Segmentation fault when using qsort

Question

I am working on a c program to read from a txt file and sort the strings.

data.txt:

jk ef ab cd bc gh fg ij hi de 

Here is my code:

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

int cmp(const void *p1, const void *p2) {
    return strcmp(*(const char **)p1,  *(const char **)p2);
}

int main() {
    FILE *f = fopen("data.txt", "r");
    char s[255][255];
    char tmp[255];
    int n = 0;

    while (!feof(f)) {
        fscanf(f, "%s", tmp);
        strcpy(s[n], tmp);
        n++;
    }

    fclose(f);

    qsort(s, n, sizeof(char *), cmp);

    int i = 0;
    for (; i < n; i++) {
        printf("%s ", s[i]);
    }

    return EXIT_SUCCESS;
} 

I ran the code on Ubuntu and it breaks on a segfault. Believe this segfault happened in qsort and I could not figure out why.

Anyone can give me some suggestions?

Answer 1

The comparison function is incorrect, as you are sorting an array of arrays of char, you can pass the pointers to the elements to strcmp directly:

int cmp(const void *p1, const void *p2) {
    return strcmp(p1, p2);
}

Note however that your parsing loop is also incorrect: feof() is not the correct way to check for end of file. Use this instead:

  n = 0;
  while (n < 255 && fscanf(f, "%254s", s[n]) == 1) {
      n++;
  }

The qsort invokation should specify the size of the array element:

  qsort(s, n, sizeof(*s), cmp);

Here is a corrected version:

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

int cmp(const void *p1, const void *p2) {
    return strcmp(p1, p2);
}

int main(void) {
    FILE *f = fopen("data.txt", "r");
    char s[255][255];
    char tmp[255];
    int n = 0;

    if (f == NULL)
        return EXIT_FAILURE;

    while (n < 255 && fscanf(f, "%254s", s[n]) == 1) {
        n++;
    }
    fclose(f);

    qsort(s, n, sizeof(*s), cmp);

    for (int i = 0; i < n; i++) {
        printf("%s ", s[i]);
    }
    printf("\n");

    return EXIT_SUCCESS;
}

Answer 2

Many people gave a good answer.

Here is how you could have found it by yourself, step by step, with standard GNU tools:

We suppose the source file is named q.c.

Compile with debugging symbols (note that no need to have a Makefile here):

% make CFLAGS=-g q
cc -g  q.c  -o q

Now, run the program with a debugger (gdb):

% gdb q
(gdb) run
Starting program: /usr/home/fenyo/tmp/qs/q
Program received signal SIGSEGV, Segmentation fault.
0x00000008009607a6 in strcmp () from /lib/libc.so.7

Now look at the stack frame:

(gdb) where
#0  0x00000008009607a6 in strcmp () from /lib/libc.so.7
#1  0x00000000004009b5 in cmp (p1=0x7ffffffeeb60, p2=0x7ffffffeeb88) at q.c:8
#2  0x000000080093b834 in qsort () from /lib/libc.so.7
#3  0x0000000000400af5 in main () at q.c:26

So your problem is in a call to your function cmp by the qsort library, that calls strcmp with bad pointers.

So we go up from one stack frame, to be at your cmp function level:

(gdb) up
#1  0x00000000004009b3 in cmp (p1=0x7ffffffeeb60, p2=0x7ffffffeeb88) at q.c:8
8           return strcmp( *(const char **) p1,  *(const char **) p2);

We look at the type of p1:

(gdb) ptype p1
type = void *

Since p1 is a pointer, we examine its content displaying 10 first bytes:

(gdb) print (*(char *) p1)@10
$43 = "jk\000\000\000\000\000\000\000"

So we discover it is a null terminated string containing jk.

So your cast is invalid: *(const char **) p1.

This should have been (const char*) p1.

We change the cast and then it works.