ARM 原生程序的生成过程

Android 平台上的 ARM 原生程序是用 Android NDK 开发的，整个原生程序的编译生成工作由 Android NDK 提供的编译工具链完成
Android NDK 目前提供了两套 ARM 编译器，分别是基于 GNU GCC 编译器的 gcc 编译器和基于LLVM 编译套件的 Clang 编译器
Android NDK 打算从 r17 版本开始移除对 gcc 的支持，只提供 Clang 编译开发 Android 原生程序，但实际却是仍可用 gcc 开发
现在用 Android NDK 写一个 ARM 原生程序，然后分析编译器执行的每一步操作，从而了解原生程序的整个生成过程。代码如下：
系统：Ubuntu 18.04
Android NDK 版本：r10e
用传统的 gcc 工具编译这段代码，只要执行 gcc hello.c -o hello 命令；用 Android NDK 要麻烦一些
在命令行下指定 CC 环境变量，用 arm-linux-androideabi-gcc 编译器单独编译代码时要指定 --sysroot 选项（官方解释）
执行如下命令，可在命令行下编译 hello.c 代码：
- 第一行：指定 Android NDK 的根目录，方便在下面的命令中引用
- 第二行：指定 SYSROOT 环境变量，arm-linux-androideabi-gcc 编译器在编译代码时会到此目录下查找头文件和库。此处使用的是 android-21，表示系统版本是 Android 5.0（这是第一个引入 arm64-v8a 架构的系统版本）
- 第三行：指定 CC 环境变量，指向具体的编译器路径
- 第四行：使用由 CC 环境变量指定的编译器编译代码。后面的 -fPIE -pie 参数，作用是生成与位置无关的代码，在 Android 5.0 及以上版本的系统中必须设置此参数，不然程序运行时会产生异常，提示信息为“error: only position independent executables (PIE) are supported”。编译成功后，会生成 hello 可执行文件
- 若是编译 C++ 代码，第四行命令可改为 export CXX="$NDK/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-g++ --sysroot=$SYSROOT"
直接执行 hello 程序会提示“该文件无法执行”
可将文件传入 Android 手机或模拟器执行，命令和输出如下：
上面用 $CC 编译的 Android 原生程序没指定 -march 参数，故默认的是 ARM 架构，对应的处理器是 armeabi，采用的指令集是 ARMv5TE
若想生成其他 ARM 架构的程序，如 ARMv7-A，可执行如下命令：
$CC -march=armv7-a hello.c -o hello
除了用 arm-linux-androideabi-gcc 编译器，还可用 Android NDK 提供的 Clang 编译器生成程序，二者只在指定编译工具链的环境变量设置上有所不同
（本书成书时）Android NDK 提供的 LLVM 编译套件中虽有 Clang 前端，但没用于处理代码的后端工具，如汇编器 as 和链接器 ld。用 Clang 只能完成编译工作，要想完成汇编和链接工作，还得借助 gcc 的 arm-linux-androideabi-as 和 arm-linux-androideabi-ld。在设置 CC 环境变量时，指定 -gcc-toolchain 即可将后端部分传给 gcc。除此之外，用 Clang 编译器编译不同架构的代码应通过 -target 参数指定，而不应通过 gcc 的 -march。指定 -target 的值为“armv7-linux-androideabi”，表示生成 ARMv7-A 架构的程序。若没指定 -target 的值，默认情况下其值为当前编译系统平台架构的名称
CC 环境变量的设置和 hello.c 的编译可执行如下命令：
无论 gcc 或 Clang，执行一次完整的编译工作通常分为预处理、编译、汇编、链接四个步骤，各步骤的工作细节有所不同

预处理

在预处理阶段，编译器将处理代码中的预处理指令。如，#include 中包含的头文件会全部被编译出来，#define 预定义、#if 预条件处理等也都会被编译器处理。详细的输出信息可通过向编译器传递 -E 选项查看。以 hello.c 为例，gcc 和 Clang 都可执行 $CC -E -fPIE -pie hello.c -o hello.i 命令完成代码的预处理
hello.i 内容如下：

