在ARM汇编中使用C函数

本文介绍了在ARM汇编中使用C函数的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我已经看到一些人在他们的代码中使用C库中的printf的例子，

I have seen examples of people using printf from the C library in their code like so:

.data .balign 4 hello: .asciz "Hello\n" .text .global main .func main main: ldr r0, hello_msg bl printf mov r7, #1 swi 0 hello_msg: .word hello .global printf

他们如何确定r0将作为字符串arg传递给printf?我知道每次调用子例程时r0-r3都会作为args传递，但是我不知道哪个寄存器映射到哪个arg.示例:使用scanf时，r0是字符串格式，而r1存储用户输入.我们应该如何知道这一点?我唯一的猜测是使用-S选项使用gcc编译我的* .s文件并查看汇编文件...但是有更好的方法吗?

How did they figure out r0 would be passed as a string arg to printf? I know r0-r3 are passed as args whenever a subroutine is called, but I don't know which register maps to which arg. Example: when using scanf, r0 is the string formatting and r1 stores the user input. How are we supposed to know this? My only guess is to compile my *.s file with gcc using the -S option and viewing the assembled file... but is there a better way?

推荐答案

我将尝试解释我学到的东西.我拥有的唯一Linux是Raspbian Jessie(Raspberry Pi 3).有一个文件/usr/include/arm-linux-gnueabihf/asm/unistd.h(其他Linux应该在某个地方有unistd.h)，该文件显示了为内置在Linux中的函数定义的功能[例如:#define __NR_write(__NR_SYSCALL_BASE + 4)].查看Linux手册页 man7/linux/man- pages/man3/write.3p.html 的格式.

I will try to explain what I have learned. The only Linux that I have is Raspbian Jessie (Raspberry Pi 3). There is a file /usr/include/arm-linux-gnueabihf/asm/unistd.h (other Linux should have unistd.h somewhere) that shows define for functions build into Linux [ex: #define __NR_write (__NR_SYSCALL_BASE+ 4)]. Look at Linux Man Page man7/linux/man-pages/man3/write.3p.html at the format.

ssize_t write(int fildes, const void *buf, size_t nbyte) return in r0 [4] ( r0 , r1 , r2 )

对此我有点陌生，所以我提供了一个程序来帮助我理解它.另请参见 man7/linux/man-pages/dir_by_project .html#man-pages 用于其他格式.

I am kind of new at this, so I have included a program that helps me understand it. Also see man7/linux/man-pages/dir_by_project.html#man-pages for other formats.

