character string memory allocation

[hughLg]

in the function:

void function()
{
   puts("hello world...");
}

how's the character string "hello world..." allocated in memory? is the momory can be freed properly if i use like that? or i have to use this:

void function()
{
   static const char s[] = "Hello world...";
   puts(s);
}

or declare that character string as a global constant or variable will make it better?

and 1 thing, when i declare a character pointer:

{
   char *s = "hello world...";
}

in this case, we're legally access the value of this string, but cannot modify it. how is the memory is allocated? the pointer is just set to point to a beginning of that string, so is the memory allocated for it? and am i have to release its memory to return memory...

 

[brudnakm]

Dennis,

There are four places that variables are stored.
1) The data segment (globals, static, const)
2) The stack (local variables)
3) The heap (malloc)
4) Processor registers (compiler-chosen local variables)

I believe that a literal string will always be in the data segment, i.e. the actual characters.

This will then be referenced by a pointer:

's' in your second example is a pointer to a constant array of chars. In other words

s[n] = foo

is not allowed. 's' would then be a local varaible.

's' in your third example is a local variable.

Mark.

 

[cluelessNewbie]

more rarely...
5) In the code segment. I don't know about c, but in assembly some people put their constant data strings in the code & jmp around it. It jarred me when I first saw it but it does make some sense for readability.

when in doubt look at the assember output.

Also note that the c compiler is going to "optimize away" some of the implied pointers & stuff. It's what they get paid to do. So in the 3rd example, the address may not actually exist as a distinct entity distinct from the code. & the loader would then determine the final address for the string reference.

 

[hughLg]

s in my second example is a array of constant character, but not a pointer.

do you tell me why some person prefer to use static character pointer like in my third example than mine?

 

[brudnakm]

Dennis,

You are right. s is an arrary of const chars, my mistake. Since arrays and pointer are so similar in C I sometimes use them synonomously.

Now to your question.

In your second example s is an array so

s = foo ; [code]

is illegal.

In your third example s is a pointer so

[code]s = foo ;

is legal.

Most people prefer to use the thrid because is more closely represents what is acutally going on. You have a pointer which references memory which is allocated elsewhere (in this case by the compiler). Use the array syntax to actually allocate space that you can use, for example

void function()
{
   char s[] = "Hello world...";
   puts(s);
}

will allocate just enough space to store the string and a null character. When the function exits the space disappears.

Mark.