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:

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username[0] = '\0';

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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;

Quote:

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Originally Posted by Bladactania

username = NULL;

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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.

Quote:

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Originally Posted by Meldreth

if the 0 is at the beginning, it's an empty string. :)

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Just in order to maintain my image of being a nitpicker: (char *)NULL is the empty string, "\0" is a string of size 0.

Greets,
Philip

Quote:

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Originally Posted by Snafuist

Just in order to maintain my image of being a nitpicker: (char *)NULL is the empty string, "\0" is a string of size 0.

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I think your nitpicking is incorrect: (char*)NULL is a null pointer, but "\0" is an empty string. "" is also an empty string, as is "\0abc".

Quote:

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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.

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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:

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

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

Quote:

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Originally Posted by Bladactania

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

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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.

Quote:

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Originally Posted by Bladactania

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.

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No, they are not the same. Setting the pointer to be a null pointer does not free memory, and using free() does not set the pointer to be a null pointer.

Hi laserlight,

Quote:

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I think your nitpicking is incorrect: (char*)NULL is a null pointer, but "\0" is an empty string. "" is also an empty string, as is "\0abc".

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We have a different opinion about what a string is. To me, there's a difference between a string and a mere character array. A string is a memory area, terminated by the first occurrence of '\0'. Hence, I don't regard "\0abc" as being a string of size 4. To me, this is either a string of size 0 or a character array of size 4.

Thus, "" is a string of unknown size, probably far from being empty.

My main interest is complexity theory, where some languages (language as simply a set of words) contain the empty word, often called epsilon, which is different from a word of size 0 (in much the same way as NULL is different from "\0"). I might add that programming is just a hobby for me, so I'm happy with whatever meaning is in use for "empty string" around here. Just nitpicking...

Greets,
Philip

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

Quote:

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Originally Posted by Snafuist

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

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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.

Quote:

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Originally Posted by Bladactania

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

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

Code:

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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

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Quote:

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Originally Posted by Snafuist

To me, there's a difference between a string and a mere character array.

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Definitely.

Quote:

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Originally Posted by Snafuist

Hence, I don't regard "\0abc" as being a string of size 4.

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It is not a string of size 4. Rather, it is a string of length 0 (or not a string at all, if you consider string literals with embedded null characters as merely char arrays).

Quote:

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Originally Posted by Snafuist

To me, this is either a string of size 0 or a character array of size 4.

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It is not a char array of size 4. It is a char array of size 5, since the terminating null character must be counted.

Quote:

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Originally Posted by Snafuist

Thus, "" is a string of unknown size, probably far from being empty.

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"" is a string literal consisting of a null character. As such, it is of length 0, and is an empty string.

Quote:

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Originally Posted by Bladactania

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

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It is legal to do so, but it is a no-op (it does not actually free any memory).

Quote:

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Originally Posted by Snafuist

Thus, "" is a string of unknown size, probably far from being empty.

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Technically in C, "" is null terminated and has a length of zero.

I read somewhere the other day that Chomsky's "mathematical model for grammars" (1956) was practically applied more in computing than it ever was in linguistics. But I don't want to start anything nasty...

laserlight, I love you. I could talk about stuff like that all day long.

Quote:

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Originally Posted by laserlight

It is not a string of size 4. Rather, it is a string of length 0 (or not a string at all, if you consider string literals with embedded null characters as merely char arrays).

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No, I'm happy to regard the first part of it as a string of length 0. The whole thing altogether is still either a string of length 0 or a character array of size 4 or 5 (see below) or two strings concatenated together with their null-bytes or (just to make it more complicated) a string of size 0 concatenated with a character array of size 3.

Quote:

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It is not a char array of size 4. It is a char array of size 5, since the terminating null character must be counted.

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Oh, this is probably my fault. I was thinking that you wrote "\0abc" as shorthand for

Code:

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{'\0', 'a', 'b', 'c'}

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as in "\0abc" is part of "foo\0abcdefg". If the "" signs denote C strings, you are of course right. Then, according to my understanding, the size is 4, and according to your intention, the size is 5. I'm completely with you so far.


BTW, I'm in the process of writing you (among others) a private message. Shall I use PM or email?

Greets,
Philip