1.gcc的有关知识

1.gcc的有关知识

运行环境：

Vbox and redhat 6.4

1.1 gcc特点：

    Gcc(GNU C Compiler)是GNU推出的功能强大、性能优越的多平台编译器，是GNU的代表作之一。Gcc可以在多种硬体平台上编译出可执行程序，其执行效率与一般的编译器相比平均效率要高20%~30%。

1.2 gcc基本用法：

Gcc最基本的用法是：

    Gcc [options] filenames

Options:编译器所需要的编译选项。

Filenames：要编译的文件名。

新建一个hello.c文件，内容如下：

#include <stdio.h>

int main(void)

{

printf("Hello forfish! ");

return 0;

}

编辑的过程如下：

1.3

2.1编译的流程：

Gcc编译器能将C/C++语言源程序、汇编程序形成可执行文件。整个形成过程可以被细分为四个阶段：

1.预处理：

Hello.i的内容：

# 1 "hello.c"

# 1 "<built-in>"

# 1 "<command-line>"

# 1 "hello.c"

# 1 "/usr/include/stdio.h" 1 3 4

# 28 "/usr/include/stdio.h" 3 4

# 1 "/usr/include/features.h" 1 3 4

# 361 "/usr/include/features.h" 3 4

# 1 "/usr/include/sys/cdefs.h" 1 3 4

# 365 "/usr/include/sys/cdefs.h" 3 4

# 1 "/usr/include/bits/wordsize.h" 1 3 4

# 366 "/usr/include/sys/cdefs.h" 2 3 4

# 362 "/usr/include/features.h" 2 3 4

# 385 "/usr/include/features.h" 3 4

# 1 "/usr/include/gnu/stubs.h" 1 3 4

# 1 "/usr/include/bits/wordsize.h" 1 3 4

# 5 "/usr/include/gnu/stubs.h" 2 3 4

# 1 "/usr/include/gnu/stubs-32.h" 1 3 4

# 8 "/usr/include/gnu/stubs.h" 2 3 4

# 386 "/usr/include/features.h" 2 3 4

# 29 "/usr/include/stdio.h" 2 3 4

# 1 "/usr/lib/gcc/i686-redhat-linux/4.4.7/include/stddef.h" 1 3 4

# 211 "/usr/lib/gcc/i686-redhat-linux/4.4.7/include/stddef.h" 3 4

typedef unsigned int size_t;

# 35 "/usr/include/stdio.h" 2 3 4

# 1 "/usr/include/bits/types.h" 1 3 4

# 28 "/usr/include/bits/types.h" 3 4

# 1 "/usr/include/bits/wordsize.h" 1 3 4

# 29 "/usr/include/bits/types.h" 2 3 4

typedef unsigned char __u_char;

typedef unsigned short int __u_short;

typedef unsigned int __u_int;

typedef unsigned long int __u_long;

typedef signed char __int8_t;

typedef unsigned char __uint8_t;

typedef signed short int __int16_t;

typedef unsigned short int __uint16_t;

typedef signed int __int32_t;

typedef unsigned int __uint32_t;

