fork and fopen example

[djlordas]

Hello,

I've got two questions from the code below::

#include <stdio.h>
#include <unistd.h>
#include <assert.h>
 
int main(void){
        int status;                             
        int ch;
        FILE *f1 = fopen("input.txt", "r");
 
        assert(f1 != NULL);
        if(fork()){                            
                printf("ProgA_Father: father's PID: %d\n", getpid());
                printf("ProgA_Father: file: %d\n", f1);
                ch=getc(f1);
                printf("ProgA_Father: data from file: %c\n", ch);
                wait(&status);         
                printf("ProgA_Father: child's exiting code: %d\n", status);
                ch=getc(f1);
                printf("ProgA_Father: data from file: %c\n", ch);
                system("ps -al");      
        }else{
                printf("ProgA_Child: child proccess created; PID: %d\n", getpid());
                printf("ProgA_Child: file: %d\n", f1);
                ch=getc(f1);
                printf("ProgA_Child: data from file: %c\n", ch);
                execl("b", "1", "2", "3", NULL);               
 }
        printf("\nProgA: all done.\n");
        fclose(f1);
        return 0;
}
 
//-----------------------------------------------
 
 
#include <stdio.h>
 
int main(int argc, char* argv[]){
        int i;  
        printf("ProgB: print arguments: \n");
        for(i=0; i<argc; printf("%s\n", argv[i++]));
                printf("ProgB: all done!\n");
        return 0;
}
 
 

here are my questions:
1. Why does 'execl("b", "1", "2", "3", NULL);' string terminates child process? any functions placed below it were not completed.

2. Why does child proccess read faulty data from text file? f.e. files contains '123', so father reads '1' and '2', and child reads '`'.

thanks in advance.

[sathishc58]

Hi

I can answer one of your two questions. You raised a question why execl() ends the child process.

All exec() family of function wil work like that. Whenver you call some other other program through execl() actually the called program will be replaced by the calling program. For example

Program A:
execl("program b");
printf("Program A after execl\n");
Program B:
printf("program b called and executed\n");

In the above example after calling execl() "Program A after exec" statement will never get printed.

This is because "A's" program memory will now be replaced by "B's" program. Now B will be running in the same memory area where "A" was residing.

If you are getting "Program A after execl" then it means that your execl() system call failed.

I hope I have given the correct information. If you find anything wrong please let me know. Whatever I studied I explained that.

And regarding your 2nd question I did not try that. May be after sometime I may reply for that question.

Thanks & Regards
Sathish Kumar

Hello,

I've got two questions from the code below::

#include <stdio.h>
#include <unistd.h>
#include <assert.h>
 
int main(void){
        int status;                             
        int ch;
        FILE *f1 = fopen("input.txt", "r");
 
        assert(f1 != NULL);
        if(fork()){                            
                printf("ProgA_Father: father's PID: %d\n", getpid());
                printf("ProgA_Father: file: %d\n", f1);
                ch=getc(f1);
                printf("ProgA_Father: data from file: %c\n", ch);
                wait(&status);         
                printf("ProgA_Father: child's exiting code: %d\n", status);
                ch=getc(f1);
                printf("ProgA_Father: data from file: %c\n", ch);
                system("ps -al");      
        }else{
                printf("ProgA_Child: child proccess created; PID: %d\n", getpid());
                printf("ProgA_Child: file: %d\n", f1);
                ch=getc(f1);
                printf("ProgA_Child: data from file: %c\n", ch);
                execl("b", "1", "2", "3", NULL);               
 }
        printf("\nProgA: all done.\n");
        fclose(f1);
        return 0;
}
 
//-----------------------------------------------
 
 
#include <stdio.h>
 
int main(int argc, char* argv[]){
        int i;  
        printf("ProgB: print arguments: \n");
        for(i=0; i<argc; printf("%s\n", argv[i++]));
                printf("ProgB: all done!\n");
        return 0;
}
 
 

here are my questions:
1. Why does 'execl("b", "1", "2", "3", NULL);' string terminates child process? any functions placed below it were not completed.

2. Why does child proccess read faulty data from text file? f.e. files contains '123', so father reads '1' and '2', and child reads '`'.

thanks in advance.

[djlordas]

I did some research on my own and I noticed that everything works fine with printf("blah blah", getc(f1)); before calling fork(). I wonder why it is so.