本手册页内容摘自 GNU C 编译器的完整文档,仅限于解释选项的含义.
如果发现手册页和Info 文件之间有所矛盾,请查对 Info 文件(权威文档)
GCC
gcc GNU 工程的 C 编译器
g++ GNU 工程的 C++编译器
gcc[option|filename ]...
g++[option|filename ]...
描述(DESCRIPTION)
C 和 C++编译器是集成的
都要用4个步骤中的一个或多个处理输入文件:
预处理 (preprocessing) cpp -E 或gcc -E
编译(compilation) cc1 -S 或gcc -S
汇编(assembly) as
链接(linking) ld
源文件后缀名标识源文件的语言,但是对编译器来说,后缀名控制着缺省设置：
-
gcc 认为预处理后的文件(.i)是 C 文件,并且设定 C 形式的连接.
-
g++ 认为预处理后的文件(.i)是 C++文件,并且设定 C++形式的连接.
源文件后缀名指出语言种类以及后期的操作:
.c C源程序 预处理，编译，汇编
.C C++源程序 预处理，编译，汇编
.cc C++源程序 预处理，编译，汇编
.cxx C++源程序 预处理，编译，汇编
.m Objective-C源程序 预处理，编译，汇编
.i 预处理后的C文件 编译，汇编
.ii 预处理后的C++文件 编译，汇编
.s 汇编语言源程序 汇编
.S 汇编语言源程序 预处理，汇编
.h 预处理器文件 通常不出现在命令行上
其他后缀名的文件被传递给连接器(linker).通常包括:
.o 目标文件(Object file)
.a 归档库文件(Archive file)
除非使用了-c, -S,或-E 选项(或者编译错误阻止了完整的过程),否则连接总是最后的步骤.
在连接阶段中,所有对应于源程序的.o 文件, -l 库文件,无法识别的文件名(包括指定的 .o 目 标文件和.a 库文件)按命令行中的顺序传递给连接器.
例子
hello.c源代码：
#include<stdio.h>
int main()
{
printf("Hello! This is our embedded world!\n");
return 0;
}
（1）预处理阶段
预处理阶段 主要处理#include和#define
-
#include编译器把#include.h文件 包含到源代码中 -
#define定义的 源代码中实际使用到的 宏 用实际的字符串代替
预处理阶段 可理解为简单的文本相加/替换：
hello.c + .h文件源码 替换宏定义define ---> hello.i
gcc命令的一般格式
gcc [选项] 要编译的文件 [选项] [目标文件]
（ -o 参数省略时，Gcc默认生成可执行的文件：编译文件.out ）
例子
Gcc –E hello.c –o hello.i
gcc -E 表示只进行预处理(不进行后面的3个阶段)
-o 指目标文件（gcc输出的文件）
.i 文件为已经过预处理的C原始程序 hello.i的内容如下：
# 1 "hello.c"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 330 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "hello.c" 2
# 1 "/usr/include/stdio.h" 1 3 4
# 64 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/sys/cdefs.h" 1 3 4
# 587 "/usr/include/sys/cdefs.h" 3 4
# 1 "/usr/include/sys/_symbol_aliasing.h" 1 3 4
# 588 "/usr/include/sys/cdefs.h" 2 3 4
# 653 "/usr/include/sys/cdefs.h" 3 4
# 1 "/usr/include/sys/_posix_availability.h" 1 3 4
# 654 "/usr/include/sys/cdefs.h" 2 3 4
# 65 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/Availability.h" 1 3 4
# 184 "/usr/include/Availability.h" 3 4
# 1 "/usr/include/AvailabilityInternal.h" 1 3 4
# 185 "/usr/include/Availability.h" 2 3 4
# 66 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/_types.h" 1 3 4
# 27 "/usr/include/_types.h" 3 4
# 1 "/usr/include/sys/_types.h" 1 3 4
# 33 "/usr/include/sys/_types.h" 3 4
# 1 "/usr/include/machine/_types.h" 1 3 4
# 32 "/usr/include/machine/_types.h" 3 4
# 1 "/usr/include/i386/_types.h" 1 3 4
# 37 "/usr/include/i386/_types.h" 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 long __darwin_intptr_t;
typedef unsigned int __darwin_natural_t;
# 70 "/usr/include/i386/_types.h" 3 4
typedef int __darwin_ct_rune_t;
typedef union {
char __mbstate8[128];
long long _mbstateL;
} __mbstate_t;
typedef __mbstate_t __darwin_mbstate_t;
typedef long int __darwin_ptrdiff_t;
typedef long unsigned int __darwin_size_t;
typedef __builtin_va_list __darwin_va_list;
typedef int __darwin_wchar_t;
typedef __darwin_wchar_t __darwin_rune_t;
typedef int __darwin_wint_t;
typedef unsigned long __darwin_clock_t;
typedef __uint32_t __darwin_socklen_t;
typedef long __darwin_ssize_t;
typedef long __darwin_time_t;
# 33 "/usr/include/machine/_types.h" 2 3 4
# 34 "/usr/include/sys/_types.h" 2 3 4
# 55 "/usr/include/sys/_types.h" 3 4
typedef __int64_t __darwin_blkcnt_t;
