Problem with Fork()

This is a discussion on Problem with Fork() within the C Programming forums, part of the General Programming Boards category; After I fork I appear to lose my variable the first time I try to execute a program. It will ...

  1. #1
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    Problem with Fork()

    After I fork I appear to lose my variable the first time I try to execute a program. It will run the 2nd, 3rd,.... time but not the first. Below is my code.
    This is part of the shell code I am writing, so everything is in a while loop. it appears to lose the execCommand variable. For instance with I run a ps command the first time and print execCommand after the for it comes out as a ?. Every other time though it comes out ok.

    thank you.
    Code:
    childPid = fork();
          
          if (childPid < 0)
          {
            perror ("fork");
            return 0;
          }
          
          if (childPid == 0)
          {
            
            if (execlp (execCommand, execCommand,(char*)0) == -1)
              perror("exec");
    
          }
          /* Have the parent wait for child to complete */
          if (wait (&status) < 0)
            perror ("wait");

  2. #2
    and the hat of wrongness Salem's Avatar
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    How exactly did you declare execCommand ?

    Whichever way you do it, either the first or the second parameter of execlp() will be wrong
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
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    I don't know if that's the issue.. when I print execCommand it comes up as ??'s.. it appears to lose it at the fork... anyway I delcared execCommand as a char by char * execCommand=malloc(100).

    I don't understand why it keeps losing the variables at the fork.. I didn't know that was an issue with forks.

  4. #4
    twm
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    >I didn't know that was an issue with forks.
    It isn't, but you're most likely not using execlp correctly. The first argument is the program to run, the second argument is the arguments to pass to the program. For example, if execCommand is "ls", then would you honestly expect
    Code:
    $ ls ls
    to work? Anyway, try giving us some code that compiles and shows your problem (as short as possible, please). That way we can be more helpful.
    The information given in this message is known to work on FreeBSD 4.8 STABLE.
    *The above statement is false if I was too lazy to test it.*
    Please take note that I am not a technical writer, nor do I care to become one.
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  5. #5
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    Isn't ls ls what I want? when I hard code ls in there then it works... again the variable is losing its value before it gets there


    I just don't understand why it is losing its value ... I can get code here but I have to relogin under Linux ...

    I don't think the problem is with the execlp. execlp("ls", "ls".. works...
    Code:
    execCommand = "ls";
    fork();
    execlp(execCommand, execCommand ...
    does not work.

    furthermore...
    Code:
    execCommand = "ls";
    fork()
    printf("%s",execCommand);
    does not work it displayes ??'s for the characters...
    Last edited by Schwarzhelm; 10-31-2003 at 02:11 PM.

  6. #6
    twm
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    >Isn't ls ls what I want?
    Not exactly. Try this and see if fork does something to your variable, the following works perfectly for me:
    Code:
    Fri Oct 31 4:12:58pm
    sangut: ~/misc
    > <c.c less
    #include <unistd.h>
    #include <sys/types.h>
    #include <stdio.h>
    
    int main ( ) {
            char *p = "ls";
    
            switch (fork()) {
            case -1:
                    perror("No forkie");
                    break;
            case 0:
                    printf("Child: %s\n", p);
                    break;
            default:
                    printf("Parent: %s\n", p);
                    break;
            }
            wait(NULL);
    
            return 0;
    }
    The information given in this message is known to work on FreeBSD 4.8 STABLE.
    *The above statement is false if I was too lazy to test it.*
    Please take note that I am not a technical writer, nor do I care to become one.
    If someone finds a mistake, gleaming error or typo, do me a favor...bite me.
    Don't assume that I'm ever entirely serious or entirely joking.

  7. #7
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    That worked ... hmm that leaves me in a mess now.. ok it works if I define the variable directly like you did. hmm what could i be doing wrong?

    The user inputs a commad and it holds until fork. If I hardcode it works fine. Any ideas.. this is kind of hard to explain.

  8. #8
    twm
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    >Any ideas.. this is kind of hard to explain.
    Source code, something we can compile. Just give me a quick program like I posted so I can test it.
    The information given in this message is known to work on FreeBSD 4.8 STABLE.
    *The above statement is false if I was too lazy to test it.*
    Please take note that I am not a technical writer, nor do I care to become one.
    If someone finds a mistake, gleaming error or typo, do me a favor...bite me.
    Don't assume that I'm ever entirely serious or entirely joking.

  9. #9
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    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <dirent.h>
    #include <fcntl.h>
    #include <time.h>
    #include <sys/param.h>
    
    char history[100][100];
    int historyCount = 1;
    char background[100][100];
    char globalCommand[100];
    
    
    
    
    int main(int argc, char *argv[])
    {
      while (1)
      {
        pid_t childPid;
        int    status;
        char * cmdLine=malloc(100);
        char * path=malloc(100);
        char * execCommand=malloc(100);
        path = printPrompt();
        printf("%s# ",path);
    
        
        cmdLine = readCommandLine();
       
        //if( isBuiltInCommand(cmdLine, path) == 1)
        {
          if (strcmp("exit", cmdLine) == 0)
          {
            exit(8);
          }
          else
            //executeBuiltInCommand(cmdLine, path);
        }
        
        else
        {
          
          execCommand = cmdLine;
          printf("%s\n", execCommand);
       
          childPid = fork();
          