@---------------------------------- @ asfileio.s @ @ Raspbian Jessie assembly program using @ SVC for file operations on Raspberry Pi 3 @ @ pi@RPi:~/Programs $ as -o asfileio.o asfileio.s @ pi@RPi:~/Programs $ gcc -o asfileio asfileio.o @ pi@RPi:~/Programs $ ./asfileio; echo $?; ls -l /tmp/test* @ Hello world @ A quick brown fox jumped over the lazy dog. @ 0 @ -rw-r--r-- 1 pi pi 26 Jun 18 14:58 /tmp/testfile01.txt @ -rw-r--r-- 1 pi pi 45 Jun 18 14:58 /tmp/testfile02.txt @ pi@RPi:~/Programs $ @---------------------------------- @ @ man7/linux/man-pages/dir_by_project.html#man-pages @ @ CREATE int creat(const char *pathname, mode_t mode); @ Mode rwx {owner, group, other} @ OPEN int open(const char *pathname, int flags); @ Flags O_RDONLY, O_WRONLY, or O_RDWR @ | O_APPEND @ READ ssize_t read(int fd, void *buf, size_t count); @ WRITE ssize_t write(int fd, const void *buf, size_t count); @ CLOSE int close(int fd); @ SYNC void sync(void); @ EXIT void _Exit(int status); @ @---------------------------------- .data @ See /usr/include/arm-linux-gnueabihf/asm/unistd.h @ See /usr/include/arm-linux-gnueabihf/bits/fcntl-linux.h .equ create, 8 .equ Mode, 0644 @ -rw-r--r-- .equ open, 5 .equ Rd, 00 .equ Wr, 01 .equ RdWr, 02 .equ Apnd, 02000 .equ read, 3 .equ write, 4 .equ close, 6 .equ sync, 36 .equ exit, 1 .equ sfile, 187 @---------------------------------- .balign 4 Create: .word dir_file, Mode, create .balign 4 Open: .word dir_file, RdWr | Apnd, open .balign 4 Write: .word data, after_data - data, write .balign 4 Write2: .word data2, after_data2 - data2, write .balign 4 Read: .word Buf, 80, read .balign 4 Buf: .space 80 @---------------------------------- data: .asciz "Hello world\n" after_data: .balign 4 data2: .asciz "A quick brown fox jumped over the lazy dog.\n" after_data2: .balign 4 dir_file: .asciz "/tmp/testfile01.txt" @---------------------------------- .text .global main, _start @_start: @ Uncommit if using ld as linker main: push {r4, r5, r7, lr} b M_Program S_Write: @ err 4 if error @ subroutine or function @ System call to write to file (or stdout) @ Write amt_wrote=(write( fd, &data, sizeof(data))) @ r0=(r7=4( r0, r1=&data, r2=len(data))) ldr r3, =Write @ address of parameters S_Write2: ldm r3, {r1, r2, r7} @ load write parameters svc #0 @ Linux kernel writes cmp r0, r2 @ check amt = len movne r0, #4 @ set error code to 4 bne exit @ exit if error mov r0, #0 @ success mov pc, lr @ return to program S_Create: @ err 8 if error @ Create fd=(creat( &dir_file, -rw-r--r--)) @ r0=(r7=8 (r0=&dir_file, r1=0644 )) ldr r3, =Create ldm r3, {r0, r1, r7} svc #0 mov r4, r0 @ save fd in r4 cmp r0, #3 @ err if < 3 movlt r0, #8 blt exit mov r0, #0 mov pc, lr S_Open: @ err 5 if error ldr r3, =Open ldm r3, {r0, r1, r7} svc #0 mov r4, r0 cmp r0, #3 movlt r0, #5 blt exit mov r0, #0 mov pc, lr S_Close: @ err 6 if error mov r7, #close svc #0 cmp r0, #0 movne r0, #6 bne exit mov pc, lr S_Read: ldr r3, =Read ldm r3, {r1, r2, r7} svc #0 mov pc, lr M_Program: @ S_Write call to write to stdout mov r0, #1 @ fd=1=stdout bl S_Write @ Create, write and close file in /tmp bl S_Create @ create file mov r0, r4 @ move fd to r0 bl S_Write @ write to file mov r0, r4 @ move fd to r0 bl S_Close @ close the file @ Open same file for write append bl S_Open @ open file mov r0, r4 @ move fd to r0 bl S_Write @ write to file mov r0, r4 @ move fd to r0 bl S_Close @ close file @ Change variables for new file ldr r3, =dir_file add r3, #12 ldr r2, [r3] eor r2, #0x30000 str r2, [r3] @ Create new file, write and close bl S_Create mov r0, r4 ldr r3, =Write2 bl S_Write2 mov r0, r4 bl S_Close @ Open file, read, write to stdout and close bl S_Open mov r0, r4 bl S_Read mov r2, r0 @ r0 has amt read mov r0, #1 @ fd = 1 = stdout ldr r1, =Buf @ addr data just read mov r7, #write svc #0 mov r0, r4 bl S_Close exit: pop {r4, r5, r7, lr} bx lr @ Exit if use gcc as linker @ mov r7, #1 @ Exit if use ld as linker @ svc #0 @ Exit if use ld as linker

我在臂架上找到的最好的文档是 caxapa.ru/thumbs/656023/IHI0042F_aapcs.pdf 第14页.

The best docs that I have found on arm registers are caxapa.ru/thumbs/656023/IHI0042F_aapcs.pdf page 14.