typedef __int32_t __darwin_blksize_t;
typedef __int32_t __darwin_dev_t;
typedef unsigned int __darwin_fsblkcnt_t;
typedef unsigned int __darwin_fsfilcnt_t;
typedef __uint32_t __darwin_gid_t;
typedef __uint32_t __darwin_id_t;
typedef __uint64_t __darwin_ino64_t;
typedef __darwin_ino64_t __darwin_ino_t;
typedef __darwin_natural_t __darwin_mach_port_name_t;
typedef __darwin_mach_port_name_t __darwin_mach_port_t;
typedef __uint16_t __darwin_mode_t;
typedef __int64_t __darwin_off_t;
typedef __int32_t __darwin_pid_t;
typedef __uint32_t __darwin_sigset_t;
typedef __int32_t __darwin_suseconds_t;
typedef __uint32_t __darwin_uid_t;
typedef __uint32_t __darwin_useconds_t;
typedef unsigned char __darwin_uuid_t[16];
typedef char __darwin_uuid_string_t[37];
# 1 "/usr/include/sys/_pthread/_pthread_types.h" 1 3 4
# 57 "/usr/include/sys/_pthread/_pthread_types.h" 3 4
struct __darwin_pthread_handler_rec {
void (*__routine)(void *);
void *__arg;
struct __darwin_pthread_handler_rec *__next;
};
struct _opaque_pthread_attr_t {
long __sig;
char __opaque[56];
};
struct _opaque_pthread_cond_t {
long __sig;
char __opaque[40];
};
struct _opaque_pthread_condattr_t {
long __sig;
char __opaque[8];
};
struct _opaque_pthread_mutex_t {
long __sig;
char __opaque[56];
};
struct _opaque_pthread_mutexattr_t {
long __sig;
char __opaque[8];
};
struct _opaque_pthread_once_t {
long __sig;
char __opaque[8];
};
struct _opaque_pthread_rwlock_t {
long __sig;
char __opaque[192];
};
struct _opaque_pthread_rwlockattr_t {
long __sig;
char __opaque[16];
};
struct _opaque_pthread_t {
long __sig;
struct __darwin_pthread_handler_rec *__cleanup_stack;
char __opaque[8176];
};
typedef struct _opaque_pthread_attr_t __darwin_pthread_attr_t;
typedef struct _opaque_pthread_cond_t __darwin_pthread_cond_t;
typedef struct _opaque_pthread_condattr_t __darwin_pthread_condattr_t;
typedef unsigned long __darwin_pthread_key_t;
typedef struct _opaque_pthread_mutex_t __darwin_pthread_mutex_t;
typedef struct _opaque_pthread_mutexattr_t __darwin_pthread_mutexattr_t;
typedef struct _opaque_pthread_once_t __darwin_pthread_once_t;
typedef struct _opaque_pthread_rwlock_t __darwin_pthread_rwlock_t;
typedef struct _opaque_pthread_rwlockattr_t __darwin_pthread_rwlockattr_t;
typedef struct _opaque_pthread_t *__darwin_pthread_t;
# 81 "/usr/include/sys/_types.h" 2 3 4
# 28 "/usr/include/_types.h" 2 3 4
# 39 "/usr/include/_types.h" 3 4
typedef int __darwin_nl_item;
typedef int __darwin_wctrans_t;
typedef __uint32_t __darwin_wctype_t;
# 68 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/sys/_types/_va_list.h" 1 3 4
# 31 "/usr/include/sys/_types/_va_list.h" 3 4
typedef __darwin_va_list va_list;
# 72 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/sys/_types/_size_t.h" 1 3 4
# 30 "/usr/include/sys/_types/_size_t.h" 3 4
typedef __darwin_size_t size_t;
# 73 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/sys/_types/_null.h" 1 3 4
# 74 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/sys/stdio.h" 1 3 4
# 39 "/usr/include/sys/stdio.h" 3 4
int renameat(int, const char *, int, const char *) __attribute__((availability(macosx,introduced=10.10)));