# 1 "hello.c"# 1 "" 1# 1 "" 3# 325 "" 3# 1 "" 1# 1 "" 2# 1 "hello.c" 2# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 1 3 4# 49 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/cdefs.h" 1 3 4# 77 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/cdefs.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/cdefs_elf.h" 1 3 4# 78 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/cdefs.h" 2 3 4# 547 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/cdefs.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/android/api-level.h" 1 3 4# 548 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/cdefs.h" 2 3 4# 50 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 2 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/types.h" 1 3 4# 31 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/types.h" 3 4# 1 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stddef.h" 1 3 4# 47 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stddef.h" 3 4typedef int ptrdiff_t;# 58 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stddef.h" 3 4typedef unsigned int size_t;# 86 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stddef.h" 3 4typedef unsigned int wchar_t;# 32 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/types.h" 2 3 4# 1 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stdint.h" 1 3 4# 63 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stdint.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdint.h" 1 3 4# 32 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdint.h" 3 4# 1 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stddef.h" 1 3 4# 33 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdint.h" 2 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/machine/wchar_limits.h" 1 3 4# 34 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdint.h" 2 3 4typedef signed char __int8_t;typedef unsigned char __uint8_t;typedef short __int16_t;typedef unsigned short __uint16_t;typedef int __int32_t;typedef unsigned int __uint32_t;typedef long long __int64_t;typedef unsigned long long __uint64_t;typedef int __intptr_t;typedef unsigned int __uintptr_t;typedef __int8_t int8_t;typedef __uint8_t uint8_t;typedef __int16_t int16_t;typedef __uint16_t uint16_t;typedef __int32_t int32_t;typedef __uint32_t uint32_t;typedef __int64_t int64_t;typedef __uint64_t uint64_t;typedef __intptr_t intptr_t;typedef __uintptr_t uintptr_t;typedef int8_t int_least8_t;typedef uint8_t uint_least8_t;typedef int16_t int_least16_t;typedef uint16_t uint_least16_t;typedef int32_t int_least32_t;typedef uint32_t uint_least32_t;typedef int64_t int_least64_t;typedef uint64_t uint_least64_t;typedef int8_t int_fast8_t;typedef uint8_t uint_fast8_t;typedef int64_t int_fast64_t;typedef uint64_t uint_fast64_t;typedef int32_t int_fast16_t;typedef uint32_t uint_fast16_t;typedef int32_t int_fast32_t;typedef uint32_t uint_fast32_t;typedef uint64_t uintmax_t;typedef int64_t intmax_t;# 64 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stdint.h" 2 3 4# 33 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/types.h" 2 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/types.h" 1 3 4# 21 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/types.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm/types.h" 1 3 4# 19 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm/types.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/types.h" 1 3 4# 21 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/types.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/int-ll64.h" 1 3 4# 21 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/int-ll64.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm/bitsperlong.h" 1 3 4# 19 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm/bitsperlong.