error when freeing an array within a struct

Question

I'm dealing with a struct that contains an int and an array of ints

here are my two functions:

//stores array of char numbers in array of ints
//returns pointer to SomeStruct
SomeStruct *fromString(char *str){

    /*get string length*/
    int length = strlen(str);


    SomeStruct *huge;
    huge = malloc(sizeof(SomeStruct));

    /* incase malloc() fails */
    if ( huge == NULL) {
        return NULL;
    }
    huge->length = length;
    huge->digits = malloc((sizeof(int))*length);

    /*store chars as ints*/
    int i;
    for(i=0;i<length;i++){
        if (str[i] == ""){
            huge->digits[i] = 0;
        }
        if (str[i] == '0'){
            huge->digits[i] = 0;
        }
        if (str[i] == '1'){
            huge->digits[i] = 1;
        }
        if (str[i] == '2'){
            huge->digits[i] = 2;
        }
        if (str[i] == '3'){
            huge->digits[i] = 3;
        }
        if (str[i] == '4'){
            huge->digits[i] = 4;
        }
        if (str[i] == '5'){
            huge->digits[i] = 5;
        }
        if (str[i] == '6'){
            huge->digits[i] = 6;
        }
        if (str[i] == '7'){
            huge->digits[i] = 7;
        }
        if (str[i] == '8'){
            huge->digits[i] = 8;
        }
        if (str[i] == '9'){
            huge->digits[i] = 9;
        }
    }

    return huge;
}

and

//frees mem
//returns NULL ptr to struct
SomeStruct *destroy(SomeStruct *p){


    /*free each cell*/
    int i;
    for(i=0;i<getLength(p);i++){
        free(p->digits[i]);
    }
    /*free p  & return NULL*/
    free(p);
    p=NULL;

    return p;
}

and heres my sample main()

int main(){

    SomeStruct *z;
    z = malloc(sizeof(SomeStruct));

    z = fromString("100100987654321");

    //printcheck
    int i;
    for(i=0;i<getLength(z);i++){
      printf("%d", z->digits[i]);
    }
    printf("\n");

    destroy(z);

    return 0;
}

After I compile, I get the proper printout of 100100987654321, but once its hits the destroy function i get the following error: malloc: * error for object 0x2: pointer being freed was not allocated.

I played around with the destroy function and free(p) alone doesn't throw an error, but when i try to free anything within the array (p->digits) i get that error message. I assume the error has to do with my malloc within the fromString function, but how? thanks

You are either not enabling warnings or not paying attention to them. Bad practice. You want zero warnings at the highest possible warning level. — n. 1.8e9-where's-my-share m.

user1118321 user1118321 · Accepted Answer · 2014-03-02T04:47:16

The problem is this:

/*free each cell*/
int i;
for(i=0;i<getLength(p);i++){
    free(p->digits[i]);
}

You don't want to free each cell, you want to free all of them at once. The above should be replaced with

free(p->digits);

Remember how you allocated the memory was:

huge->digits = malloc((sizeof(int))*length);

You didn't allocate a different pointer for each element of digits, so no need to free every element separately.

If you are using unix/linux system, In terminal:

man 3 malloc

man 3 free

you will get more information about this.

error when freeing an array within a struct

1 Answers