int renamex_np(const char *, const char *, unsigned int) __attribute__((availability(macosx,introduced=10.12))) __attribute__((availability(ios,introduced=10.0))) __attribute__((availability(tvos,introduced=10.0))) __attribute__((availability(watchos,introduced=3.0)));
int renameatx_np(int, const char *, int, const char *, unsigned int) __attribute__((availability(macosx,introduced=10.12))) __attribute__((availability(ios,introduced=10.0))) __attribute__((availability(tvos,introduced=10.0))) __attribute__((availability(watchos,introduced=3.0)));
# 76 "/usr/include/stdio.h" 2 3 4
typedef __darwin_off_t fpos_t;
# 88 "/usr/include/stdio.h" 3 4
struct __sbuf {
unsigned char *_base;
int _size;
};
struct __sFILEX;
# 122 "/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 (* _Nullable _close)(void *);
int (* _Nullable _read) (void *, char *, int);
fpos_t (* _Nullable _seek) (void *, fpos_t, int);
int (* _Nullable _write)(void *, const char *, int);
struct __sbuf _ub;
struct __sFILEX *_extra;
int _ur;
unsigned char _ubuf[3];
unsigned char _nbuf[1];
struct __sbuf _lb;
int _blksize;
fpos_t _offset;
} FILE;
extern FILE *__stdinp;
extern FILE *__stdoutp;
extern FILE *__stderrp;
# 231 "/usr/include/stdio.h" 3 4
void clearerr(FILE *);
int fclose(FILE *);
int feof(FILE *);
int ferror(FILE *);
int fflush(FILE *);
int fgetc(FILE *);
int fgetpos(FILE * restrict, fpos_t *);
char *fgets(char * restrict, int, FILE *);
FILE *fopen(const char * restrict __filename, const char * restrict __mode) __asm("_" "fopen" );
int fprintf(FILE * restrict, const char * restrict, ...) __attribute__((__format__ (__printf__, 2, 3)));
int fputc(int, FILE *);
int fputs(const char * restrict, FILE * restrict) __asm("_" "fputs" );
size_t fread(void * restrict __ptr, size_t __size, size_t __nitems, FILE * restrict __stream);
FILE *freopen(const char * restrict, const char * restrict,
FILE * restrict) __asm("_" "freopen" );
int fscanf(FILE * restrict, const char * restrict, ...) __attribute__((__format__ (__scanf__, 2, 3)));
int fseek(FILE *, long, int);
int fsetpos(FILE *, const fpos_t *);
long ftell(FILE *);
size_t fwrite(const void * restrict __ptr, size_t __size, size_t __nitems, FILE * restrict __stream) __asm("_" "fwrite" );
int getc(FILE *);
int getchar(void);
char *gets(char *);
void perror(const char *);
int printf(const char * restrict, ...) __attribute__((__format__ (__printf__, 1, 2)));
int putc(int, FILE *);
int putchar(int);
int puts(const char *);
int remove(const char *);
int rename (const char *__old, const char *__new);
void rewind(FILE *);
int scanf(const char * restrict, ...) __attribute__((__format__ (__scanf__, 1, 2)));
void setbuf(FILE * restrict, char * restrict);
int setvbuf(FILE * restrict, char * restrict, int, size_t);
int sprintf(char * restrict, const char * restrict, ...) __attribute__((__format__ (__printf__, 2, 3))) __attribute__((__availability__(swift, unavailable, message="Use snprintf instead.")));
int sscanf(const char * restrict, const char * restrict, ...) __attribute__((__format__ (__scanf__, 2, 3)));
FILE *tmpfile(void);
