free() runtime error while working with structure pointer arrays

Question

Hello i have been struggling with my code for a while and finally found that the free() funtion is the cause. I think i am missing something about the details of how free() works.

My output is :

test test test test test 
ID: 200
RELEASE YEAR: 2006


ID: 201
RELEASE YEAR: 2006


ID: 202
RELEASE YEAR: 2006


ID: 203
RELEASE YEAR: 2006


ID: 204
RELEASE YEAR: 2006


AB

Edit : Added the full code

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

#define MAX 1000                                        //Do not edit this macro.

typedef struct{
    int film_id;
    char title[255];
    char description[1023];
    unsigned int release_year;
    char rental_duration;
    float rental_rate;
    unsigned char length;
    float replacement_cost;
    char rating[10];
    char last_update[30];
} RECORD_t, *RECORD;                                    //Do not edit this struct.


RECORD *find_by_year(int release_year, RECORD film_array, int start, int end,int *found);


int main(){
    RECORD rec = (RECORD)malloc(sizeof(RECORD_t)*MAX);  //Do not edit this line.
    FILE *file = fopen("data.txt", "rb");               //Do not edit this line.
    if (file == NULL) {                                 //Do not edit this line.
        printf("Cannot open the file.\n");              //Do not edit this line.
        exit(0);                                        //Do not edit this line.
    }                                                   //Do not edit this line.
    fread(rec, sizeof(RECORD_t)*MAX, 1, file);          //Do not edit this line.
    fclose(file);                                       //Do not edit this line.


    int i,test;
    RECORD *rec_arr;

    rec_arr=find_by_year(2006,rec,200,203,&test);
    for(i=0;i<test;i++){
            printf("ID: %d\n", rec_arr[i]->film_id);
            printf("RELEASE YEAR: %d\n", rec_arr[i]->release_year);
            printf("\n\n");
            fflush(stdout);
    }
    printf("A");
                fflush(stdout);

//  file = fopen("data.txt", "wb");                     //Do not edit this line.
//  fwrite(rec, sizeof(RECORD_t)*MAX, 1, file);         //Do not edit this line.
//  fclose(file);                                       //Do not edit this line.
    free(rec);                                          //Do not edit this line.


    printf("B");
    fflush(stdout);

    free(rec_arr);

printf("C");
    fflush(stdout);

    return 1;                                           //Do not edit this line.
}

RECORD *find_by_year(int release_year, RECORD film_array, int start, int end,int *found) {

    RECORD *rec_arr=malloc(sizeof(RECORD)*1);
    RECORD *narray;//for realloc check
    int size=1,i,j;
    start--;
    if(rec_arr==NULL){//if malloc fails
        printf("MALLOC FAILED find_by_year returning NULL\n");
        fflush(stdout);
        return NULL;
    }

    for(i=start;i<=end;i++){
        if(film_array[i].release_year==release_year){
            rec_arr[size-1]=&film_array[i];
            size++;
            narray=realloc(rec_arr,size);//increment the size by 1


            //ERROR HANDLING
            if(narray==NULL){//if realloc fails
                printf("INNER REALLOC FAILED find_by_year");
                fflush(stdout);
                narray =malloc(sizeof(RECORD) * size);

                if(narray==NULL){ //if malloc fails
                    printf("INNER MALLOC ALSO FAILED find_by_year returning NULL\n");
                    fflush(stdout);
                    return NULL;
                }

                for(j=1;j<size;j++){//copy
                    narray[size-1]=rec_arr[size-1];
                    free(rec_arr);
                }
            }
            printf("test ");
            fflush(stdout);

            rec_arr=narray;
        }
    }
    printf("\n");
    fflush(stdout);

    *found=size-1;


    if(size==1)//if not found anything
        return NULL;

    return rec_arr;
}

From the results of debugging free(rec_arr) fails everytime what could be the problem here. I cropped the code and i am almost sure that the cropped parts are working from the debugging.

Answer 1

This line is incorrect:

narray=realloc(rec_arr,size);//increment the size by 1

The second argument to realloc should be the number of bytes, not the number of records. You should multiply size by sizeof(RECORD) .

---

Here's some general recommendations about the rest of the code:

• I suggest just aborting in the if(narray=NULL) case. That code will almost never happen anyway , and probably will fail anyway even if it does enter. Recovering from out-of-memory is an advanced topic (especially taking into account that modern operating systems overcommit).
• You mentioned using Eclipse+CDT -- you should be able to step through the code in the debugger instead of having to use printf debugging statements.
• It would be good to check for rec being NULL on the first line of main, and also check the return value of find_by_year -- if it returns NULL then you don't want to go on to do the i loop. Your function does not set *found in the case of returning NULL, sometimes, so the caller has to do the null check before using the record count.
• I don't really agree with the other suggestion to change the realloc strategy. Keeping the code simple to read is not a bad plan , either for beginners or experts. And I doubt it is really inefficient as modern operating systems have a minimum allocation size so the realloc calls will basically be doing nothing unless your search returns more than say 1000 records.

Answer 2

The main thing wrong with your code is the approach. Repeatedly incrementing the size of an array by one using realloc() is horrifically inefficient. Instead, just do one pass over the data to check how many records match, then allocate the entire storage at once and do a second pass to populate it. Or, allocate an array as large as all the records, populate just the part you need, and don't worry about the "wasted" memory because it probably doesn't matter.

Either of the above approaches will result in much simpler code with fewer bugs.