在C中释放重新分配的数组

Question

I am just starting to learn about malloc'd and realloc'd arrays. Can anyone help explain to me how to properly free my following array? I have tried looking at other posts, but I have a hard time understanding memory allocation in C.

char ** result = NULL;
int numSpaces = 0;

char * p = strtok(command, " ");

/* split string and append tokens to 'result' */
while (p)
{
    result = realloc (result, sizeof (char*) * ++numSpaces);

    if (result == NULL)
        exit (-1); /* memory allocation failed */

    result[numSpaces-1] = p;

    p = strtok(NULL, " ");
}

Answer 1

Freeing realloc -ed memory is not different from freeing malloc -ed memory, in that all you need is to call free on the result at the end, when you no longer need the memory.

Your code, however, has a pattern that may lead to a memory leak: you are assigning realloc -ed block back to result without checking it for NULL . If realloc fails, the previous value of the result becomes unrecoverable, because it has been overwritten by NULL .

Here is how you could fix this issue:

char **temp = realloc(result, sizeof (char*) * ++numSpaces);

if (temp == NULL) {
    free(result); // Free the old memory block
    exit (-1); /* memory allocation failed */
}
result = temp;

Answer 2

At some point after you are done using result , you need to call free(result); .

This might look like:

char ** result = NULL;
int numSpaces = 0;

char * p = strtok(command, " ");

while (p) {
    result = realloc (result, sizeof (char*) * ++numSpaces);
    result[numSpaces-1] = p;
    p = strtok(NULL, " ");
}

for (i=0; i<numSpaces; ++i)
    printf("word %d: %s\n", i, result[i]);

free(result);