__attribute__((__availability__(swift, unavailable, message="Use mkstemp(3) instead.")))
__attribute__((deprecated("This function is provided for compatibility reasons only. Due to security concerns inherent in the design of tmpnam(3), it is highly recommended that you use mkstemp(3) instead.")))
char *tmpnam(char *);
int ungetc(int, FILE *);
int vfprintf(FILE * restrict, const char * restrict, va_list) __attribute__((__format__ (__printf__, 2, 0)));
int vprintf(const char * restrict, va_list) __attribute__((__format__ (__printf__, 1, 0)));
int vsprintf(char * restrict, const char * restrict, va_list) __attribute__((__format__ (__printf__, 2, 0))) __attribute__((__availability__(swift, unavailable, message="Use vsnprintf instead.")));
# 297 "/usr/include/stdio.h" 3 4
char *ctermid(char *);
FILE *fdopen(int, const char *) __asm("_" "fdopen" );
int fileno(FILE *);
# 321 "/usr/include/stdio.h" 3 4
int pclose(FILE *) __attribute__((__availability__(swift, unavailable, message="Use posix_spawn APIs or NSTask instead.")));
FILE *popen(const char *, const char *) __asm("_" "popen" ) __attribute__((__availability__(swift, unavailable, message="Use posix_spawn APIs or NSTask instead.")));
# 342 "/usr/include/stdio.h" 3 4
int __srget(FILE *);
int __svfscanf(FILE *, const char *, va_list) __attribute__((__format__ (__scanf__, 2, 0)));
int __swbuf(int, FILE *);
# 353 "/usr/include/stdio.h" 3 4
inline __attribute__ ((__always_inline__)) int __sputc(int _c, FILE *_p) {
if (--_p->_w >= 0 || (_p->_w >= _p->_lbfsize && (char)_c != '\n'))
return (*_p->_p++ = _c);
else
return (__swbuf(_c, _p));
}
# 379 "/usr/include/stdio.h" 3 4
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);
int getw(FILE *);
int putw(int, FILE *);
__attribute__((__availability__(swift, unavailable, message="Use mkstemp(3) instead.")))
__attribute__((deprecated("This function is provided for compatibility reasons only. Due to security concerns inherent in the design of tempnam(3), it is highly recommended that you use mkstemp(3) instead.")))
char *tempnam(const char *__dir, const char *__prefix) __asm("_" "tempnam" );
# 417 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/sys/_types/_off_t.h" 1 3 4
# 30 "/usr/include/sys/_types/_off_t.h" 3 4
typedef __darwin_off_t off_t;
# 418 "/usr/include/stdio.h" 2 3 4
int fseeko(FILE * __stream, off_t __offset, int __whence);
off_t ftello(FILE * __stream);
int snprintf(char * restrict __str, size_t __size, const char * restrict __format, ...) __attribute__((__format__ (__printf__, 3, 4)));
int vfscanf(FILE * restrict __stream, const char * restrict __format, va_list) __attribute__((__format__ (__scanf__, 2, 0)));
int vscanf(const char * restrict __format, va_list) __attribute__((__format__ (__scanf__, 1, 0)));
