strcpy pass initialized null pointer c

Question

I'm trying out the following code:

int main()
{
   char *yo = "yo";
   char *whaddup = NULL;
   strcpy(whaddup,yo);
}

and I get a segfault. Complete C noob here - other places say I should initialize whaddup as an array. Why can't I pass in a pointer to null?

Same reason you can't do `char *whaddup = NULL; *whaddup = 'a';` for example. Which is also the same reason you can't do `int *whaddup = NULL; *whaddup = 4;` — user253751, Jun 28 '16 at 02:07
@immibis but we can do `char *in = NULL; in = fgets(you.first_name, 100, stdin);` where `you` is some struct. Does this mean we can assign a pointer to the variable `in` to make it point somewhere else, but not a literal? — user2635088, Sep 24 '16 at 12:47
Well `in` doesn't contain NULL after you assign something that isn't NULL to it. If I do `int in = 5; in = 4;` then in contains 4 after this runs, and doesn't contain 5. `in = something;` is not the same as `*in = something;` - the latter doesn't modify `in` — user253751, Sep 24 '16 at 21:18

score 3 · Accepted Answer · answered Jun 28 '16 at 01:45

3

Just any strcpy documentation will tell you that the destination string should be a char array large enough to hold the string and the NUL terminator.

So what you need is something like

char* yo = "yo";
char* whaddup = malloc(strlen(yo)+1);
strcpy(whaddup, yo);

Or alternatively you could use strdup function which does this for you, but it's not standard as it's POSIX only.

answered Jun 28 '16 at 01:45

Jack

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I only included the sizeof(char) to demonstrate the conceptual memory allocation, but yes @user2635088 a character is one byte so you don't need it. Safe to get in the habit of using it when you begin allocating int, double, etc. – Jack Ryan Jun 28 '16 at 01:49

score 1 · Answer 2 · answered Jun 28 '16 at 01:44

You have to either declare whaddup as a character array or ideally allocate space for it with malloc.

int main()
{
   char *yo = "yo";
   char *whaddup = malloc(sizeof(char) * 8); // buffer for 8 characters
   strcpy(whaddup,yo);
}

By initializing whaddup to NULL you are not giving it any space in memory, so even copying one character into it will result in a segmentation fault.

score 0 · Answer 3 · answered Jun 28 '16 at 01:50

0

You can pass in a pointer to null, but you can not copy string from pointer to null.

The function char* strcpy(char* dest, const char* src) is copy string from address src to address dest, your dest is null.

answered Jun 28 '16 at 01:50

killer.vaan

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strcpy pass initialized null pointer c

3 Answers3

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