Problem with execl() and redirection.

This is a discussion on Problem with execl() and redirection. within the C Programming forums, part of the General Programming Boards category; Hi, I am trying to execute the program "upper" (code in upper.c) from "useupper" (code in useupper.c) with input redirection ...

  1. #1
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    Problem with execl() and redirection.

    Hi,

    I am trying to execute the program "upper" (code in upper.c) from "useupper" (code in useupper.c) with input redirection for "upper".
    Both the codes compile fine. But when I am running "useupper", the program waits for input and does not convert and show in uppercase the contents of "new.txt" (in which I have put some lowercase text).

    linux:~/prog # gcc -o useupper useupper.c
    linux:~/prog # gcc -o upper upper.c
    linux:~/prog #./useupper new.txt
    ... waits for some input here ...

    Code:
    // useupper.c
    
    #include <stdio.h>
    #include <unistd.h>
    #include <stdlib.h>
    
    
    int main(int argc, char* argv[])
    {
         if(argc != 2){
              fprintf(stderr,"usage: useupper file\n");
              exit(1);
         }
         execl("./upper", "upper", "<", argv[1], 0);
         perror("could not exec ./upper");
         exit(3);
    }
    Code:
    // upper.c
    
    #include <stdio.h>
    #include <ctype.h>
    #include <stdlib.h>
    
    
    int main()
    {
         int ch;
         while((ch = getchar()) != EOF)
              putchar(toupper(ch));
         exit(0);
    }
    "upper" itself works fine.

    linux:~/prog # ./upper<new.txt
    THIS TEXT WAS IN LOWERCASE IN FILE.
    linux:~/prog #


    Why is the text in "new.txt" not converted to uppercase when "upper" is run with execl() and redirection by "useupper"?

    Thanks and regards,

    Arun

  2. #2
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    What happens if you do?
    Code:
         execl("./upper", "upper", "<new.txt", 0);

  3. #3
    cas
    cas is offline
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    Using < for redirection is a shell feature; execl() knows nothing about it. You'll need to do the dirty work by hand. Look at fork() and pipe() and dup2() (and, if you're not familiar with them, open() and read() and write()).

    If you don't want to do all that, you can just use system(), which will spawn a shell and so your < character will do what you expect. You'll just have to build up the string using snprintf().

    In addition, you should use (char *)0 (or (char *)NULL) to terminate the list you pass to execl(), not just a plain zero.

  4. #4
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    Hi,

    Thanks for the responses and clarifying.

    Regards,

    Arun

  5. #5
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    >Using < for redirection is a shell feature; execl() knows nothing about it.
    Just curious ... would this work?
    Code:
         execl("sh <new.txt", "./upper", "upper", (char *) 0);

  6. #6
    cas
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    Quote Originally Posted by swoopy View Post
    >Using < for redirection is a shell feature; execl() knows nothing about it.
    Just curious ... would this work?
    Code:
         execl("sh <new.txt", "./upper", "upper", (char *) 0);
    No; that would look for a program called "sh <new.txt". You could do:
    Code:
    execl("/bin/sh", "sh", "-c", "./upper < new.txt", (char *)NULL);
    This is like using system() except, of course, execl() won't return on success.

  7. #7
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    Ah ok, thanks.

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