freeing substring in c - loop

Question

I'm trying to get a sub-string for each member of the struct 'structs' and then assign that sub-string to a new member of the temp_struct. The problem I'm having is how to free the sub-string on each iteration, for some reason the code runs, however valgrind throws an Invalid read of size 1, which I assume I'm reading off the block of memory.

How could I free the sub-string?

Thanks

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

struct st_ex {
    char product[16];
    float price;
};
struct st_temp {
    char *prod;
};

char *temp = NULL;

// from stackoverflow
char* substr( const char* source, size_t start, size_t end )
{
    char* dest = malloc( end - start + 1) ;
    memcpy( dest, &source[start], end - start ) ;
    dest[end - start] = 0 ;
    return dest ;
}

int main()
{
    struct st_ex structs[] = {{"mp3 player", 2.0f}, {"plasma tv", 20.0f},
                              {"notebook", 10.0f},  {"smartphone", 49.9f},
                              {"dvd player", 10.0f}, {"matches", 0.2f }};
    struct st_temp **temp_struct;

    size_t j, i;
    temp_struct = malloc(sizeof *temp_struct * 6);
    for (j = 0; j < 6; j++)
        temp_struct[j] = malloc(sizeof *temp_struct[j]);

    size_t structs_len = sizeof(structs) / sizeof(struct st_ex);

    for(i=0; i<structs_len; i++){
        temp = substr(structs[i].product, 0, 4);
        temp_struct[i]->prod = temp;
        free(temp);
        temp = NULL;
    }
    for(i=0; i<6; i++ )
        printf("%s\n",temp_struct[i]->prod);

    for(i=0; i<6; i++ )
        free(temp_struct[i]);

    free(temp_struct);
    return 0;
}

Answer 1

Don't free substrings. In C, a substring is part of the original string. If you want to obtain a substring independent of a string, use strdup.

Answer 2

1) You are freeing the substring

    temp = substr(structs[i].product, 0, 4); 
    temp_struct[i]->prod = temp; 
    free(temp); 

The third line above frees the memory you malloc'd in substr.

2) because you're freeing the memory here, you've introduced a bug.
It's invalid to access the malloc'd memory after you free it, therefore it's invalid to try to print temp_struct[i]->prod.

The solution?
Don't free(temp), instead in your loop to free temp_struct[i], you first need to free temp_struct[i]->prod, like this

for(i=0; i<6; i++ )     
{
    free(temp_struct[i]->prod);
    free(temp_struct[i]);    
}

Answer 3

Josh, your temp_struct is holding on to the substring... you don't want to free the substring where you currently are freeing it. The invalid read is coming from this line:

for(i=0; i<6; i++ )
    printf("%s\n",temp_struct[i]->prod);

Instead, you want to free the substring when you free your temp structs, like so:

for(i=0; i<6; i++ )
{
    free(temp_struct[i]->prod);
    free(temp_struct[i]);
}