C – Segmentation Fault with strcmp?

by

Apart from the possibility that the values in your htable may be invalid pointers (i.e., neither NULL nor a pointer to a decent C string), you have a serious problem of encountering an infinite loop if it contains neither a NULL nor the string you’re looking for.

For the immediate problem, try changing the code to:

#define FLUSH fflush (stdout); fsync (fileno (stdout))

int linear_probe (htable h, char *item, int k) {
    int pos = k;
    do {
        pos = (pos + 1) % h->capacity;
        printf ("========\n");                    FLUSH;
        printf ("inpk: %d\n",   k);               FLUSH;
        printf ("posn: %d\n",   pos);             FLUSH;
        printf ("cpct: %d\n",   h->capacity);     FLUSH;
        printf ("keyp: %p\n",   h->keys[pos]);    FLUSH;
        printf ("keys: '%s'\n", h->keys[pos]);    FLUSH;
        printf ("item: '%s'\n", item);            FLUSH;
        printf ("========\n");                    FLUSH;
    } while ((pos != k)
          && (h->keys[pos] != NULL)
          && (strcmp (h->keys[pos], item) != 0));
    return pos;
}

Those debug statements should give you an indication as to what’s going wrong.

Since you’re getting:

inpk: -2055051140
posn: -30
cpct: 113
keyp: 0x100000001

right before the crash, it’s evident that someone is passing in a bogus value for k. The modulo operation on negative numbers is implementation defined in the C standard so you’re getting a negative value for pos as well. And since h->pos[-30] is going to be undefined behaviour, all bets are off.

Either find and fix the code that’s passing in that bogus value (probably an uninitialised variable) or protect your function by changing:

int pos = k;

into:

int pos;
if ((k < 0) || (k >= h->capacity))
    k = 0;
pos = k;

at the start of your function. I’d actually do both but then I’m pretty paranoid 🙂

And, based on yet another update (the hash key calculation, if you generate an unsigned int and then blindly use that as a signed int, you’ve got a good chance of getting negative values:

#include <stdio.h>

int main (void) {
    unsigned int x = 0xffff0000U;
    int y = x;
    printf ("%u %d\n", x, y);
    return(0);
}

This outputs:

4294901760 -65536

My suggestion is to use unsigned integers for values that are clearly meant to be unsigned.

More Related Contents:

  1. C/SDL – Segmentation fault when program run multiple times in succession [closed]
  2. Why is this segfaulting? Can someone explain the valgrind error?
  3. Segmentation fault, I don’t know why
  4. Seg fault on code that should work according to the professor [duplicate]
  5. Making an array using pointers in c
  6. my code does not work and I don’t know why ;-;
  7. Last word is Failing …?
  8. Why do I get a segmentation fault with strcmp [closed]
  9. Why do I get a segmentation fault when writing to a “char *s” initialized with a string literal, but not “char s[]”?
  10. Why is this string reversal C code causing a segmentation fault? [duplicate]
  11. hash function for string
  12. What is a bus error? Is it different from a segmentation fault?
  13. segmentation fault using scanf [duplicate]
  14. Execution of printf() and Segmentation Fault
  15. What is the difference between a segmentation fault and a stack overflow?
  16. Getting Segmentation Fault
  17. Returning pointer from a function
  18. casting char[][] to char** causes segfault?
  19. What is a hash table and how do you make it in C? [closed]
  20. Segmentation Fault before main
  21. Segmentation Fault in strcpy()
  22. strtok segmentation fault
  23. Segmentation Fault While Creating Large Arrays in C
  24. Returning this pointer from a function
  25. segfault only when NOT using debugger
  26. Why is strtok causing a segmentation fault?
  27. Segmentation fault in btree implementation
  28. strlen not checking for NULL
  29. Segmentation fault when allocating large arrays on the stack
  30. strncpy leading to segmentation fault

Leave a Comment