spawnvp: bad parameter

This is a discussion on spawnvp: bad parameter within the C Programming forums, part of the General Programming Boards category; Somehow the spawnvp function doesn't like the second parameter (>). Any ideas how to solve this? Thanks in advance, Monster ...

  1. #1
    Me want cookie! Monster's Avatar
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    spawnvp: bad parameter

    Somehow the spawnvp function doesn't like the second parameter (>). Any ideas how to solve this?

    Thanks in advance,
    Monster
    Code:
    #include <stdio.h>
    #include <string.h>
    #include <process.h>
    #include <errno.h>
    
    int main(void)
    {
      char *my_args[5];
      
      my_args[0] = "ping";
      my_args[1] = "125.125.125.125";
      my_args[2] = ">";
      my_args[3] = "c:\\output.txt";
      my_args[4] = NULL;
       
      if(spawnvp(P_WAIT, my_args[0], my_args) == -1)
      {		
        puts(strerror(errno));
      }
    
      getchar();
      return 0;
    }
    Output: Bad parameter >

  2. #2
    .
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    The reason for the problem:

    ping is a binary (executable). spawnvp and execvp call a shell when a shell script is present. ping is not a script.

    You should wrap the command in a small shell script then call that script. Or a better call,
    system("/usr/sbin/ping somenode > somefile.txt").

    Also, it's a very good idea to put the path to any file or script:
    /usr/sbin/ping

  3. #3
    and the hat of wrongness Salem's Avatar
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    > ping is a binary (executable). spawnvp and execvp call a shell when a shell script is present. ping is not a script.
    Nope - nothing to do with that at all

    exec functions run executables
    If you want to run a script, you run the executable with the name of a script as a parameter

    system() allows you to run scripts directly, where it can figure out which script interpreter to run for a given script.

    > my_args[2] = ">";
    This is the real problem
    I/O redirection is a function of whatever command shell you are running.
    At this low level, it's just another parameter for the ping command (which it complains about)

    > Any ideas how to solve this?
    The short answer is use system()

    The long answer is to get involved in opening and closing streams to make stdout goto a file within the process you exec()
    This is reasonably easy to do in unix/linux, but I've no idea what you need to do to make it work in DOS.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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