将字符串分配给char *变量

Question

A code I am reviewing uses following string assignment

char *str;
str ="";

The coder then uses this 'str' to temporarily hold a string like.

str = "This is a message";
fwrite(str, 1 ,strlen(str), fp);

Then this str is used again at some other place to assign a new string with a similar use.

I know that this works, I want to find out how exactly does this work. How can you declare a char pointer and make it point to a string like that? What could be the maximum string length such a pointer can hold? Where is this string stored? Is it automatically malloc 'd?

Answer 1

A pointer doesn't "hold" a string, it just points to where the original string is located. In this case the string literal is kept as part of the program and the pointer is set to it; when you reassign the pointer, you're not making any copies, just setting the pointer to a different address.

The maximum size of the string is thus the maximum size of a string literal, which will depend on the compiler and the amount of available program space.

If you want to actually make a copy of a string, first you must allocate some storage for it which must be one greater than the number of characters. Then use strcpy to make the copy.

Answer 2

This string is statically contained in the object module. You don't need to malloc memory for such strings, because they already have a memory assigned by the compiler. Because of this, you also can not free such a pointer. If you look with an hex editor in your exe file, you can see that such a string is contained inside it, as opposed to a dynamically allocated string, which only exists in memory as long as the executable runs.

The maximum size of such a string depends on your compiler.

Answer 3

char* is just a pointer to a char (or series of them).

You can have it pointing to any "string" you like. In the examples given, they are just changing the pointer's value (ie what str points to).

Answer 4

char *name;

name="some string"; //the name points to the address of location of the string.

is not similar to :

char str[];

str="some string";// remember this type of statement won't work,because str is about to store characters but you are assigning the pointer.

A constant character string always represents a pointer to that string.