memory leakage on asprintf using inside a loop in C - c

Question

I have a piece of code that looks like this
#include <stdio.h>
int main()
{
int i;
int number_of_chunks = 12;
char *final_string = NULL;
for(i = 0; i < number_of_chunks; i++)
{
char *chunk = some_hash_table.pop(i);
asprintf(&final_string, "%s%s", (final_string==NULL?"":final_string), chunk);
}
free(final_string);
return 0;
}
Here I am concatinating string chunks dynamically, meaning I don't know the size of each chunk in advance. For this I am using asprintf. The code works fine, however rise some serious memory issue. My doubt is asprintf allocates memory in each iteration and the code loses pointer in each iteration. If there is any other way I can concate string inside loop please guide me

Answer 1

To put your question in the simplest possible way, what you are essentially trying to do with the above code is
1. Allocate memory to a pointer continuously(in your case 12 times in the for loop) and
2. free it at the end only once, which is causing memory leak.
Like in the below code
#include <stdio.h>
int main()
{
int i;
int number_of_chunks = 12;
char *final_string = NULL;
for(i = 0; i < number_of_chunks; i++)
{
/*For example: similar to what asprintf does, allocate memory to the pointer*/
final_string = malloc(1);
}
free(final_string);
return 0;
}
From the above example it is easily visible that you have allocated the memory 12 times but freed only once.
code snippet:
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
int i;
int number_of_chunks = 12;
char *final_string = NULL;
char *tmp = NULL;
for(i = 0; i < number_of_chunks; i++)
{
char *chunk = some_hash_table.pop(i);
asprintf(&final_string, "%s%s", (tmp==NULL?"":tmp), chunk);
if (tmp)
free(tmp);
tmp = final_string;
}
printf("%s\n", final_string);
free(final_string);
return 0;
}

Answer 2

Others have already pointed out that you lose the reference to all but the last allocation and that having the same string that is written to as printf argument is probably undefined behaviour, even more so as re-allocations might occur and invalidate the format argument.
You don't use asprintf's formatting capabilities, you use it only to concatenate strings, so you might want to take another approach. You could either collect the strings in an array, determine the needed length, allocate as appropriate and fill the allocated buffer with memcpy.
Or you could write a self-allocating string buffer similar to C++'s std::stringstream, for example:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
struct append_t {
char *str; /* string */
size_t len; /* length of string */
size_t size; /* allocated size */
};
void append(struct append_t *app, const char *str)
{
size_t len = strlen(str);
while (app->len + len + 1 >= app->size) {
app->size = app->size ? app->size * 2 : 0x100;
app->str = realloc(app->str, app->size);
/* error handling on NULL re-allocation */
}
strcpy(app->str + app->len, str);
app->len += len;
}
int main(int argc, char **argv)
{
struct append_t app = {NULL};
for (int i = 1; i < argc; i++) {
append(&app, argv[i]);
}
if (app.str) puts(app.str);
free(app.str);
return 0;
}