Segmentation Fault? getc function and arrays - c

Question

I am trying to write a program to compile with Xeon Phi and it says there is a segmentation fault? I think it is when I try to fill the arrays with the getc function. I have written this code several different formats, and I understand that this might not be the most efficient, but I need to test it out to see if it will work by parallelizing it
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
//#include <omp.h>
int main()
{
struct stat buf1;
struct stat buf2;
FILE *fp1, *fp2;
int ch1, ch2;
clock_t elapsed;
char fname1[40], fname2[40];
printf("Enter name of first file:");
fgets(fname1, 40, stdin);
while (fname1[strlen(fname1) - 1] == '\n')
{
fname1[strlen(fname1) -1] = '\0';
}
printf("Enter name of second file:");
fgets(fname2, 40, stdin);
while (fname2[strlen(fname2) - 1] == '\n')
{
fname2[strlen(fname2) -1] = '\0';
}
fp1 = fopen(fname1, "rb");
if (fp1 == NULL)
{
printf("Cannot open %s for reading\n", fname1);
exit(1);
}
fp2 = fopen(fname2, "rb");
if (fp2 == NULL)
{
printf("Cannot open %s for reading\n", fname2);
exit(1);
}
stat(fname1, &buf1);
size_t size1 = buf1.st_size;
stat(fname2, &buf2);
size_t size2 = buf2.st_size;
printf("Size of file 1: %zd\n", size1);
printf("Size of file 2: %zd\n", size2);
elapsed = clock(); // get starting time
size_t smallest = 0;
if(size1 < size2)
{
smallest = size1;
}
else
{
smallest = size2;
}
printf("Smallest Value: %zu\n", smallest);
size_t i, j, k;
size_t data[smallest];
size_t arry1[smallest];
size_t arry2[smallest];
unsigned long long counter = 0;
for(i = 0; i < smallest; i++)
{
data[i] = 1;
arry1[i] = getc(fp1);
arry2[i] = getc(fp2);
}
//#pragma omp for //reduction(+:counter)
for(k = 0; k < smallest; k++)
{
if((arry1[k] ^ arry2[k]) == 0)
{
counter+= data[k];
}
}
fclose (fp1); // close files
fclose (fp2);
float percent = (float)counter / (float)smallest * 100.0f;
printf("Counter: %zu Total: %zu\n", counter, smallest);
printf("Percentage: %.2f%\n", percent);
elapsed = clock() - elapsed; // elapsed time
printf("That took %.2f seconds.\n", (float)elapsed/CLOCKS_PER_SEC);
return 0;
}
Thanks for your help in advance!

Answer 1

You cannot declare an array with a size that's not known at compile time:
int smallest;
smallest = .... // some computation
size_t data[smallest]; // this is wrong!
You should instead use malloc() to accomplish that:
size_t *data;
smallest = ... // whatever
data = malloc(smallest * sizeof(size_t));

Answer 2

This loop:
while (fname1[strlen(fname1) - 1] == '\n')
fname1[strlen(fname1) -1] = '\0';
will read off the start of the string if the line was blank (i.e. "\n"). Change while to if.
Also, check that smallest > 0 before declaring the VLAs.
It might be insightful to output the value of smallest, typical systems default to a stack size of somewhere between 1MB and 8MB, so perhaps you cause a stack overflow here. You could eliminate this possibility by using malloc, as ocho88 suggests (but without the bogus cast):
size_t *data = malloc(smallest * sizeof *data);
size_t *arry1 = malloc(smallest * sizeof *arry1);
size_t *arry2 = malloc(smallest * sizeof *arry2);
if ( !data || !arry1 || !arry2 )
// exit with out-of-memory error
I'm not sure why you use a size_t to store the result of getc.
If this does not solve the problem then it would be useful to identify which line is segfaulting. If you can't get a debugger working, then you can output (to stderr, or to stdout with fflush) to find out where it is getting up to.