C malloc/free double pointer of char via a function

Question

I already read these links: link1 and link2 .

However, if I execute the following piece of code inside valgrind:

valgrind --tool=memcheck --leak-check=full --num-callers=40 --show-possibly-lost=no

I can see that the memory is not correctly freed.

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

void printVector(char ** vector, int N);
void allocateVector(char *** vector, int N, int M);
void deallocateVector(char *** vector, int N);

int main(int argc, char * argv[]) { 
    char ** vector;
    int N=6;
    int M=200;
    allocateVector(&vector,N,M);
    printVector(vector,N);
    deallocateVector(&vector,N);    
}

void allocateVector(char *** vector, int N, int M) {
    *vector=(char **) malloc(N*sizeof(char *));
    int i;
    for(i=0; i<N; i++) {
        (*vector)[i]=(char *) malloc(M*sizeof(char));
        (*vector)[i]="Empty";
    }
}

void deallocateVector(char *** vector, int N) {
    int i;
    char ** temp=*vector;
    for(i=0; i<N; i++) {
        if(temp[i]!=NULL) {
            free(temp[i]);
        }
    }
    if(temp!=NULL) {
        free(temp);
    }
    *vector=NULL;
}

I cannot find where is the mistake.

Answer 1

The problem is here:

for(i=0; i<N; i++) {
    (*vector)[i]=(char *) malloc(M*sizeof(char));
    (*vector)[i]="Empty";
}

You allocate space and store a pointer to it in (*vector)[i] . Then you overwrite that pointer with the address of the string constant "Empty" .

This causes two issues:

• The memory returned by malloc is leaked, as you no longer have a reference to it.
• When you later call free , you're passing it the address of a string constant instead of the address of an allocated block of memory. Calling free in this way invokes undefined behavior .

You need to use the strcpy function to copy the string constant into the memory you allocated:

for(i=0; i<N; i++) {
    (*vector)[i]=malloc(M);
    strcpy((*vector)[i],"Empty");
}

Also, don't cast the return value of malloc , and sizeof(char) is defined to be 1 and can be omitted.