Thread: Setting a char * to null

Hello,

I have a pointer which holds a variable for example:

char * username;
strcpy(username, "ted");

How do you set the pointer so its empty and does not hold the name ted?


I've tried setting the pointer to null but keep getting a segmentation error.

Thanks

for starters that code won't work anyway because the pointer doesn't point to memory you own. you have to allocate memory to the pointer first, or point it to an array. once you have that you can null out a string by setting the first character to 0.

Code:

username[0] = '\0';

that works because strings are just arrays of char with a 0 at the end. if the 0 is at the beginning, it's an empty string.

you need to allocate memory for the pointer before you do the strcpy function...

username = (char *) malloc=(sizeof(char) * strlen("ted"));
strcpy(username, "ted");
username = NULL;

Originally Posted by Bladactania

username = NULL;

that doesn't work. well it works, but it doesn't work well. if you allocate memory to username then set it to null, you lose the only reference you had to the memory and can't free it. that's called a memory leak. if you point username to an existing array you can set it to null but that doesn't change the actual string. it just means the pointer no longer points to the string.

Do you actually want to remove the data or just free the memory? These aren't the same things if I recall correctly.

Originally Posted by Meldreth

that doesn't work. well it works, but it doesn't work well. if you allocate memory to username then set it to null, you lose the only reference you had to the memory and can't free it. that's called a memory leak. if you point username to an existing array you can set it to null but that doesn't change the actual string. it just means the pointer no longer points to the string.

Well I had assumed he would free the memory later... Guess that was a bad assumption since he didn't know the need to allocate it in the first place.

that's wrong though. (char*)NULL isn't a string at all, it's a null pointer. this isn't a string either even though it's an array of char.

Code:

char im_not_a_string[] = {'a', 'b', 'c'};

the definition of a string is a contiguous sequence of characters terminated by '\0'. no '\0', no string.

Originally Posted by Bladactania

Well I had assumed he would free the memory later...

how can he free it if he doesn't have a pointer to it? setting username to NULL means you don't have a pointer to free anymore.

You can't have it both ways... if null means I don't have a pointer to free anymore then freeing it is the same as setting it to NULL.

so you can't free a pointer that has been set to NULL or 0?

Originally Posted by Snafuist

A string is a memory area, terminated by the first occurrence of '\0'.

that contradicts what you said before. NULL can't be an empty string if a string is a memory area terminated by the first '\0'. a null pointer is neither a memory area nor terminated by '\0'. it's a pointer to nowhere, with a predictable value so you know you're pointed to nowhere.

Originally Posted by Bladactania

so you can't free a pointer that has been set to NULL or 0?

you can always say free(NULL) or free(0). but that doesn't change anything.

Code:

p = malloc(n); // allocate memory
p = NULL; // forget where you allocated the memory
free(p); // does nothing
// the memory is still allocated!
// you can't free it because you pointed p somewhere else