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/bitsperlong.h" 1 3 4# 20 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm/bitsperlong.h" 2 3 4# 22 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/int-ll64.h" 2 3 4typedef __signed__ char __s8;typedef unsigned char __u8;typedef __signed__ short __s16;typedef unsigned short __u16;typedef __signed__ int __s32;typedef unsigned int __u32;__extension__ typedef __signed__ long long __s64;__extension__ typedef unsigned long long __u64;# 22 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/types.h" 2 3 4# 20 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm/types.h" 2 3 4# 22 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/types.h" 2 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/posix_types.h" 1 3 4# 21 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/posix_types.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/stddef.h" 1 3 4# 19 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/stddef.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/compiler.h" 1 3 4# 20 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/stddef.h" 2 3 4# 22 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/posix_types.h" 2 3 4typedef struct { unsigned long fds_bits[1024 / (8 * sizeof(long))];} __kernel_fd_set;typedef void (*__kernel_sighandler_t)(int);typedef int __kernel_key_t;typedef int __kernel_mqd_t;# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm/posix_types.h" 1 3 4# 21 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm/posix_types.h" 3 4typedef unsigned short __kernel_mode_t;typedef unsigned short __kernel_ipc_pid_t;typedef unsigned short __kernel_uid_t;typedef unsigned short __kernel_gid_t;typedef unsigned short __kernel_old_dev_t;# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/posix_types.h" 1 3 4# 21 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/posix_types.h" 3 4# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm/bitsperlong.h" 1 3 4# 22 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/posix_types.h" 2 3 4typedef long __kernel_long_t;typedef unsigned long __kernel_ulong_t;typedef __kernel_ulong_t __kernel_ino_t;typedef int __kernel_pid_t;# 49 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/posix_types.h" 3 4typedef __kernel_long_t __kernel_suseconds_t;typedef int __kernel_daddr_t;typedef unsigned int __kernel_uid32_t;typedef unsigned int __kernel_gid32_t;typedef __kernel_uid_t __kernel_old_uid_t;typedef __kernel_gid_t __kernel_old_gid_t;# 71 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/posix_types.h" 3 4typedef unsigned int __kernel_size_t;typedef int __kernel_ssize_t;typedef int __kernel_ptrdiff_t;# 84 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm-generic/posix_types.h" 3 4typedef struct { int val[2];} __kernel_fsid_t;typedef __kernel_long_t __kernel_off_t;typedef long long __kernel_loff_t;typedef __kernel_long_t __kernel_time_t;typedef __kernel_long_t __kernel_clock_t;typedef int __kernel_timer_t;typedef int __kernel_clockid_t;typedef char * __kernel_caddr_t;typedef unsigned short __kernel_uid16_t;typedef unsigned short __kernel_gid16_t;# 33 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/asm/posix_types.h" 2 3 4# 33 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/posix_types.h" 2 3 4# 25 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/linux/types.h" 2 3 4typedef __u16 __le16;typedef __u16 __be16;typedef __u32 __le32;typedef __u32 __be32;typedef __u64 __le64;typedef __u64 __be64;typedef __u16 __sum16;typedef __u32 __wsum;# 36 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/types.h" 2 3 4typedef __kernel_gid32_t __gid_t;typedef __gid_t gid_t;typedef __kernel_uid32_t __uid_t;typedef __uid_t uid_t;typedef __kernel_pid_t __pid_t;typedef __pid_t pid_t;typedef uint32_t __id_t;typedef __id_t id_t;typedef unsigned long blkcnt_t;typedef unsigned long blksize_t;typedef __kernel_caddr_t caddr_t;typedef __kernel_clock_t clock_t;typedef __kernel_clockid_t __clockid_t;typedef __clockid_t clockid_t;typedef __kernel_daddr_t