__extension__ typedef signed long long int __int64_t;

__extension__ typedef unsigned long long int __uint64_t;

__extension__ typedef long long int __quad_t;

__extension__ typedef unsigned long long int __u_quad_t;

# 131 "/usr/include/bits/types.h" 3 4

# 1 "/usr/include/bits/typesizes.h" 1 3 4

# 132 "/usr/include/bits/types.h" 2 3 4

__extension__ typedef __u_quad_t __dev_t;

__extension__ typedef unsigned int __uid_t;

__extension__ typedef unsigned int __gid_t;

__extension__ typedef unsigned long int __ino_t;

__extension__ typedef __u_quad_t __ino64_t;

__extension__ typedef unsigned int __mode_t;

__extension__ typedef unsigned int __nlink_t;

__extension__ typedef long int __off_t;

__extension__ typedef __quad_t __off64_t;

__extension__ typedef int __pid_t;

__extension__ typedef struct { int __val[2]; } __fsid_t;

__extension__ typedef long int __clock_t;

__extension__ typedef unsigned long int __rlim_t;

__extension__ typedef __u_quad_t __rlim64_t;

__extension__ typedef unsigned int __id_t;

__extension__ typedef long int __time_t;

__extension__ typedef unsigned int __useconds_t;

__extension__ typedef long int __suseconds_t;

__extension__ typedef int __daddr_t;

__extension__ typedef long int __swblk_t;

__extension__ typedef int __key_t;

__extension__ typedef int __clockid_t;

__extension__ typedef void * __timer_t;

__extension__ typedef long int __blksize_t;

__extension__ typedef long int __blkcnt_t;

__extension__ typedef __quad_t __blkcnt64_t;

__extension__ typedef unsigned long int __fsblkcnt_t;

__extension__ typedef __u_quad_t __fsblkcnt64_t;

__extension__ typedef unsigned long int __fsfilcnt_t;

__extension__ typedef __u_quad_t __fsfilcnt64_t;

__extension__ typedef int __ssize_t;

typedef __off64_t __loff_t;

typedef __quad_t *__qaddr_t;

typedef char *__caddr_t;

__extension__ typedef int __intptr_t;

__extension__ typedef unsigned int __socklen_t;

# 37 "/usr/include/stdio.h" 2 3 4

# 45 "/usr/include/stdio.h" 3 4

struct _IO_FILE;

typedef struct _IO_FILE FILE;

# 65 "/usr/include/stdio.h" 3 4

typedef struct _IO_FILE __FILE;

# 75 "/usr/include/stdio.h" 3 4

# 1 "/usr/include/libio.h" 1 3 4

# 32 "/usr/include/libio.h" 3 4

# 1 "/usr/include/_G_config.h" 1 3 4

# 15 "/usr/include/_G_config.h" 3 4

# 1 "/usr/lib/gcc/i686-redhat-linux/4.4.7/include/stddef.h" 1 3 4

# 16 "/usr/include/_G_config.h" 2 3 4

# 1 "/usr/include/wchar.h" 1 3 4

# 83 "/usr/include/wchar.h" 3 4

typedef struct

{

int __count;

union

{

unsigned int __wch;

char __wchb[4];

} __value;

} __mbstate_t;

# 21 "/usr/include/_G_config.h" 2 3 4

typedef struct

{

__off_t __pos;

__mbstate_t __state;

} _G_fpos_t;

typedef struct

{

__off64_t __pos;

__mbstate_t __state;

} _G_fpos64_t;

# 53 "/usr/include/_G_config.h" 3 4

typedef int _G_int16_t __attribute__ ((__mode__ (__HI__)));

typedef int _G_int32_t __attribute__ ((__mode__ (__SI__)));

typedef unsigned int _G_uint16_t __attribute__ ((__mode__ (__HI__)));

typedef unsigned int _G_uint32_t __attribute__ ((__mode__ (__SI__)));

# 33 "/usr/include/libio.h" 2 3 4

# 53 "/usr/include/libio.h" 3 4

# 1 "/usr/lib/gcc/i686-redhat-linux/4.4.7/include/stdarg.h" 1 3 4

# 40 "/usr/lib/gcc/i686-redhat-linux/4.4.7/include/stdarg.h" 3 4

typedef __builtin_va_list __gnuc_va_list;

# 54 "/usr/include/libio.h" 2 3 4

# 170 "/usr/include/libio.h" 3 4

struct _IO_jump_t; struct _IO_FILE;

# 180 "/usr/include/libio.h" 3 4

typedef void _IO_lock_t;

struct _IO_marker {

struct _IO_marker *_next;

struct _IO_FILE *_sbuf;

int _pos;

# 203 "/usr/include/libio.h" 3 4

};

enum __codecvt_result

{

__codecvt_ok,

__codecvt_partial,

__codecvt_error,

__codecvt_noconv

};

