Academic Question About Compilers

How would something like this:

int x;
char hello[6] = "hello";
x = atoi(hello);

be compiled? I'm asking what the compiler will do internally to compile something like this. From generation of symbols, the code to pass parameters to library functions, how the stack is involved etc.

Thanks a lot!

Maybe there is an option on your compiler to produce the assembler code?

Anyway I was interested in having a go at producing the assembler code myself
I tried it on this short program I wrote earlier.

Code:

---

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

main(argc,argv)
        int  argc;
        char *argv[];
{
        system("testbat");
}

---

compiled with

Code:

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set DJGPP=C:\DJGPP\DJGPP.ENV
set PATH=C:\DJGPP\BIN;%PATH%

gcc -S  testb.c

---

Which produces this:-
Which may answer some of your question (or not).

Code:

---


        .file        "testb.c"
        .section .text
LC0:
        .ascii "testbat\0"
.globl _main
_main:
        pushl        %ebp
        movl        %esp, %ebp
        subl        $8, %esp
        andl        $-16, %esp
        movl        $0, %eax
        addl        $15, %eax
        addl        $15, %eax
        shrl        $4, %eax
        sall        $4, %eax
        subl        %eax, %esp
        subl        $12, %esp
        pushl        $LC0
        call        _system
        addl        $16, %esp
        leave
        ret
        .ident        "GCC: (GNU) 4.0.1"

---

Not sure what all that is about lol,

Code:

---

pushl        $LC0
        call        _system

---

Seems to be the passing of parameter to the 'system' call, I think the other stuff
is to do with the program arguements.

Anyone know what 'leave' does?

Code:

---

main(argc,argv)
        int  argc;
        char *argv[];

---

Don't use the pre-ANSI way of function signatures. Use this:

Code:

---

int main(int argc,char *argv[] )
{
    // code
}

---

Code:

---

int x; // int x is put ion the stack
char hello[6] = "hello"; // some memory of 6 bytes is put on the stack, and hello is copied into it.
x = atoi(hello); // address of hello is put in a register, then atoi is called,
//then the address of x is put in a register and the return value register (whatever that is) is put into the address of x

---

this is just the usual way of doing things, not how a compiler must do. it might change with different optimizations or compilers.

Quote:

---

Originally Posted by esbo

Code:

---

        .file        "testb.c"
        .section .text

---

---

.file is an informational directive which tells the assembler what the original source file name is. .section .text tells the assembler to place the following data and instructions into the object section called ".text."

Quote:

---

Code:

---

LC0:
        .ascii "testbat\0"

---

---

LC0 is a label generated by the compiler which is used to reference the string literal "testbat\0", which you used in the source code. In other words, it is storing the string literal and giving it the name "LC0."

Quote:

---

Code:

---

.globl _main
_main:

---

---

.globl tells the assembler that the symbol "_main" should be exported for linking.

Quote:

---

Code:

---

        pushl        %ebp
        movl        %esp, %ebp

---

---

Standard function prolog. Save the base pointer of the parent function, and set up a new base pointer at the current stack frame.

Quote:

---

Code:

---

        subl        $8, %esp
        andl        $-16, %esp
        movl        $0, %eax
        addl        $15, %eax
        addl        $15, %eax
        shrl        $4, %eax
        sall        $4, %eax
        subl        %eax, %esp
        subl        $12, %esp

---

---

A lot of manipulation of the stack pointer. This is creating space for local variables and making sure the stack is aligned properly. If you had compiled with optimization, this code should be much simpler than this.

Quote:

---

Code:

---

        pushl        $LC0

---

---

Push a pointer to the string literal "testbat\0" onto the stack in preparation for the function call:

Quote:

---

Code:

---

        call        _system

---

---

Call the system() function.

Quote:

---

Code:

---

        addl        $16, %esp

---

---

Fix up the stack pointer, discarding the previously pushed parameters as well as the local variables.

Quote:

---

Code:

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        leave

---

---

This undoes the effect of the function prolog.

Quote:

---

Code:

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        ret

---

---

And return from main().

Quote:

---

Code:

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        .ident        "GCC: (GNU) 4.0.1"

---

---

Identify the compiler program.