CODEWITHSUNDEEP
cbsearch
sclee1
sclee1

Reputation: 1281

Odd point issue by null pointer

I am doing a code practice using C language.

As below codes,

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

int ACDSort(const void *p1, const void *p2);
int Compare(const void *pKey, const void *pValue);
int main(void)
{
    char * strAry[4] = {"Hardware","Cookie","Boy","Power"};
    char * destStr = "Cookie";

    //qsort((void*)strAry, sizeof(strAry) / sizeof(char*), sizeof(char*), ACDSort);

    char **ptrAdr = (char**)bsearch((void*)destStr, strAry, sizeof(strAry) / sizeof(char*), sizeof(char*), Compare);

    printf("%s\n", *ptrAdr);
}

int Compare(const void *pKey, const void *pValue) {
    char *key = ((char*)pKey);
    char *value = *((char**)pValue);
    return strcmp(key, value);
}

int ACDSort(const void *p1, const void *p2) {

    char * n1 = *((char**)p1);
    char * n2 = *((char**)p2);
    int ret;

    if (strlen(n1) > strlen(n2))
        ret = 1;
    else if (strlen(n1) < strlen(n2))
        ret = -1;
    else
        ret = 0;
    return ret;
}

I called bsearch to find the string of cookie. The problem is that the error occurred when I erased the // in order to sort the array based on the string length. I don't know why the error was executed because I think that qsort couldn't be able to significant affect to my codes.

Could you tell me the reason why the error occurred returning null pointer when erasing the //?

Ps. I used qsort and bsearch to get familiar with pointer variables.

Upvotes: 0

Views: 125

Answers (2)

VolAnd
VolAnd

Reputation: 6407

Type of destStr can be changed to be the same as type of strAry, e.g.:

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

int Compare(const void *pKey, const void *pValue)
{
    char *key = *((char**)pKey);
    char *value = *((char**)pValue);
    return strcmp(key, value);
}

int main(void)
{
    char * strAry[4] = { "Hardware", "Cookie", "Boy", "Power" };
    char * destStr[1] = { "Cookie" }; // Type changing

    qsort(strAry, sizeof(strAry) / sizeof(char*), sizeof(char*), Compare);

    char **ptrAdr = (char**)bsearch((void*)destStr, strAry, sizeof(strAry) / sizeof(char*), sizeof(char*), Compare);

    printf("%s\n", *ptrAdr);
}

Also, if you use C language (and compiler) consider using strcmp directly as callback function that compares two elements.

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

/*int Compare(const void *pKey, const void *pValue)
{
    char *key = *((char**)pKey);
    char *value = *((char**)pValue);
    return strcmp(key, value);
}*/

int main(void)
{
    char * strAry[4] = { "Hardware", "Cookie", "Boy", "Power" };
    char * destStr[1] = { "Cookie" }; // Type changing

    //qsort(strAry, sizeof(strAry) / sizeof(char*), sizeof(char*), Compare);
    qsort(strAry, sizeof(strAry) / sizeof(char*), sizeof(char*), strcmp);

    //char **ptrAdr = (char**)bsearch((void*)destStr, strAry, sizeof(strAry) / sizeof(char*), sizeof(char*), Compare);
    char **ptrAdr = (char**)bsearch(destStr, strAry, sizeof(strAry) / sizeof(char*), sizeof(char*), strcmp);

    printf("%s\n", *ptrAdr);
}

Note:

This solution has one disadvantage that appears as ...

warning: passing argument 5 of ‘bsearch’ from incompatible pointer type [enabled by default]

But it works (I have tried with GCC version 4.8.2 and MS Visual studio 12.0)

Upvotes: -1

Lundin
Lundin

Reputation: 213950

bsearch uses binary search, that's why. Binary search requires the data to be sorted. Sort the string array in alphabetic order and it will work.

As a side note, you need to get rid of all those superfluous casts, all they do is to hide potential bugs.

Working program after fixes and clean-up:

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

int compare (const void* p1, const void* p2);

int main (void)
{
    const char* strAry[4] = {"Boy", "Cookie", "Hardware", "Power"};
    const char* key = "Cookie";

    char** ptrAdr = bsearch(key, 
                            strAry, 
                            sizeof(strAry)/sizeof(*strAry), 
                            sizeof(*strAry), 
                            compare);

    printf("%s\n", *ptrAdr);
}

int compare (const void* p1, const void* p2) 
{
  const char* s1 = p1;
  const char* s2 = *(const char* const*)p2;

  return strcmp(s1, s2);
}

p2 will end up a const void pointer to a const char*, which is why we get that weird-looking cast when striving for const-correctness.

Upvotes: 5

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