Do you need to free a string from 'strcpy' if you copy a string generated from malloc?-CodePudding

Say I have some code snippet

char *str = malloc(sizeof(char)*10)
// some code to add content to the string in some way

To create a string of 10 chars. If I then copy str with strcpy from the standard string library into a new variable like so

char *copy;
strcpy(copy, str);

I'm aware I then need to free str using free(str), but is that enough? Or does strcpy also dynamically allocate memory for copy if used on a string created from malloc?

CodePudding user response：

Or does strcpy also dynamically allocate memory for copy

No, strcpy knows nothing about memory, so your code copies a string into an uninitialized pointer pointing at la-la land.

If you want allocation copy in the same call, there is non-standard strdup for that (which looks like it will be added to the C standard in the next version of the language).

Alternatively just do char *copy = malloc(strlen(str) 1); and then strcpy. Keep in mind to always leave room for the null terminator.

CodePudding user response：

strcpy does not allocate, thus your second snippet is invalid unless copy is initialized with some buffer (regardless of it being stack- or heap allocated) or is is a large enough array.

Side note: if you don't know the exact length of the source string, you need to make sure the target buffer size is not exceed (e.g. by using strncpy or providing a large enough target buffer).

I guess documentation should answer your question in detail.

CodePudding user response：

For starters the pointer copy is not initialized

char *copy;

So this call of strcoy

strcpy(copy, str);

results in undefined behavior.

The pointer copy must point to a character array where the string pointed to by the pointer str will be copied.

You need to free what was allocated with malloc, calloc or realloc.

So if the target array pointed to by the pointer copy was dynamically allocated like for example

char *copy = malloc( 10 * sizeof( char ) );

then of course you will need to free the allocated memory when it is not required any more.

This has nothing common with the function strcpy that just copies a string from one character array to another character array.