Unexplained Segfault

This is a discussion on Unexplained Segfault within the C Programming forums, part of the General Programming Boards category; Hey all, I can't for the life of me figure out why this is segfaulting... sorry to bother you all ...

  1. #1
    Registered User crepincdotcom's Avatar
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    Unhappy Unexplained Segfault

    Hey all,

    I can't for the life of me figure out why this is segfaulting... sorry to bother you all with such a lame question. Here's the source:

    Code:
    #include <stdlib.h>
    #include <stdio.h>
    //#include <math.h>
    
    #define NUM_IN     2
    #define NUM_HIDDEN 3
    #define NUM_OUT    1
    
    //double v[3][3];         //the widest line across has 3 neurons, and 3 layers
    double w[3][3][3];      //for each connection [line][neuron][connection]
    int num[3]={NUM_IN,NUM_HIDDEN,NUM_OUT};
    
    void init_weights() {
      int i,j,k;
      double t;
      
      for (i=1;i<3;i++) {               //layer
        for (j=0;j<num[i];j++) {        //neurons is layer
          for (k=0;k<num[i-1];k++) {    //connections per neuron
            t=((double)(rand()))/((double)(RAND_MAX));
            w[i][j][k]=(t-0.5);         //vals -0.5 <= w <= 0.5
            printf("Setting for layer %d, neuron %d, input %d: %f\n",
                   i,j,k,(t-0.5));
          }
        }
      }
    }
    
    int main( int argc, char *argv[] ) {
      srand(getpid()*time());
    
      init_weights();
    
      return 0;
    }
    And when I run it:

    Code:
    [jack@jacklaptop1 neural-net]$ uname -a;id
    Linux jacklaptop1.crepinc.com 2.6.15-1.2054_FC5smp #1 SMP Tue Mar 14 16:05:46 EST 2006 i686 i686 i386 GNU/Linux
    uid=500(jack) gid=500(jack) groups=500(jack) context=user_u:system_r:unconfined_t
    [jack@jacklaptop1 neural-net]$ gcc -o neural1 neural1.c -lm
    [jack@jacklaptop1 neural-net]$ ./neural1
    Setting for layer 1, neuron 0, input 0: -0.480149
    Setting for layer 1, neuron 0, input 1: -0.267299
    Setting for layer 1, neuron 1, input 0: -0.104947
    Setting for layer 1, neuron 1, input 1: -0.483809
    Setting for layer 1, neuron 2, input 0: 0.079405
    Setting for layer 1, neuron 2, input 1: 0.422783
    Setting for layer 2, neuron 0, input 0: -0.258891
    Setting for layer 2, neuron 0, input 1: 0.237996
    Setting for layer 2, neuron 0, input 2: 0.243387
    Segmentation fault
    [jack@jacklaptop1 neural-net]$
    Thanks for your help,


    -Jack Carrozzo
    jack {-@-} crepinc.com

  2. #2
    and the hat of wrongness Salem's Avatar
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    > srand(getpid()*time());
    time() has a parameter, but the compiler doens't know that because you didn't include the right headers.
    So the unpassed pointer is just any old thing which is lying around on the stack, which time() duly trashes (probably the return address from main back to the OS).
    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
    Registered User crepincdotcom's Avatar
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    ahhhh you guys are great. I removed the time() call... I knew it was something stupid.

    Thanks again,
    -Jack C
    jack {at} crepinc.com
    http://www.crepinc.com

  4. #4
    ... kermit's Avatar
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    Just as an afterthought, if you had compiled with the -Wall flag, you would have caught that you did not include <time.h>, which would have been quite helpful for you.

    Code:
    gcc -Wall -o neural1 neural1.c -lm

  5. #5
    Registered User crepincdotcom's Avatar
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    is -Wall like "use strict;" in perl then?
    -Jack C
    jack {at} crepinc.com
    http://www.crepinc.com

  6. #6
    Gawking at stupidity
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    That would probably be more like -ansi. -Wall just turns all the warnings on.
    If you understand what you're doing, you're not learning anything.

  7. #7
    Frequently Quite Prolix dwks's Avatar
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    Typically, to make the compiler really picky, I use
    Code:
    gcc -W -Wall -ansi -pedantic -O2 other-arguments
    dwk

    Seek and ye shall find. quaere et invenies.

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