Question about fork and exec

This is a discussion on Question about fork and exec within the Linux Programming forums, part of the Platform Specific Boards category; I'm a newbie to processes in general and i have this code: Code: #include <sys/stat.h> #include <fcntl.h> #include <sys/types.h> #include ...

  1. #1
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    Question about fork and exec

    I'm a newbie to processes in general and i have this code:
    Code:
    #include <sys/stat.h>
    #include <fcntl.h>
    #include <sys/types.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <stdlib.h>
    
    
    int main() {
    	char buffer[100];
    	int pid, status;
    	while(1) {
    		printf("> ");
    		fgets(buffer, sizeof(buffer), stdin);
    		printf("here\n");
    		switch(pid = fork()) {
    			case 0 :
    				system(buffer);
    		}
    		waitpid(pid, &status, 0);
    	}
    }
    ( there are too many includes, i know ).

    I have to simulate a shell for a university homework and i have the following problem:
    Wherever i run this, i read a command from the keyboard and the "here" print always comes AFTER the command's output. Why is this happening? It's very important for my homework.

    Stelian

  2. #2
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    Try to flush the output buffer using fflush after printf("here")

  3. #3
    spurious conceit MK27's Avatar
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    Very likely the command's output is to stderr, and not stdout. Stderr is not buffered, and is a separate stream, so it is possible for stderr output to appear while stdout still has unflushed content.

    Use fflush like Shaki says or:
    Code:
    fprintf(stderr, "here\n");
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  4. #4
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    Thank you! fflush seems to be the solution

  5. #5
    Cat without Hat CornedBee's Avatar
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    On a side note, system() returns normally, so you're leaving a lot of zombies behind. You need to terminate the child process after calling system(), or better yet, use some exec() variant instead.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

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