Array declaration: C Segmentation fault

问题

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.

回答1:

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.

回答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.

来源：https://stackoverflow.com/questions/38443757/array-declaration-c-segmentation-fault

标签

arrays

file-io