malloc, free, and memmove inside a subfunction

Question

I want to use a subfunction to copy a char array. it is like this:

void NSV_String_Copy (char *Source, char *Destination)
{
    int len = strlen(Source);
    if (*Destination != NULL)
        free(Destination);
    Destination = malloc(len + 1);
    memmove(*Destination, Source, len);
    Destination[len] = '\0';             //null terminate
}

that way, I can call it from the main function and perform the operation this way:

char *MySource = "abcd";
char *MyDestination;

NSV_String_Copy (MySource, MyDestination);

However, it does not work as intended. please help!

FWIW, that whitespace style is *highly* idiosyncratic, and thus very hard to read. I've taken the liberty of editing to make it readable to the rest of the world... — Oliver Charlesworth, Feb 19 '15 at 22:42
Also, "it does not work as intended" is not a useful problem description. — Oliver Charlesworth, Feb 19 '15 at 22:43
I think you need pointers to pointers here (double-star) - at least for `Destination`. — 500 - Internal Server Error, Feb 19 '15 at 22:43
it's interesting how you use `*Destination` only at two places. — Karoly Horvath, Feb 19 '15 at 22:43
@LeeDanielCrocker, my actual code has the malloc initialization. you picked that out right tho. — CaTx, Feb 20 '15 at 14:22

user3386109 · Accepted Answer · 2015-02-20T01:39:08.607

C passes arguments by value, which means that you can't change the caller's MyDestination using the function prototype in the question. Here are two ways to update the caller's copy of MyDestination.

Option a) pass the address of MyDestination

void NSV_String_Copy (char *Source, char **Destination)
{
    int len = strlen(Source);
    if (*Destination != NULL)
        free(*Destination);
    *Destination = malloc(len + 1);
    memmove(*Destination, Source, len);
    (*Destination)[len] = '\0';             //null terminate
}

int main( void )
{
    char *MySource = "abcd";
    char *MyDestination = NULL;

    NSV_String_Copy(MySource, &MyDestination);
    printf("%s\n", MyDestination);
}

Option b) return Destination from the function, and assign it to MyDestination

char *NSV_String_Copy (char *Source, char *Destination)
{
    if (Destination != NULL)
        free(Destination);

    int len = strlen(Source);
    Destination = malloc(len + 1);
    memmove(Destination, Source, len);
    Destination[len] = '\0';             //null terminate

    return Destination;
}

int main( void )
{
    char *MySource = "abcd";
    char *MyDestination = NULL;

    MyDestination = NSV_String_Copy(MySource, MyDestination);
    printf("%s\n", MyDestination);
}

thank you, sir. you fixed my problem. first time I have to deal with pointer to pointer. mucho appreciado. — CaTx, Feb 20 '15 at 14:19

malloc, free, and memmove inside a subfunction

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