int vsnprintf(char * restrict __str, size_t __size, const char * restrict __format, va_list) __attribute__((__format__ (__printf__, 3, 0)));
int vsscanf(const char * restrict __str, const char * restrict __format, va_list) __attribute__((__format__ (__scanf__, 2, 0)));
# 442 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/sys/_types/_ssize_t.h" 1 3 4
# 30 "/usr/include/sys/_types/_ssize_t.h" 3 4
typedef __darwin_ssize_t ssize_t;
# 443 "/usr/include/stdio.h" 2 3 4
int dprintf(int, const char * restrict, ...) __attribute__((__format__ (__printf__, 2, 3))) __attribute__((availability(macosx,introduced=10.7)));
int vdprintf(int, const char * restrict, va_list) __attribute__((__format__ (__printf__, 2, 0))) __attribute__((availability(macosx,introduced=10.7)));
ssize_t getdelim(char ** restrict __linep, size_t * restrict __linecapp, int __delimiter, FILE * restrict __stream) __attribute__((availability(macosx,introduced=10.7)));
ssize_t getline(char ** restrict __linep, size_t * restrict __linecapp, FILE * restrict __stream) __attribute__((availability(macosx,introduced=10.7)));
# 458 "/usr/include/stdio.h" 3 4
extern const int sys_nerr;
extern const char *const sys_errlist[];
int asprintf(char ** restrict, const char * restrict, ...) __attribute__((__format__ (__printf__, 2, 3)));
char *ctermid_r(char *);
char *fgetln(FILE *, size_t *);
const char *fmtcheck(const char *, const char *);
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)));
FILE *zopen(const char *, const char *, int);
FILE *funopen(const void *,
int (* _Nullable)(void *, char *, int),
int (* _Nullable)(void *, const char *, int),
fpos_t (* _Nullable)(void *, fpos_t, int),
int (* _Nullable)(void *));
# 498 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/secure/_stdio.h" 1 3 4
# 31 "/usr/include/secure/_stdio.h" 3 4
# 1 "/usr/include/secure/_common.h" 1 3 4
# 32 "/usr/include/secure/_stdio.h" 2 3 4
# 42 "/usr/include/secure/_stdio.h" 3 4
extern int __sprintf_chk (char * restrict, int, size_t,
const char * restrict, ...);
# 52 "/usr/include/secure/_stdio.h" 3 4
extern int __snprintf_chk (char * restrict, size_t, int, size_t,
const char * restrict, ...);
extern int __vsprintf_chk (char * restrict, int, size_t,
const char * restrict, va_list);
extern int __vsnprintf_chk (char * restrict, size_t, int, size_t,
const char * restrict, va_list);
# 499 "/usr/include/stdio.h" 2 3 4
# 2 "hello.c" 2
int main()
{
printf("Hello! This is our embedded world!\n");
return 0;
}
（2）编译阶段(最重要的阶段)
可接收.c和.i类型的文件 ---> .s文件
gcc -S hello.i -o hello.s
gcc -S只进行编译(而不进行汇编，生成汇编代码)
编译阶段gcc首先要检查代码的规范性、是否有语法错误等，以确定代码的实际要做的工作。
检查无误后gcc把代码翻译成汇编语言 *.i编译成*.s文件
hello.s 内容如下：
.section __TEXT,__text,regular,pure_instructions
.macosx_version_min 10, 12
.globl _main
.align 4, 0x90
_main: ## @main
.cfi_startproc
## BB#0:
pushq %rbp
Ltmp0:
.cfi_def_cfa_offset 16
Ltmp1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp2:
.cfi_def_cfa_register %rbp
subq $16, %rsp
leaq L_.str(%rip), %rdi
movl $0, -4(%rbp)
movb $0, %al
callq _printf
xorl %ecx, %ecx
movl %eax, -8(%rbp) ## 4-byte Spill
movl %ecx, %eax
addq $16, %rsp
popq %rbp
retq
.cfi_endproc
.section __TEXT,__cstring,cstring_literals
L_.str: ## @.str
.asciz "Hello! This is our embedded world!\n"
.subsections_via_symbols
（2）编译阶段(最重要的阶段)
可接收.c和.i类型的文件 ---> .s文件
gcc -S hello.i -o hello.s
gcc -S只进行编译(而不进行汇编，生成汇编代码)
编译阶段gcc首先要检查代码的规范性、是否有语法错误等，以确定代码的实际要做的工作。
检查无误后gcc把代码翻译成汇编语言 *.i编译成*.s文件
hello.s 内容如下：
.section __TEXT,__text,regular,pure_instructions
.macosx_version_min 10, 12
.globl _main
.align 4, 0x90
_main: ## @main
.cfi_startproc
## BB#0:
pushq %rbp
Ltmp0:
.cfi_def_cfa_offset 16
Ltmp1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp2:
.cfi_def_cfa_register %rbp
subq $16, %rsp
leaq L_.str(%rip), %rdi
movl $0, -4(%rbp)
movb $0, %al
callq _printf
xorl %ecx, %ecx
movl %eax, -8(%rbp) ## 4-byte Spill
movl %ecx, %eax
addq $16, %rsp
popq %rbp
retq
.cfi_endproc
.section __TEXT,__cstring,cstring_literals
L_.str: ## @.str
.asciz "Hello! This is our embedded world!\n"
.subsections_via_symbols
（3）汇编阶段
汇编阶段可接收 .c .i .s 类型的文件 ---> .o文件
把 .s文件 翻译成 二进制机器指令 .o文件:
gcc -c hello.s -o hello.o
-c告诉gcc进行汇编处理。
这步生成的文件是二进制文件，用文本工具打开看到的将是“乱码”，使用反汇编工具（如GDB）才能读懂它。
（4）链接阶段
链接阶段。
在这里涉及到一个重要的概念：函数库。
链接的作用
例子的源码中并没有定义”printf”的函数实现，且预编译中包含进的”stdio.h”中也只有该函数的声明，而没有定义函数的实现。
那么哪里实现了”printf”函数？动态库文件libc.so.6
系统把这些函数实现都做到名为libc.so.6的库文件中，在没有特别指定时，Gcc会到系统默认的搜索路径”/usr/lib”下进行查找，链接到libc.so.6库函数中就能实现函数”printf”
函数库一般分为静态库和动态库两种。Gcc在编译时默认使用动态库
静态库：指编译链接时，把库文件的代码全部加入到可执行文件中，运行时也就不再需要库文件，因此生成的文件比较大。
后缀名一般为”.a”
动态库：编译链接时并没有把库文件的代码加入到可执行文件中，而是在程序执行时由运行时链接文件加载库，节省系统的开销。
动态库一般后缀名为”.so”
如前面所述的libc.so.6就是动态库。
生成静态库：
·ar cr libxxx.a file1.o file2.o
把file1.o和file2.o打包 生成libxxx.a静态库
生成动态库：
gcc -fPIC -shared file1.c -o libtest.so
也可以分成两部来写：
gcc -fPIC file1.c -c //这一步生成file1.o
gcc -shared file1.o -o libtest.so
使用
gcc test.c -L/path -libxxx -o test
效果是一样的。
使用动态库，运行程序时要指定动态库的位置，
用环境变量来指定export LD_LIBRARY_PATH=path
否则会提示找不到动态库的位置
链接动态库/静态库 使用的方法是一样的，
如果在库中有同名的静态库文件和动态库文件，如libtest.a和libtest.so
gcc链接时默认优先选择动态库 即 链接libtest.so
强制gcc链接静态库文件（如libtest.a）:加选项-static
静态库链接时搜索路径顺序：
- ld会去找GCC命令中的参数-L
- 再找gcc的环境变量LIBRARY_PATH
- 再找内定目录 /lib /usr/lib /usr/local/lib 这是当初compile gcc时写在程序内的
动态链接时、执行时搜索路径顺序: - 编译目标代码时指定的动态库搜索路径
- 环境变量LD_LIBRARY_PATH指定的动态库搜索路径
- 配置文件/etc/ld.so.conf中指定的动态库搜索路径
- 默认的动态库搜索路径/lib
- 默认的动态库搜索路径/usr/lib
有关环境变量：
LIBRARY_PATH环境变量：指定程序静态链接库文件搜索路径
LD_LIBRARY_PATH环境变量：指定程序动态链接库文件搜索路径
完成了链接之后，Gcc就可以生成可执行文件，如下所示。
gcc hello.o -o hello
运行该可执行文件，出现正确的结果如下。
./hello
Hello! This is our embedded world!
选项(OPTIONS)
选项必须分立给出: -dr 完全不同于-d -r
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