# 271 "/usr/include/libio.h" 3 4

struct _IO_FILE {

int _flags;

char* _IO_read_ptr;

char* _IO_read_end;

char* _IO_read_base;

char* _IO_write_base;

char* _IO_write_ptr;

char* _IO_write_end;

char* _IO_buf_base;

char* _IO_buf_end;

char *_IO_save_base;

char *_IO_backup_base;

char *_IO_save_end;

struct _IO_marker *_markers;

struct _IO_FILE *_chain;

int _fileno;

int _flags2;

__off_t _old_offset;

unsigned short _cur_column;

signed char _vtable_offset;

char _shortbuf[1];

_IO_lock_t *_lock;

# 319 "/usr/include/libio.h" 3 4

__off64_t _offset;

# 328 "/usr/include/libio.h" 3 4

void *__pad1;

void *__pad2;

void *__pad3;

void *__pad4;

size_t __pad5;

int _mode;

char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];

};

typedef struct _IO_FILE _IO_FILE;

struct _IO_FILE_plus;

extern struct _IO_FILE_plus _IO_2_1_stdin_;

extern struct _IO_FILE_plus _IO_2_1_stdout_;

extern struct _IO_FILE_plus _IO_2_1_stderr_;

# 364 "/usr/include/libio.h" 3 4

typedef __ssize_t __io_read_fn (void *__cookie, char *__buf, size_t __nbytes);

typedef __ssize_t __io_write_fn (void *__cookie, __const char *__buf,

size_t __n);

typedef int __io_seek_fn (void *__cookie, __off64_t *__pos, int __w);

typedef int __io_close_fn (void *__cookie);

# 416 "/usr/include/libio.h" 3 4

extern int __underflow (_IO_FILE *);

extern int __uflow (_IO_FILE *);

extern int __overflow (_IO_FILE *, int);

# 460 "/usr/include/libio.h" 3 4

extern int _IO_getc (_IO_FILE *__fp);

extern int _IO_putc (int __c, _IO_FILE *__fp);

extern int _IO_feof (_IO_FILE *__fp) __attribute__ ((__nothrow__));

extern int _IO_ferror (_IO_FILE *__fp) __attribute__ ((__nothrow__));

extern int _IO_peekc_locked (_IO_FILE *__fp);

extern void _IO_flockfile (_IO_FILE *) __attribute__ ((__nothrow__));

extern void _IO_funlockfile (_IO_FILE *) __attribute__ ((__nothrow__));

extern int _IO_ftrylockfile (_IO_FILE *) __attribute__ ((__nothrow__));

# 490 "/usr/include/libio.h" 3 4

extern int _IO_vfscanf (_IO_FILE * __restrict, const char * __restrict,

__gnuc_va_list, int *__restrict);

extern int _IO_vfprintf (_IO_FILE *__restrict, const char *__restrict,

__gnuc_va_list);

extern __ssize_t _IO_padn (_IO_FILE *, int, __ssize_t);

extern size_t _IO_sgetn (_IO_FILE *, void *, size_t);

extern __off64_t _IO_seekoff (_IO_FILE *, __off64_t, int, int);

extern __off64_t _IO_seekpos (_IO_FILE *, __off64_t, int);

extern void _IO_free_backup_area (_IO_FILE *) __attribute__ ((__nothrow__));

# 76 "/usr/include/stdio.h" 2 3 4

typedef __gnuc_va_list va_list;

# 91 "/usr/include/stdio.h" 3 4

typedef __off_t off_t;

# 103 "/usr/include/stdio.h" 3 4

typedef __ssize_t ssize_t;

typedef _G_fpos_t fpos_t;

# 161 "/usr/include/stdio.h" 3 4

# 1 "/usr/include/bits/stdio_lim.h" 1 3 4

# 162 "/usr/include/stdio.h" 2 3 4

extern struct _IO_FILE *stdin;

extern struct _IO_FILE *stdout;

extern struct _IO_FILE *stderr;

extern int remove (__const char *__filename) __attribute__ ((__nothrow__));

extern int rename (__const char *__old, __const char *__new) __attribute__ ((__nothrow__));

extern int renameat (int __oldfd, __const char *__old, int __newfd,

__const char *__new) __attribute__ ((__nothrow__));

extern FILE *tmpfile (void) ;