daddr_t;typedef unsigned long fsblkcnt_t;typedef unsigned long fsfilcnt_t;typedef __kernel_mode_t __mode_t;typedef __mode_t mode_t;typedef __kernel_key_t __key_t;typedef __key_t key_t;typedef __kernel_ino_t __ino_t;typedef __ino_t ino_t;typedef uint32_t __nlink_t;typedef __nlink_t nlink_t;typedef void* __timer_t;typedef __timer_t timer_t;typedef __kernel_suseconds_t __suseconds_t;typedef __suseconds_t suseconds_t;typedef uint32_t __useconds_t;typedef __useconds_t useconds_t;typedef uint32_t dev_t;typedef __kernel_time_t __time_t;typedef __time_t time_t;typedef __kernel_off_t off_t;typedef __kernel_loff_t loff_t;typedef loff_t off64_t;# 122 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/types.h" 3 4typedef int32_t __socklen_t;typedef __socklen_t socklen_t;typedef __builtin_va_list __va_list;# 138 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/types.h" 3 4typedef __kernel_ssize_t ssize_t;typedef unsigned int uint_t;typedef unsigned int uint;# 1 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/sysmacros.h" 1 3 4# 36 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/sysmacros.h" 3 4static __inline__ int major(dev_t _dev){  return (_dev >> 8) & 0xfff;}static __inline__ int minor(dev_t _dev){  return (_dev & 0xff) | ((_dev >> 12) & 0xfff00);}static __inline__ dev_t makedev(int __ma, int __mi){  return ((__ma & 0xfff) << 8) | (__mi & 0xff) | ((__mi & 0xfff00) << 12);}# 146 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/sys/types.h" 2 3 4typedef unsigned char u_char;typedef unsigned short u_short;typedef unsigned int u_int;typedef unsigned long u_long;typedef uint32_t u_int32_t;typedef uint16_t u_int16_t;typedef uint8_t u_int8_t;typedef uint64_t u_int64_t;# 51 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 2 3 4# 1 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stdarg.h" 1 3 4# 30 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stdarg.h" 3 4typedef __builtin_va_list va_list;# 50 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stdarg.h" 3 4typedef __builtin_va_list __gnuc_va_list;# 53 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 2 3 4# 1 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stddef.h" 1 3 4# 54 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 2 3 4# 1 "/root/android-ndk-r10e/toolchains/llvm-3.5/prebuilt/linux-x86_64/bin/../lib/clang/3.5/include/stddef.h" 1 3 4# 57 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 2 3 4typedef off_t fpos_t;# 75 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 3 4struct __sbuf { unsigned char *_base; int _size;};# 108 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 3 4typedef struct __sFILE { unsigned char *_p; int _r; int _w; short _flags; short _file; struct __sbuf _bf; int _lbfsize; void *_cookie; int (*_close)(void *); int (*_read)(void *, char *, int); fpos_t (*_seek)(void *, fpos_t, int); int (*_write)(void *, const char *, int); struct __sbuf _ext; unsigned char *_up; int _ur; unsigned char _ubuf[3]; unsigned char _nbuf[1]; struct __sbuf _lb; int _blksize; fpos_t _offset;} FILE;extern FILE __sF[];# 220 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 3 4void clearerr(FILE *);int fclose(FILE *);int feof(FILE *);int ferror(FILE *);int fflush(FILE *);int fgetc(FILE *);char *fgets(char * __restrict, int, FILE * __restrict);FILE *fopen(const char * __restrict , const char * __restrict);int fprintf(FILE * __restrict , const char * __restrict, ...)  __attribute__((__format__(printf, 2, 3))) __attribute__((__nonnull__ (2)));int fputc(int, FILE *);int fputs(const char * __restrict, FILE * __restrict);size_t fread(void * __restrict, size_t, size_t, FILE * __restrict);FILE *freopen(const char * __restrict, const char * __restrict,     FILE * __restrict);int fscanf(FILE * __restrict, const char * __restrict, ...)  __attribute__((__format__(scanf, 2, 3))) __attribute__((__nonnull__ (2)));int fseek(FILE *, long, int);long ftell(FILE *);size_t fwrite(const void * __restrict, size_t, size_t, FILE * __restrict);int getc(FILE *);int getchar(void);ssize_t getdelim(char ** __restrict, size_t * __restrict, int,     FILE * __restrict);ssize_t getline(char ** __restrict, size_t * __restrict, FILE * __restrict);void perror(const char *);int printf(const char * __restrict, ...)  __attribute__((__format__(printf, 1, 2))) __attribute__((__nonnull__ (1)));int putc(int, FILE *);int putchar(int);int puts(const char *);int remove(const char *);void rewind(FILE *);int scanf(const char * __restrict, ...)  __attribute__((__format__(scanf, 1, 2))) __attribute__((__nonnull__ (1)));void setbuf(FILE * __restrict, char * __restrict);int setvbuf(FILE * __restrict, char * __restrict, int, size_t);int sscanf(const char * __restrict, const char * __restrict, ...)  __attribute__((__format__(scanf, 2, 3))) __attribute__((__nonnull__ (2)));FILE *tmpfile(void);int ungetc(int, FILE *);int vfprintf(FILE * __restrict, const char * __restrict, __va_list)  __attribute__((__format__(printf, 2, 0))) __attribute__((__nonnull__ (2)));int vprintf(const char * __restrict, __va_list)  __attribute__((__format__(printf, 1, 0))) __attribute__((__nonnull__ (1)));int dprintf(int, const char * __restrict, ...) __attribute__((__format__(printf, 2, 3))) __attribute__((__nonnull__ (2)));int vdprintf(int, const char * __restrict, __va_list) __attribute__((__format__(printf, 2, 0))) __attribute__((__nonnull__ (2)));char* gets(char*) ;int sprintf(char* __restrict, const char* __restrict, ...)    __attribute__((__format__(printf, 2, 3))) __attribute__((__nonnull__ (2)));char* tmpnam(char*) ;int vsprintf(char* __restrict, const char* __restrict, __va_list)    __attribute__((__format__(printf, 2, 0))) __attribute__((__nonnull__ (2)));char* tempnam(const char*, const char*)                                                                        ;extern int rename(const char*, const char*);extern int renameat(int, const char*, int, const char*);int fgetpos(FILE * __restrict, fpos_t * __restrict);int fsetpos(FILE *, const fpos_t *);int fseeko(FILE *, off_t, int);off_t ftello(FILE *);int snprintf(char * __restrict, size_t, const char * __restrict, ...)  __attribute__((__format__(printf, 3, 4))) __attribute__((__nonnull__ (3)));int vfscanf(FILE * __restrict, const char * __restrict, __va_list)  __attribute__((__format__(scanf, 2, 0))) __attribute__((__nonnull__ (2)));int vscanf(const char *, __va_list)  __attribute__((__format__(scanf, 1, 0))) __attribute__((__nonnull__ (1)));int vsnprintf(char * __restrict, size_t, const char * __restrict, __va_list)  __attribute__((__format__(printf, 3, 0))) __attribute__((__nonnull__ (3)));int vsscanf(const char * __restrict, const char * __restrict, __va_list)  __attribute__((__format__(scanf, 2, 0))) __attribute__((__nonnull__ (2)));# 317 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 3 4FILE *fdopen(int, const char *);int fileno(FILE *);int pclose(FILE *);FILE *popen(const char *, const char *);void flockfile(FILE *);int ftrylockfile(FILE *);void funlockfile(FILE *);int getc_unlocked(FILE *);int getchar_unlocked(void);int putc_unlocked(int, FILE *);int putchar_unlocked(int);# 349 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 3 4int asprintf(char ** __restrict, const char * __restrict, ...)  __attribute__((__format__(printf, 2, 3))) __attribute__((__nonnull__ (2)));char *fgetln(FILE * __restrict, size_t * __restrict);int fpurge(FILE *);void setbuffer(FILE *, char *, int);int setlinebuf(FILE *);int vasprintf(char ** __restrict, const char * __restrict,    __va_list)  __attribute__((__format__(printf, 2, 0))) __attribute__((__nonnull__ (2)));FILE *funopen(const void *,  int (*)(void *, char *, int),  int (*)(void *, const char *, int),  fpos_t (*)(void *, fpos_t, int),  int (*)(void *));# 2 "hello.c" 2int main(int argc, char const *argv[]){ printf("Hello ARM!\n"); return 0;}

