what is the correct way to define a string in C?

Question

what is the correct way to define a string in C? using:

char string[10];

or

char *string="???";

If I use an array, I can use any pointer to point to it and then manipulate it. It seems like using the second one will cause trouble because we didn't allocate memory for that. I am taught that array is just a pointer value, I thought these two are the same before. Until I did something like string* = *XXXX , and realize it didn't work like a pointer.

Answer 1

As @affenlehrer points out, how you "define" a string depends on how you want to use it. In reality, 'defining' a string in C really just amounts to putting it in quotes somewhere in your program. You should probably read more about how memory works and is allocated in C, but if you write:

char *ptr = "???"

What happens is that the compiler will take the string "???" (which is really four bytes of data, three '?'s followed by one zero byte for the NUL terminator). It will insert that at some static place in your program (in something called the .bss segment ), and when your program starts running, the value of ptr will be initialized to point to that location in memory. This means you have a pointer to four bytes of memory, and if you try to write outside of those bytes, your program is doing something bad (and probably violating memory safety).

On the other hand, if you write

char string[10];

Then this basically tells the compiler to go allocate some space in your program of 10 bytes, and make the variable 'string' point to it. It depends where you put this: if you put it inside a function, then you will have a stack allocated buffer of 10 bytes. If you manipulate this buffer inside a function, and then don't do anything with the pointer afterwards, you're all fine. However, if you pass back the address of string -- or use the pointer in any way -- after the function returns, you're in the wrong. This is because, after the function returns, you lose all of the stack allocated variables.

Answer 2

There are even more ways to create strings in C (eg using malloc). What is your usecase? Basically you need a place in memory where the data is stored (on the stack, on the heap, static as in your second example) and then a character pointer to the first character of your string. Most string related functions will "see" the end of the string by the trailing '\\0', in some other cases (mostly general purpose data related functions) you also have to provide the length of the string.