Question

When you make a string out of char pointers how does this work?

char *name = "ben";

Is this ‘hidden’ pointer arithmetic?

Answer 1

There isn’t any hidden pointer arithmetic, but I suspect you want a more detailed answer than that.

If you have a function:

void foo() {
    char * bar = "Hello World";
}

There are actually two chunks of memory that come in to play:

• The first is where the 12 bytes are used to store “Hello World” (1 byte for each letter plus a NULL byte at the end). The compiler will put this in the Data segment. This memory (location and values) is set at compile time and cannot be modified at run time (if you try it will segfault).
• The second location is the pointer to the data, this is the bar variable. When your program calls foo(), it allocates enough stack space (4 bytes on 32 bit) to house this memory location and it gets initialized to the location of the actual data. This happens every time you fun foo().

Further more, if you execute a statement like this later in the function:

bar = "Good bye";

You aren’t changing the data “Hello World” to “Good bye”. You actually just end up with a 3rd chunk of memory in the data segment with “Good bye” in it (still allocated at compile time), then the pointer (bar) gets set to that location when that line executes.

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Another method to create “strings” (character arrays) is:

void foo() {
    char bar[] = "Hello World";
}

This is not the same as the first (close, though). In this method, you still have two variables, except the actual data you’re concerned about (“Hello World” + null byte) is allocated and initialized on the program stack.

You can see the difference in the compiled assembly by running gcc -S test.c and then reading test.s.

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At some point you will want to look at C’s string functions.

They key thing to remember when using these functions is that they don’t know how long your character arrays are at all, they figure that out based on where the first null character is (a sentinel value).