# 208 "/usr/include/stdio.h" 3 4

extern char *tmpnam (char *__s) __attribute__ ((__nothrow__)) ;

extern char *tmpnam_r (char *__s) __attribute__ ((__nothrow__)) ;

# 226 "/usr/include/stdio.h" 3 4

extern char *tempnam (__const char *__dir, __const char *__pfx)

__attribute__ ((__nothrow__)) __attribute__ ((__malloc__)) ;

extern int fclose (FILE *__stream);

extern int fflush (FILE *__stream);

# 251 "/usr/include/stdio.h" 3 4

extern int fflush_unlocked (FILE *__stream);

# 265 "/usr/include/stdio.h" 3 4

extern FILE *fopen (__const char *__restrict __filename,

__const char *__restrict __modes) ;

extern FILE *freopen (__const char *__restrict __filename,

__const char *__restrict __modes,

FILE *__restrict __stream) ;

# 294 "/usr/include/stdio.h" 3 4

# 305 "/usr/include/stdio.h" 3 4

extern FILE *fdopen (int __fd, __const char *__modes) __attribute__ ((__nothrow__)) ;

# 318 "/usr/include/stdio.h" 3 4

extern FILE *fmemopen (void *__s, size_t __len, __const char *__modes)

__attribute__ ((__nothrow__)) ;

extern FILE *open_memstream (char **__bufloc, size_t *__sizeloc) __attribute__ ((__nothrow__)) ;

extern void setbuf (FILE *__restrict __stream, char *__restrict __buf) __attribute__ ((__nothrow__));

extern int setvbuf (FILE *__restrict __stream, char *__restrict __buf,

int __modes, size_t __n) __attribute__ ((__nothrow__));

extern void setbuffer (FILE *__restrict __stream, char *__restrict __buf,

size_t __size) __attribute__ ((__nothrow__));

extern void setlinebuf (FILE *__stream) __attribute__ ((__nothrow__));

extern int fprintf (FILE *__restrict __stream,

__const char *__restrict __format, ...);

extern int printf (__const char *__restrict __format, ...);

extern int sprintf (char *__restrict __s,

__const char *__restrict __format, ...) __attribute__ ((__nothrow__));

extern int vfprintf (FILE *__restrict __s, __const char *__restrict __format,

__gnuc_va_list __arg);

extern int vprintf (__const char *__restrict __format, __gnuc_va_list __arg);

extern int vsprintf (char *__restrict __s, __const char *__restrict __format,

__gnuc_va_list __arg) __attribute__ ((__nothrow__));

extern int snprintf (char *__restrict __s, size_t __maxlen,

__const char *__restrict __format, ...)

__attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, 3, 4)));

extern int vsnprintf (char *__restrict __s, size_t __maxlen,

__const char *__restrict __format, __gnuc_va_list __arg)

__attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, 3, 0)));

# 416 "/usr/include/stdio.h" 3 4

extern int vdprintf (int __fd, __const char *__restrict __fmt,

__gnuc_va_list __arg)

__attribute__ ((__format__ (__printf__, 2, 0)));

extern int dprintf (int __fd, __const char *__restrict __fmt, ...)

__attribute__ ((__format__ (__printf__, 2, 3)));

extern int fscanf (FILE *__restrict __stream,

__const char *__restrict __format, ...) ;

extern int scanf (__const char *__restrict __format, ...) ;

extern int sscanf (__const char *__restrict __s,

__const char *__restrict __format, ...) __attribute__ ((__nothrow__));

# 447 "/usr/include/stdio.h" 3 4

extern int fscanf (FILE *__restrict __stream, __const char *__restrict __format, ...) __asm__ ("" "__isoc99_fscanf")

;

extern int scanf (__const char *__restrict __format, ...) __asm__ ("" "__isoc99_scanf")

;

extern int sscanf (__const char *__restrict __s, __const char *__restrict __format, ...) __asm__ ("" "__isoc99_sscanf")

