Array declaration: C Segmentation fault

Question

I am trying to write an array (2x20000) on C. The test code is:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>  

double test( int smod )
{
//
// test subroutine
//
double  vect_fma[2][20000];
int i;

// write on file //
FILE *f = fopen("file.txt", "w");
///////////////////

   for( i = 1; i < 20001; i = i + 1 ){     
    // allocate the vector for the fma analysis
    vect_fma[1][i] = i*smod;
    vect_fma[2][i] = i*smod;
    if ( i%smod == 0 ) 
    fprintf(f, "%f %f %f  \n", 1.0*i, vect_fma[1][i],vect_fma[2][i] );   
}    
    fclose(f);
    return 0;
}  


int smod;
void main()
{  
  smod  = 10; // every 10 print the output 
    test(smod);    // call the function  
}

I compiled the code with gcc test.c -lm -o test and I received Segmentation fault (core dumped) .

As far as I am new on C, I understand that "the compiler tries to store it on the stack" and a solution could be the one presented in the linked page....but that solution looks quite weird (and complex to understand) if compared with more simple fortran declaration of array real(8), dimension(n:m) :: vect_fma which I can put in a subroutine or in a function without problems. Is maybe that the declaration I wrote in the code is similar to the fortran real(8), dimension(n,m),allocatable :: vect_fma one ?

So the question is, it exist a simpler way in C to declare an array inside a function ? Many thanks to everybody.

@EdHeal something like that 2d_array = (int *)malloc(sizeof(int)*N*M); ? — Fornax-A
– Fornax-A, Commented Jul 18, 2016 at 18:36
Did you receive the segmentation fault during compilation (which is what your answer implies) or during execution? — CorbinMc
– CorbinMc, Commented Jul 18, 2016 at 18:36
C arrays are always 0-based indexes, not 1-based indexes. vect_fma[2] is outside the bounds of the array, and so is vect_fma[1][i] when i == 20000. — Dark Falcon
– Dark Falcon, Commented Jul 18, 2016 at 18:37

P.P · Accepted Answer · 2016-07-18 18:40:28Z

2

You have out of bounds access in multiple places, which is undefined behaviour. In C, an array index ranges from 0 to N-1, not from 1 to N. That means, rewriting the loop part to:

   for( i = 0; i < 20000; i = i + 1 ){     
    // allocate the vector for the fma analysis
    vect_fma[0][i] = i*smod;
    vect_fma[1][i] = i*smod;
    if ( i%smod == 0 ) 
       fprintf(f, "%f %f %f  \n", 1.0*i, vect_fma[0][i],vect_fma[1][i] );   
}

It's possible 2x20000 doubles might be too big for the stack size on your system, you'd better off fixing the undefined behaviours first and see if the problem disappears.

answered Jul 18, 2016 at 18:40

P.P

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2 Comments

Fornax-A Over a year ago

thanks for your answer it was correct about the integers and thanks for the example posted. I mixed up with fortran which start from 1 to 20000 which means I have 20000 elements, instead in C I have to start from 0 to 20000 .

Michi Over a year ago

0 to 19999 not 20000, because is < 20000

CorbinMc · Accepted Answer · 2016-07-18 18:41:01Z

2

The problem is your for loop. You should begin with an iteration where i=0 and end with an iteration where i=19999. Your code begins with an iteration where i=1 and ends with an iteration where i=20000.

The problem is that that there is no 20000th element of your array, only a 19999th (zero indexed). When you access the the 20000th element your accessing system memory that was never allocated for your program which is causing a segmentation fault.

Fix your for loop and you should be good.

answered Jul 18, 2016 at 18:41

CorbinMc

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