c - Why am I getting a segmentation fault?

Question

I'm trying to write a program that takes in a plaintext file as it's argument and parses through it, adding all the numbers together and then print out the sum. The following is my code:

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

static int sumNumbers(char filename[])
{
    int sum = 0;
    FILE *file = fopen(filename, "r");
    char *str;

    while (fgets(str, sizeof BUFSIZ, file))
    {
        while (*str != '')
        {
            if (isdigit(*str))
            {
                sum += atoi(str);
                str++;
                while (isdigit(*str))
                    str++;
                continue;
            }
            str++;
        }
    }

    fclose(file);

    return sum;
}

int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        fprintf(stderr, "Please enter the filename as the argument.
");
        exit(EXIT_FAILURE);
    }
    else
    {
        printf("The sum of all the numbers in the file is : %d
", sumNumbers(argv[1]));
        exit(EXIT_SUCCESS);
    }

    return 0;
}

And the text file I'm using is:

This a rather boring text file with some random numbers scattered throughout it.

Here is one: 87 and here is another: 3

and finally two last numbers: 12 19381. Done. Phew.

When I compile and try to run it, I get a segmentation fault.

Answer 1

You've not allocated space for the buffer.
The pointer str is just a dangling pointer. So your program effectively dumps the data read from the file into memory location which you don't own, leading to the segmentation fault.

You need:

char *str;
str = malloc(BUFSIZ); // this is missing..also free() the mem once done using it.

or just:

char str[BUFSIZ]; // but then you can't do str++, you'll have to use another 
                  // pointer say char *ptr = str; and use it in place of str.

EDIT:

There is another bug in:

while (fgets(str, sizeof BUFSIZ, file))

The 2nd argument should be BUFSIZ not sizeof BUFSIZ.

Why?

Because the 2nd argument is the maximum number of characters to be read into the buffer including the null-character. Since sizeof BUFSIZ is 4 you can read max upto 3 char into the buffer. That is reason why 19381 was being read as 193 and then 81<space>.