C bsearch导致分段错误

Question

I have the function call userInteractive(*anangramInfo) which pass in the pointer of struct anangramInfo and this struct contains the pointer "anagramPointer" to a actual anagrams. So, i get the stdin from the user. then use that as a key of bsearch to find the pointer thoese anagram has the same anangram.sorted. then move the pointer to left until the key is not matching the anagram.sorted, then move back to the right printf the anagram.word until the anagram.sorted is not match. But it causing me the segmentation problems

#include <string.h>
#include <errno.h>
#include <ctype.h>
#include "anagrams.h"
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>

#define SIZE 80

//struct
struct anagram {
    char word[SIZE];
    char sorted[SIZE];
};

struct anagramInfo {
    struct anagram *anagramPtr;
    int             numOfAnagrams;
};



/* qsort struct comparision function (product C-string field) */ 
int sortedMemberCompare( const void *ptr1, const void *ptr2 )
{ 
    struct anagram *ia = (struct anagram *)ptr1;
    struct anagram *ib = (struct anagram *)ptr2;

    return strcmp(ia->sorted, ib->sorted);
    /* strcmp functions works exactly as expected from
    comparison function */ 
}

void userInteractive( struct anagramInfo *anagramInfoPtr ){

    struct anagram a;
    char data[SIZE];
    char *pos;

    printf("Enter a word to search for anagrams [^D to exit]:\n");
    fgets(data, SIZE, stdin);

    //get ripe of the '\n'
    pos=strchr(data, '\n');
    *pos = '\0';

    strncpy(a.word,data,sizeof(data));

    //lowercase word
    int i;
    for(i=0;data[i] != '\0';i++)
      {
        data[i]=(char)tolower(data[i]);
      }

    /* sort array using qsort functions */ 
    qsort(data,strlen(data), 1, charCompare);

    strncpy(a.sorted,data,sizeof(data));

    //struct pointer to the elements
    struct anagram *ptr= (struct anagram *)bsearch(a.sorted,anagramInfoPtr-> anagramPtr ->sorted,anagramInfoPtr-> numOfAnagrams,sizeof(struct anagram),sortedMemberCompare);

    printf(ptr->word);

    while(strncmp(ptr ->sorted,a.sorted,sizeof(a.sorted))==0) {
        ptr--;
    }

    while(strncmp(ptr ->sorted,a.sorted,sizeof(a.sorted))==0) {
        printf(ptr ->word);
        printf(" ");
        ptr++;
    }

}

Answer 1

Your oversight is that bsearch is not searching on a contiguous array ; it is not even searching in the right place, since you are searching for sorted strings , but you ought to search for sorted members of anagrams :

struct anagram *ptr = (struct anagram *)bsearch(&a,
            anagramInfoPtr-> anagramPtr,
            anagramInfoPtr-> numOfAnagrams,
            sizeof(struct anagram),
            sortedMemberCompare);

Then, when bsearch fails and returns NULL , you do not check that ptr is NULL and use it; hence the segfault. (You also do not check NULL at the Ctrl-D).

Test

This is a modified version - I added back the charCompare function and whipped up a main() with five fixed strings in order to make it work, after a fashion.

#include <string.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>

#define SIZE 80

//struct
struct anagram {
    char word[SIZE];
    char sorted[SIZE];
};

struct anagramInfo {
    struct anagram *anagramPtr;
    int             numOfAnagrams;
};

/* qsort struct comparision function (product C-string field) */
int sortedMemberCompare( const void *ptr1, const void *ptr2 )
{
    struct anagram *ia = (struct anagram *)ptr1;
    struct anagram *ib = (struct anagram *)ptr2;

    return strcmp(ia->sorted, ib->sorted);
    /* strcmp functions works exactly as expected from
    comparison function */
}

int charCompare(const void *a, const void *b)
{
        const char *aa = a, *bb = b;
        if (*aa < *bb)
                return -1;
        if (*aa == *bb)
                return 0;
        return 1;
}

void userInteractive( struct anagramInfo *anagramInfoPtr ){

    struct anagram a;
    char data[SIZE];
    char *pos;

    printf("Enter a word to search for anagrams [^D to exit]:\n");
    fgets(data, SIZE, stdin);

    //get ripe of the '\n'
    pos=strchr(data, '\n'); // In Windows this might be a "\r" maybe?
    if (NULL == pos)
       exit(0);

    *pos = '\0';

    strncpy(a.word,data,sizeof(data));

    //lowercase word
    int i;
    for(i=0;data[i] != '\0';i++)
    {
        data[i]=(char)tolower(data[i]);
    }

    /* sort array using qsort functions */
    qsort(data, strlen(data), 1, charCompare);
    strncpy(a.sorted,data,sizeof(data));

    //struct pointer to the elements
    struct anagram *ptr = (struct anagram *)bsearch(&a,
                anagramInfoPtr-> anagramPtr,
                anagramInfoPtr-> numOfAnagrams,
                sizeof(struct anagram),
                sortedMemberCompare);

    // You must check that ptr is not NULL.
    if (NULL == ptr)
    {
        printf("Not found\n");
        return;
    }

    printf("Found: %s\n", ptr->word);

    /* Here there is a subtle error. If you find the FIRST item of the list,
       this code will position itself BEFORE THE BEGINNING of the list, and
       run a strncmp() against who knows what. Other segfault lurking! */

    while(ptr > anagramInfoPtr -> anagramPtr && strncmp(ptr ->sorted,a.sorted,strlen(a.sorted))==0) {
        ptr--;
    }

    // while(strncmp(ptr ->sorted,a.sorted,sizeof(a.sorted))==0) {
    //    ptr--;
    //}

    // Here too you should check you do not pass max number of anagrams,
    // if you hit the last item of the lot.
    while(strncmp(ptr ->sorted,a.sorted,sizeof(a.sorted))==0) {
        printf(ptr ->word);
        printf(" ");
        ptr++;
    }

}

int main()
{
        int i;
        struct anagramInfo *root;

        root = malloc(sizeof(struct anagramInfo));

        root->numOfAnagrams     = 5;
        root->anagramPtr        = malloc(root->numOfAnagrams * sizeof(struct anagram));

        strcpy(root->anagramPtr[0].word, "oriental");
        strcpy(root->anagramPtr[1].word, "teaching");
        strcpy(root->anagramPtr[2].word, "senator");
        strcpy(root->anagramPtr[3].word, "admirer");
        strcpy(root->anagramPtr[4].word, "rescued");

        for (i = 0; i < root->numOfAnagrams; i++)
        {
            int j;
            for (j = 0; root->anagramPtr[i].word[j]; j++)
               root->anagramPtr[i].word[j] = tolower(root->anagramPtr[i].word[j]);
            strcpy(root->anagramPtr[i].sorted, root->anagramPtr[i].word);
            qsort(root->anagramPtr[i].sorted, strlen(root->anagramPtr[i].sorted), 1, charCompare);
        }
        qsort(root-> anagramPtr, root-> numOfAnagrams, sizeof(struct anagram), sortedMemberCompare);
        for (;;)
            userInteractive(root);
    }

Run

$ gcc -W -Wall -o anagram anagram.c
$ ./anagram
Enter a word to search for anagrams [^D to exit]:
treason
Found: senator

Enter a word to search for anagrams [^D to exit]:
cheating
Found: teaching

Enter a word to search for anagrams [^D to exit]:
relation
Found: oriental

Enter a word to search for anagrams [^D to exit]:

$