How to realloc some memory allocated using calloc?

Question

I've allocated a string with the calloc function:

//string1 and string2 previously declared
char *stringClone = calloc(strlen(string1) + 1, sizeof(char));

Now I want to do the same thing on stringClone with a different string. Doing:

stringClone = calloc(strlen(string2) + 1, sizeof(char));

I'm gonna have some memory leak, right? How should I use the realloc in this case?

Answer 1

You can use realloc() to reallocate memory allocated by malloc() , calloc() , realloc() , aligned_alloc() or strdup() . Note that if the reallocated block is larger than the original block returned by calloc() , the newly allocated portion will not be initialized to all bits zero.

Note however that the syntax for realloc() is not what you use: you must pass the pointer as the first argument and a single size_t for the new size. Furthermore, if a new block cannot be allocated, NULL is returned and the block is not freed, hence you should not store the return value directly to stringClone .

If you want to use realloc() , here is what you should do:

//string1 and string2 previously declared
char *stringClone = calloc(strlen(string1) + 1, 1);
...
char *newp = realloc(stringClone, strlen(string2) + 1);
if (newp == NULL) {
    // deal with out of memory condition
    free(stringClone);
}

Since you do not seem to care that the contents of stringClone be preserved in the the reallocated block, you should probably simply write:

//string1 and string2 previously declared
char *stringClone = calloc(strlen(string1) + 1, 1);
if (stringClone == NULL) {
    // deal with out of memory condition
    ...
}
strcpy(stringClone, string1);
...
free(stringClone);
stringClone = calloc(strlen(string2) + 1, 1);
if (stringClone == NULL) {
    // deal with out of memory condition
    ...
}
strcpy(stringClone, string2);

Note also that on POSIX compliant systems, there is a memory allocation function that is very useful for your use case: strdup(s) takes a pointer to a C string, allocates strlen(s) + 1 bytes, copies the string to the allocated block and returns it:

//string1 and string2 previously declared
char *stringClone = strdup(string1);
if (stringClone == NULL) {
    // deal with out of memory condition
    ...
}
...
free(stringClone);
stringClone = strdup(string2);
if (stringClone == NULL) {
    // deal with out of memory condition
    ...
}

Note also that casting the return value of malloc , calloc and realloc is unnecessary in C and considered bad style.

Answer 2

The reason to use realloc is that it keeps the original data intact. However, if I understand your use-case correctly, you intend to erase the original data. In that case, it is simpler and clearer to just write:

char *stringClone = calloc(strlen(string1) + 1, sizeof(char));
// check for calloc error
// use stringClone for string1
free(stringClone);
stringClone = calloc(strlen(string2) + 1, sizeof(char));
// check for calloc error
// use stringClone for string2

The error-checking is simpler for calloc than for realloc , since no temporary variable is needed. Also, this pattern makes it clear that the array contents for string1 and string2 are not related.