          //if (childPid == 0)
          {
            //if (execlp (globalCommand, globalCommand,(char*)0) == -1)
            //    perror("exec");
            printf("%s\n",execCommand);
          }
          if (childPid < 0)
          {
            perror ("fork");
            return 0;
          }
          
          //if (childPid == 0)
          //{
    
            //if (execlp ("ps", "ps",(char*)0) == -1)
            //  perror("exec");
            //exit(127);
            //printf("childPid %i\n", childPid);
          //}
          /* Have the parent wait for child to complete */
    
          
          else
          {
            //if (runInForeground(cmdLine))
            {
            if (wait (&status) < 0)
              perror ("wait");
    
            }
            /*else
            {
              //record list of background jobs
            } */
          } 
        } 
    
      }    
             
      return EXIT_SUCCESS;
    }

  10. #10
    End Of Line Hammer's Avatar
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    Some thoughts:

    >>char * cmdLine=malloc(100);
    >>cmdLine = readCommandLine();
    You're malloc()ing memory and assigning the pointer to cmdLine. Then, you call a function and gives it's return code to cmdLine as well, thus loosing the address of the malloc()'d memory. Instant memory leak, and potential problem.

    >>char * execCommand=malloc(100);
    >>execCommand = cmdLine;
    And now you've gone and done the same thing again.


    There's too much commented out code in your post. It's unclear what you're trying to do with it. It's better to clean the code before posting here, you'll get better help that way.

    As for the fork(), make sure you use a method like twm showed, with regards to the return value.

    As for your use of execlp(), the first two parameters are correct, it's perfectly valid to have them both set to the name of the program (despite what you've been told ). Here's an example taken from the POSIX site:

    Code:
    Using execlp()
    The following example searches for the location of the ls command 
    among the directories specified by the PATH environment variable.
    
    #include <unistd.h>
    
    int ret;
    ...
    ret = execlp ("ls", "ls", "-l", (char *)0);
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  11. #11
    ATH0 quzah's Avatar
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    Well, for starters you have a memory leak:
    Code:
    char * execCommand=malloc(100);
    ...
    
    else
    {
          execCommand = cmdLine;
    You've probably got two of them:
    Code:
    char * cmdLine=malloc(100);
    ...
    cmdLine = readCommandLine();
    [edit]
    Curses, Foiled again!
    [/edit]

    Quzah.
    Hope is the first step on the road to disappointment.

  12. #12
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    Sorry about all the commented code. I was trying to comment out all the calls to the other sub routines that I didn't post here. I also have been playing with the stuff around the fork to see if I could fix my problem. It sounds like the memory leak might be my problem. How do I declare the cmdLine value? Do I just declare it as char cmdLine[100] or is there something else I need to do to fix the memory leak?

    Thank you

  13. #13
    Code Goddess Prelude's Avatar
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    >is there something else I need to do to fix the memory leak?
    If you don't want memory leaks, don't use dynamic memory allocation. It's as simple as this (for now at least ): When you don't know the size of a string until runtime, or you expect the size of the string the change, use dynamic memory. Otherwise, don't.

    However, in this case, it depends on how readCommandLine works. Most likely it allocates memory and returns a pointer to that memory. So you would create a pointer and simply assign the return of readCommandLine to that pointer. There's no need to use malloc, but you probably need to use free later.
    My best code is written with the delete key.

  14. #14
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    It still messes up... After my fork I lose the variable even when I try to print it, it prints a ?... any other suggestions?

    Here's my main again with not as much stuff commented out... It may not work for you though you may have to comment out the sub routine calls that aren't there.

    Code:
    int main(int argc, char *argv[])
    {
      while (1)
      {
        pid_t childPid;
        int    status;
        char  * cmdLine;
        char * path;
        char * execCommand;
        path = printPrompt();
        printf("%s# ",path);
    
        
        cmdLine = readCommandLine();
       
        if( isBuiltInCommand(cmdLine, path) == 1)
        {
          if (strcmp("exit", cmdLine) == 0)
          {
            exit(8);
          }
          else
            executeBuiltInCommand(cmdLine, path);
        }
        
        else
        {
          execCommand = cmdLine;
          printf("command Line: %s",cmdLine);
          printf("test %s\n", cmdLine);
       
          childPid = fork();
          printf("balls %s\n",cmdLine);
          if (childPid == 0)
          {
            if (execlp (execCommand, execCommand,(char*)0) == -1)
                perror("exec");
            
          }
          if (childPid < 0)
          {
            perror ("fork");
            return 0;
          }
          
          
          else
          {
            //if (runInForeground(cmdLine))
            {
            if (wait (&status) < 0)
              perror ("wait");
    
            }
            /*else
            {
              //record list of background jobs
            } */
          } 
        } 
    
      }    
             
      return EXIT_SUCCESS;
    }
    Thank you
    Last edited by Schwarzhelm; 11-02-2003 at 08:21 PM.

  15. #15
    End Of Line Hammer's Avatar
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    Post this function: readCommandLine
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

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