How to write C code which is similar to String.isNullOrEmpty in C#

Question

I've been C# developer.

But Now I have to write C code.

I used to use String.IsNullOrEmpty() for preventing Errors and assign a new string.

I hope to write C code to act like String.IsNullOrEmpty. and assign a new string

so I wrote some sample code.

static char *pCtvUrl;

void set_app_url(const char* appUrl){
     if(!appUrl || !*appUrl)
         return;
     pCtvUrl = malloc(sizeof(appUrl));
     strcpy(pCtvUrl,appUrl);
}

I want to check if this is correct or not.

If you know better way to solve, please give me some advice.

thanks

Answer 1

Your check for a NULL or empty string is correct

Change:

pCtvUrl = malloc(sizeof(appUrl)); /* This would allocate sizeof(char*) only. */

to:

pCtvUrl = malloc(sizeof(char) * (strlen(appUrl) + 1)); /* '+1' is for null terminator.*/

You also need to free() memory of pCtvUrl before reassigning it:

static char *pCtvUrl = 0;

Before pCtvUrl = malloc(...); call free(pCtvUrl); .

Instead of using a global variable consider changing set_app_url() to return a copy of the string:

char* set_app_url(const char* appUrl)
{
    char* retVal = 0;

    if (appUrl && *appUrl)
    {
        retVal = malloc(strlen(appUrl) + 1);
        strcpy(retVal,appUrl);
    }

    return retVal;
}

The caller would be responsible for freeing the returned string.

Answer 2

Checking whether appUrl is NULL or "empty" is correct.

However, calling your set_app_url function more than once will lead to memory leaks. You don't free previous pCtvUrl .

Also, you may have buffor underrun. You need to malloc strlen(appUrl) + 1 . Anyway, I suggest strdup to use here, instead.