使用fork（）的单词频率

Question

I have the following program and I've got stuck at some point.(the program won't create more child processes but just one. Could anyone help me out ?

Write a program that counts occurrences of a string as a substring in another string (the two strings are given as arguments on the command line). Every time it checks if the first string appears as a substring starting from a position, checking will be done by a child process (obtained with fork) and the father process is not expecting for the child process to finish to initiate a search starting from a different position - so the verifications are being made in parallel. Each child process returns 0 = did not checked (not shown as substring from that position), 1 = has been verified. After conducting all searches, the father process is expected to finish all sons processes and gather their return codes - this value is will be printed(is the number of times as a substring).

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<string.h>
char *s1,*s2;
int verificare(char *s1, char *s2, int lungime)
{
  pid_t pid;
  int i,status;
  for (i = 0; i < lungime; i++) {
    pid = fork();
    switch (pid) {
    case -1:
      return EXIT_FAILURE;

    case 0: 
      if (strstr(s1, s2) != NULL)
        return 1;
      else
        return 0;
    _exit(0);

    default:
        waitpid(pid, &status, 0);
    if (WIFEXITED(status))
      printf("Child %d, Return code %d\n", pid,
             WEXITSTATUS(status));
    return status;
}
  }

}
int main(int argc,char **argv){
    if (argc!=3){
        printf("Too less arg");
        return 0;
    }
    s1=argv[1];
    s2=argv[2];
verificare(s1,s2,strlen(s2));
return 0;
}

Answer 1

1. You are using strstr which will do just what your own program should have done. Shouldn't you rather loop over the following characters and compare them to the pattern?
2. Your program essentially runs sequential since you are wait() ing for the child processes to finish immediately after you have started one. Shouldn't you store all child PIDs away in some array and only after all children are started begin to wait?
3. Your parent process will never see the return value of verificare in the child processes because your main ignores it and always returns 0. Maybe use exit to return from the child?
4. Your program has no way to report to the user whether a match was found.

I understand that this is a homework assignment and you are not supposed to do it differently than they asked for. Note however that this is not a very good usage of fork . Occurrence of a sub-string can be checked with a single processor in time linear in the length of the string to search (using finite automatons). So this parallel algorithm doesn't really buy you anything here.