Piping problem

This is a discussion on Piping problem within the C Programming forums, part of the General Programming Boards category; I get garbage output when printing the string s. This is after obtaining the output from xrandr -q, which will ...

  1. #1
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    Jun 2013
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    Piping problem

    I get garbage output when printing the string s.
    This is after obtaining the output from xrandr -q, which will list monitor devices and resolutions.
    I use this to determine if VGA1 has diconnected, and if so turn on display LVDS1 and exit the program.

    If you can tell me why s contains garbage i would be grateful.

    Code:
    #include <string.h>
    #include <stdio.h>
    //Searches a string for string in var 'me', returns the starting position of the found string +1 if successfull.
    //On failure it returns 0.
    unsigned int str_find(const char*str, const char* me, unsigned int s){
            
            unsigned int x = 0, l = strlen(me) - 1;
            while( (str[s] ^ '\0') ){
                if ( !(str[s]^me[x]) ){
                    //cout<<"Equal at " << s << ',' << x << endl;
                    if ( !(x^l) )
                        return s - x + 1;
                        
                    if ( (str[s+1]^me[x+1]) )
                        s -= x-2, x = 0;
                    else ++x;
                }
                ++s;
            } 
            return 0;
    }
    
    
    int main(){
      
      FILE *f;
      int c;
      unsigned short n;
      char s[1024];
      while( (f=popen("xrandr -q", "r")) ){
        
        for( n = 0; (c=fgetc(f) != EOF ); ++n )
          s[n] = c;
        s[n] = '\0';
    
    
        pclose(f);
        
        printf("%s",s);
        
        if( str_find("VGA1 disconnected", s, 0) > 0 ){
          printf("VGA1 Disconnected, igniting LVDS1 and exiting program\n");
          system("xrandr --output LVDS1 --auto");
          return 0;
          }
          
      }
      
      return 1;
    }

  2. #2
    and the hat of wrongness Salem's Avatar
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    Well you could probably start by removing all those silly ^ usage, and replace with more obvious and readable !=

    Then perhaps lookup what the strstr() function does in string.h


    > for( n = 0; (c=fgetc(f) != EOF ); ++n )
    Next, lookup operator precedence, and realise that = is lower than !=

    In which case, you probably want
    for( n = 0; (c=(fgetc(f) != EOF) ); ++n )
    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.
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  3. #3
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    Thank you, and sorry about the ^'s.

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