < 0.

Normally, in both the parent process and in the child, fork() will return 0 or a positive number (provided that the parent was able to create a child process). So in both the parent and the child process, fork() < 0 will evaluate to 0 and pid1 will be set to 0.

Since pid1 is now equal to 0, if(pid1 > 0) will have a false condition, and the block of code after it will not execute in the child or in the parent. The second fork() will never be called, not even in the parent process where it is meant to be called.

---

Another thing to investigate is the effect of opening all the files in the parent process. Referring to information in the answers to Are file descriptors shared when fork()ing?, when one child reads a character from the input file it may cause the other child to "skip over" that character. The input received by each child would then depend on the order in which the child processes' calls to read overlapped, which could be random.

You could try opening the files in the child processes instead of the parent (so the input file has to be opened twice) and see if that clears things up.