__attribute__ ((__nothrow__));

# 467 "/usr/include/stdio.h" 3 4

extern int vfscanf (FILE *__restrict __s, __const char *__restrict __format,

__gnuc_va_list __arg)

__attribute__ ((__format__ (__scanf__, 2, 0))) ;

extern int vscanf (__const char *__restrict __format, __gnuc_va_list __arg)

__attribute__ ((__format__ (__scanf__, 1, 0))) ;

extern int vsscanf (__const char *__restrict __s,

__const char *__restrict __format, __gnuc_va_list __arg)

__attribute__ ((__nothrow__)) __attribute__ ((__format__ (__scanf__, 2, 0)));

# 498 "/usr/include/stdio.h" 3 4

extern int vfscanf (FILE *__restrict __s, __const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vfscanf")

__attribute__ ((__format__ (__scanf__, 2, 0))) ;

extern int vscanf (__const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vscanf")

__attribute__ ((__format__ (__scanf__, 1, 0))) ;

extern int vsscanf (__const char *__restrict __s, __const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vsscanf")

__attribute__ ((__nothrow__)) __attribute__ ((__format__ (__scanf__, 2, 0)));

# 526 "/usr/include/stdio.h" 3 4

extern int fgetc (FILE *__stream);

extern int getc (FILE *__stream);

extern int getchar (void);

# 554 "/usr/include/stdio.h" 3 4

extern int getc_unlocked (FILE *__stream);

extern int getchar_unlocked (void);

# 565 "/usr/include/stdio.h" 3 4

extern int fgetc_unlocked (FILE *__stream);

extern int fputc (int __c, FILE *__stream);

extern int putc (int __c, FILE *__stream);

extern int putchar (int __c);

# 598 "/usr/include/stdio.h" 3 4

extern int fputc_unlocked (int __c, FILE *__stream);

extern int putc_unlocked (int __c, FILE *__stream);

extern int putchar_unlocked (int __c);

extern int getw (FILE *__stream);

extern int putw (int __w, FILE *__stream);

extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream)

;

extern char *gets (char *__s) ;

# 660 "/usr/include/stdio.h" 3 4

extern __ssize_t __getdelim (char **__restrict __lineptr,

size_t *__restrict __n, int __delimiter,

FILE *__restrict __stream) ;

extern __ssize_t getdelim (char **__restrict __lineptr,

size_t *__restrict __n, int __delimiter,

FILE *__restrict __stream) ;

extern __ssize_t getline (char **__restrict __lineptr,

size_t *__restrict __n,

FILE *__restrict __stream) ;

extern int fputs (__const char *__restrict __s, FILE *__restrict __stream);

extern int puts (__const char *__s);

extern int ungetc (int __c, FILE *__stream);

extern size_t fread (void *__restrict __ptr, size_t __size,

size_t __n, FILE *__restrict __stream) ;

extern size_t fwrite (__const void *__restrict __ptr, size_t __size,

size_t __n, FILE *__restrict __s) ;

# 732 "/usr/include/stdio.h" 3 4

extern size_t fread_unlocked (void *__restrict __ptr, size_t __size,

size_t __n, FILE *__restrict __stream) ;

extern size_t fwrite_unlocked (__const void *__restrict __ptr, size_t __size,

size_t __n, FILE *__restrict __stream) ;

extern int fseek (FILE *__stream, long int __off, int __whence);

extern long int ftell (FILE *__stream) ;

extern void rewind (FILE *__stream);

# 768 "/usr/include/stdio.h" 3 4

extern int fseeko (FILE *__stream, __off_t __off, int __whence);

extern __off_t ftello (FILE *__stream) ;

# 787 "/usr/include/stdio.h" 3 4

extern int fgetpos (FILE *__restrict __stream, fpos_t *__restrict __pos);

extern int fsetpos (FILE *__stream, __const fpos_t *__pos);

