第七章 ARM 反汇编基础（三）（ARM 原生程序的生成过程）

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 4
typedef 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 4
typedef 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 4
typedef 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 4

typedef 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 4


typedef __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 4



typedef 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 4
typedef 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 4


typedef 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 4
typedef __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 4
typedef 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 4
typedef 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 4


typedef __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 4



typedef __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 4
typedef 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 4
typedef __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 4
static __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 4


typedef 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 4
typedef __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 4
typedef __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 4



typedef off_t fpos_t;
# 75 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 3 4
struct __sbuf {
 unsigned char *_base;
 int _size;
};
# 108 "/root/android-ndk-r10e/platforms/android-21/arch-arm/usr/include/stdio.h" 3 4
typedef 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 4
void 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 4
FILE *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 4
int 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" 2

int 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
	.align	2
.LC0:
	.ascii	"Hello ARM!\000"
	.text
	.align	2
	.global	main
	.type	main, %function
main:
	@ args = 0, pretend = 0, frame = 8
	@ frame_needed = 1, uses_anonymous_args = 0
	stmfd	sp!, {fp, lr}
	add	fp, sp, #4
	sub	sp, sp, #8
	str	r0, [fp, #-8]
	str	r1, [fp, #-12]
	ldr	r3, .L3
.LPIC0:
	add	r3, pc, r3
	mov	r0, r3
	bl	puts(PLT)
	mov	r3, #0
	mov	r0, r3
	sub	sp, fp, #4
	@ sp needed
	ldmfd	sp!, {fp, pc}
.L4:
	.align	2
.L3:
	.word	.LC0-(.LPIC0+8)
	.size	main, .-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
	.cpu	arm1022e
	.eabi_attribute	6, 4	@ Tag_CPU_arch
	.eabi_attribute	8, 1	@ Tag_ARM_ISA_use
	.eabi_attribute	15, 1	@ Tag_ABI_PCS_RW_data
	.eabi_attribute	16, 1	@ Tag_ABI_PCS_RO_data
	.eabi_attribute	17, 2	@ Tag_ABI_PCS_GOT_use
	.eabi_attribute	20, 1	@ Tag_ABI_FP_denormal
	.eabi_attribute	21, 1	@ Tag_ABI_FP_exceptions
	.eabi_attribute	23, 3	@ Tag_ABI_FP_number_model
	.eabi_attribute	24, 1	@ Tag_ABI_align_needed
	.eabi_attribute	25, 1	@ Tag_ABI_align_preserved
	.eabi_attribute	18, 4	@ Tag_ABI_PCS_wchar_t
	.eabi_attribute	26, 2	@ Tag_ABI_enum_size
	.file	"hello.i"
	.globl	main
	.align	2
	.type	main,%function
main:                                   @ @main
	.fnstart
.Leh_func_begin0:
@ BB#0:                                 @ %entry
	.save	{r11, lr}
	push	{r11, lr}
	.setfp	r11, sp
	mov	r11, sp
	.pad	#16
	sub	sp, sp, #16
	ldr	r2, .LCPI0_2
.LPC0_0:
	add	r2, pc, r2
	ldr	r3, .LCPI0_1
	add	r2, r3, r2
	ldr	r3, .LCPI0_0
	str	r3, [r11, #-4]
	str	r0, [sp, #8]
	str	r1, [sp, #4]
	mov	r0, r2
	bl	printf(PLT)
	ldr	r1, .LCPI0_0
	str	r0, [sp]                @ 4-byte Spill
	mov	r0, r1
	mov	sp, r11
	pop	{r11, pc}
	.align	2
@ BB#1:
.LCPI0_0:
	.long	0                       @ 0x0
.LCPI0_1:
	.long	.L.str(GOTOFF)
.LCPI0_2:
	.long	_GLOBAL_OFFSET_TABLE_-(.LPC0_0+8)
.Ltmp0:
	.size	main, .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: nounwind
define 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*, ...) #1

attributes #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