Command line execution C file example

First write a C language code file mstore.c as follows:

#include <stdio.h>

long mult2(long, long);

void multstore(long x, long y, long *dest) {
    
    
    long t = mult2(x, y);
    *dest = t;
}

Use the "-S" option on the command line to see the assembly code generated by the C language compiler:

PS E:\VS Code\VS Code C\chapter 3> gcc -S mstore.c

This will cause GCC to run the compiler and generate an assembly file mstore.s, but does not do any further work. (Under normal circumstances, it will continue to call the assembler to generate object code files).

	.file	"mstore.c"
	.text
	.globl	multstore
	.def	multstore;	.scl	2;	.type	32;	.endef
	.seh_proc	multstore
multstore:
	pushq	%rbx
	.seh_pushreg	%rbx
	subq	$32, %rsp
	.seh_stackalloc	32
	.seh_endprologue
	movq	%r8, %rbx
	call	mult2
	movl	%eax, (%rbx)
	addq	$32, %rsp
	popq	%rbx
	ret
	.seh_endproc
	.ident	"GCC: (x86_64-posix-seh-rev0, Built by MinGW-W64 project) 8.1.0"
	.def	mult2;	.scl	2;	.type	32;	.endef

If you continue to use the "-C" command-line option, GCC will compile and assemble the code:

PS E:\VS Code\VS Code C\chapter 3> gcc -c mstore.c

This will generate the object code file mstore.o, which is in binary format, so it cannot be viewed directly.
To view the contents of a machine code file, there is a program called a disassembler that is very useful. These programs generate a format similar to assembly based on machine code. The program OBJDUMP (object dump) with the "-d" command line flag can play this role:

PS E:\VS Code\VS Code C\chapter 3> objdump -d mstore.o

The result is as follows: To


generate the actual executable code, the linker must be run on a group of object code files, and this group of object code files must contain a main function. Suppose there is the following function in the file main.c:

#include <stdio.h>

void multstore(long, long, long *);

int main() {
    
    
    long d;
    multstore(2, 3, &d);
    printf("2 * 3 --> %ld\n", d);
    return 0;
}

long mult2(long a, long b) {
    
    
    long s = a * b;
    return s;
}

Then, use the following method to generate the executable file prog:

PS E:\VS Code\VS Code C\chapter 3> gcc -o prog main.c mstore.c

For example, the

file prog not only contains the code of the two processes, but also contains the code used to start and terminate the program, as well as the code used to interact with the operating system. We can also disassemble the prog file:

PS E:\VS Code\VS Code C\chapter 3> objdump -d prog.exe

At this point, you can run the generated prog file: it is

found that the program running results of the two C files meet expectations.
In the generated mstore.s file:
all .lines beginning with ' ' are directives to instruct the assembler and linker to work, we can usually ignore these lines. If we remove these, the assembly code with explanation is as follows:
Note the following is the mstore.s file in the Linux environment

multstore:
	pushq	%rbx
	subq	$32, %rsp
	movq	%r8, %rbx
	call	mult2
	movl	%eax, (%rbx)
	addq	$32, %rsp
	popq	%rbx
	ret