Question

Here are 2 type of code snippets which have the same outputs.

char *p = "abc";
::printf("%s",p);

And

::printf("%s","abc");

Is there any difference as to where the “abc” string is stored in memory?

I once heard that in the second code, the “abc” string is placed by the compiler in read-only memory (the .text part?)

How to tell this difference from code if any?

Many thanks.

Update

My current understanding is:

when we write:

char *p="abc"

Though this seems to be only a declarative statement, but indeed the compiler will generate many imperative instructions for it. These instructions will allocate proper space within the stack frame of the containing method, it could be like this:

subl %esp, $4

then the address of “abc” string is moved to that allocated space, it could be like this:

movl $abc_string_address, -4(%ebp)

The “abc” string is stored in the executable file image. But where in the memory it (i mean the string) will be loaded totally depends on the implementation of the compiler/linker, if it is loaded into the read-only part of the process’s address space (i.e. the protection bit of the memory page is flagged as read-only), then the p is a read-only pointer, if it is loaded into the r/w part, the p is writable.

Correct me if I am wrong. Now I am looking into the assembly code generated by the gcc to have a confirmation for my understanding. I’ll update this thread again shortly.

Answer 1

Is there any difference as to where the “abc” string is stored in memory?

Nope, and that is true for both. String literals are stored in the read-only segment. However, if you declare your variable as a char[] it will be copied onto the stack, i.e., not read only.