预处理阶段 gcc 和 Clang 所做的工作差不多

编译

在编译阶段，编译器首先检查代码的规范性，及其中是否存在语法错误等，以确定代码实际要做的工作。确认无误后，编译器把代码翻译成 ARM 汇编语言的代码，输出信息可通过向编译器传递 -S 选项查看
以 hello.c 为例，执行 $CC hello.i -S -fPIE -o hello.s 命令后会生成 hello.s 汇编文件，根据 CC 环境变量指定编译器的不同，生成的汇编代码也会不同
gcc 生成的代码如下：

.arch armv5te.fpu softvfp.eabi_attribute 20, 1.eabi_attribute 21, 1.eabi_attribute 23, 3.eabi_attribute 24, 1.eabi_attribute 25, 1.eabi_attribute 26, 2.eabi_attribute 30, 6.eabi_attribute 34, 0.eabi_attribute 18, 4.file"hello.c".section.rodata.align2.LC0:.ascii"Hello ARM!\000".text.align2.globalmain.typemain, %functionmain:@ args = 0, pretend = 0, frame = 8@ frame_needed = 1, uses_anonymous_args = 0stmfdsp!, {fp, lr}addfp, sp, #4subsp, sp, #8strr0, [fp, #-8]strr1, [fp, #-12]ldrr3, .L3.LPIC0:addr3, pc, r3movr0, r3blputs(PLT)movr3, #0movr0, r3subsp, fp, #4@ sp neededldmfdsp!, {fp, pc}.L4:.align2.L3:.word.LC0-(.LPIC0+8).sizemain, .-main.ident"GCC: (GNU) 4.9 20140827 (prerelease)".section.note.GNU-stack,"",%progbits

hello.s 展示了一个 ARM 汇编程序源文件的完整结构
. 开头的汇编指令是汇编器提供的伪指令，不同的伪指令描述了汇编程序的不同部分
- .arch：指定汇编程序使用的处理器架构，默认情况下，armeabi 指定的处理器架构为 ARMv5TE
- .section：声明一个段
- .ascii：声明一个字符串
将 CC 环境变量定义为 Clang，并将 -target 设置为 armv5te-linux-androideabi，生成 hello.s
hello.s 内容如下：

.text.syntax unified.cpuarm1022e.eabi_attribute6, 4@ Tag_CPU_arch.eabi_attribute8, 1@ Tag_ARM_ISA_use.eabi_attribute15, 1@ Tag_ABI_PCS_RW_data.eabi_attribute16, 1@ Tag_ABI_PCS_RO_data.eabi_attribute17, 2@ Tag_ABI_PCS_GOT_use.eabi_attribute20, 1@ Tag_ABI_FP_denormal.eabi_attribute21, 1@ Tag_ABI_FP_exceptions.eabi_attribute23, 3@ Tag_ABI_FP_number_model.eabi_attribute24, 1@ Tag_ABI_align_needed.eabi_attribute25, 1@ Tag_ABI_align_preserved.eabi_attribute18, 4@ Tag_ABI_PCS_wchar_t.eabi_attribute26, 2@ Tag_ABI_enum_size.file"hello.i".globlmain.align2.typemain,%functionmain:                                   @ @main.fnstart.Leh_func_begin0:@ BB#0:                                 @ %entry.save{r11, lr}push{r11, lr}.setfpr11, spmovr11, sp.pad#16subsp, sp, #16ldrr2, .LCPI0_2.LPC0_0:addr2, pc, r2ldrr3, .LCPI0_1addr2, r3, r2ldrr3, .LCPI0_0strr3, [r11, #-4]strr0, [sp, #8]strr1, [sp, #4]movr0, r2blprintf(PLT)ldrr1, .LCPI0_0strr0, [sp]                @ 4-byte Spillmovr0, r1movsp, r11pop{r11, pc}.align2@ BB#1:.LCPI0_0:.long0                       @ 0x0.LCPI0_1:.long.L.str(GOTOFF).LCPI0_2:.long_GLOBAL_OFFSET_TABLE_-(.LPC0_0+8).Ltmp0:.sizemain, .Ltmp0-main.cantunwind.fnend.type.L.str,%object          @ @.str.section.rodata.str1.1,"aMS",%progbits,1.L.str:.asciz"Hello ARM!\n".size.L.str, 12.ident"clang version 3.5 "

可看出，Clang 用的指令集为 arm1022e，生成的汇编代码也与 gcc 不同
Clang 在生成 hello.s 期间还做了许多内部转换工作。Clang 作为 LLVM 套件的 C 语言编译器前端，在生成汇编代码前会将代码转换成 LLVM 特有的中间代码 Bitcode 字节码，然后将 Bitcode 字节码转换成与机器相关的汇编指令
要想生成 Bitcode 字节码，可在调用 Clang 时使用 -emit-llvm 参数
hello.c 可执行如下命令生成 hello.bc：
Bitcode 是 LLVM IR（一种程序代码的中间语法表达）的表现形式，它即可存在于二进制格式的 .bc 文件中，也可存在于可读的文本格式的 .ll 文件中。可用 llvm-dis 命令将 .bc 文件转换成可读的 .ll 文件，也可用 llvm-as 命令将 .ll 转换为 .bc
hello.ll 的内容如下：

; ModuleID = 'hello.bc'target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-n32-S64"target triple = "armv5e--linux-androideabi"@.str = private unnamed_addr constant [12 x i8] c"Hello ARM!\0A\00", align 1; Function Attrs: nounwinddefine i32 @main(i32 %argc, i8** %argv) #0 {entry:  %retval = alloca i32, align 4  %argc.addr = alloca i32, align 4  %argv.addr = alloca i8**, align 4  store i32 0, i32* %retval  store i32 %argc, i32* %argc.addr, align 4  store i8** %argv, i8*** %argv.addr, align 4  %call = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([12 x i8]* @.str, i32 0, i32 0))  ret i32 0}declare i32 @printf(i8*, ...) #1attributes #0 = { nounwind "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="true" }attributes #1 = { "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="true" }!llvm.module.flags = !{!0, !1}!llvm.ident = !{!2}!0 = metadata !{i32 1, metadata !"wchar_size", i32 4}!1 = metadata !{i32 1, metadata !"min_enum_size", i32 4}!2 = metadata !{metadata !"clang version 3.5 "}

将 hello.bc 转换为 hello.s：
这和之前生成的 hello.s 是一样的

汇编

汇编阶段，执行如下命令，生成 hello.o：
$CC -c hello.s -o hello.o
它的内部环境变量 CC 将汇编文件传给汇编器生成目标文件
arm-linux-androideabi-gcc 用的汇编器是 arm-linux-androideabi-as，用的汇编格式为 GNU ARM，它有自己的语法结构
Clang 是 LLVM 的一部分，目前（成书时）Clang 在编译时用的汇编器仍是 arm-linux-androideabi-as。因此，无论是 gcc 还是 Clang，汇编工作统一由 arm-linux-androideabi-as 完成，实际上它完成了如下工作：

链接

最后一步为链接，即将所有的目标合并，生成最终的可执行程序或动态库，命令如下：
$CC hello.o -fPIE -pie -o hello
在它的内部，其实是由 CC 指定的编译器将目标文件传给链接器，从而完成链接工作的
注意：除了在编译时要指定 -fPIE，链接时也要指定
Clang 没有自己的链接器，两个编译器都是调用 arm-linux-androideabi-ld 完成链接工作。因此，实际是进行了如下操作：
-xxx：指额外的链接参数，如库的路径与引用的库，具体的参数可通过如下命令看到：
$CC hello.o -o hello -v

第七章 ARM 反汇编基础（三）（ARM 原生程序的生成过程）

文章目录

ARM 原生程序的生成过程

预处理

编译

汇编

链接

更多相关文章

随机推荐