# 810 "/usr/include/stdio.h" 3 4

# 819 "/usr/include/stdio.h" 3 4

extern void clearerr (FILE *__stream) __attribute__ ((__nothrow__));

extern int feof (FILE *__stream) __attribute__ ((__nothrow__)) ;

extern int ferror (FILE *__stream) __attribute__ ((__nothrow__)) ;

extern void clearerr_unlocked (FILE *__stream) __attribute__ ((__nothrow__));

extern int feof_unlocked (FILE *__stream) __attribute__ ((__nothrow__)) ;

extern int ferror_unlocked (FILE *__stream) __attribute__ ((__nothrow__)) ;

extern void perror (__const char *__s);

# 1 "/usr/include/bits/sys_errlist.h" 1 3 4

# 27 "/usr/include/bits/sys_errlist.h" 3 4

extern int sys_nerr;

extern __const char *__const sys_errlist[];

# 849 "/usr/include/stdio.h" 2 3 4

extern int fileno (FILE *__stream) __attribute__ ((__nothrow__)) ;

extern int fileno_unlocked (FILE *__stream) __attribute__ ((__nothrow__)) ;

# 868 "/usr/include/stdio.h" 3 4

extern FILE *popen (__const char *__command, __const char *__modes) ;

extern int pclose (FILE *__stream);

extern char *ctermid (char *__s) __attribute__ ((__nothrow__));

# 908 "/usr/include/stdio.h" 3 4

extern void flockfile (FILE *__stream) __attribute__ ((__nothrow__));

extern int ftrylockfile (FILE *__stream) __attribute__ ((__nothrow__)) ;

extern void funlockfile (FILE *__stream) __attribute__ ((__nothrow__));

# 938 "/usr/include/stdio.h" 3 4

# 2 "hello.c" 2

int main(void)

{

printf("Hello forfish! ");

return 0;

}

2.编译生成汇编文件hello.s:

Hello.s 的内容：

    .file    "hello.c"

    .section    .rodata

.LC0:

    .string    "Hello forfish!"

    .text

.globl main

    .type    main, @function

main:

    pushl    %ebp

    movl    %esp, %ebp

    andl    $-16, %esp

    subl    $16, %esp

    movl    $.LC0, (%esp)

    call    puts

    movl    $0, %eax

    leave

    ret

    .size    main, .-main

    .ident    "GCC: (GNU) 4.4.7 20120313 (Red Hat 4.4.7-4)"

    .section    .note.GNU-stack,"",@progbits

3.汇编：

打开看不见代码，因为是2进制文件。

4.链接运行：

2.1 gcc编译流程

Gcc通过后缀来区别输入文件的类别，常见类型有：

v .c为后缀的文件: C语言源代码文件

v .a为后缀的文件: 是由目标文件构成的库文件

v .C，.cc或.cxx 为后缀的文件: 是C++源代码文件

v .h为后缀的文件: 头文件

v .o为后缀的文件: 是编译后的目标文件

v .s为后缀的文件: 是汇编语言源代码文件

2.2 gcc编译选项

掌握Gcc的使用方法，最重要的是掌握Gcc的编译选项，Gcc编译器的编译选项大约有100多个，其中多数我们根本就用不到，这里只介绍其中最基本、最常用的参数。

-o output_filename：确定可执行文件的名称为output_filename。如果不给出这个选项，gcc就给出预设的可执行文件a.out。

-O，对程序进行优化编译、链接，采用这个选项，整个源代码会在编译、连接过程中进行优化处理，这样产生的可执行文件的执行效率可以提高，但是，编译、连接的速度就相应地要慢一些。

-O2，比-O更好的优化编译、连接，当然整个编译、连接过程会更慢。

实例：

#include <stdio.h>

int main(void){

    double counter;

    double result;

    double temp;

    for(counter = 0; counter< 2000.0*2000.0*2000.0 / 20.0 + 2020;

            counter += (5-1)/4){

        temp = counter/1979;

        result = counter;

    }

    printf("Result is %lf ",result);

    return 0;

}

在上面的程序中result是没有用到的变量，该程序优化与否的时间差：

-c:条件是不链接。编译器只是由输入的.c等源代码文件编译生成.o为后缀的中间文件。

-g:条件是产生调试工具(GNU的gdb)所需要的符号信息，要想对编译出的程序进行调试，就必须加入这个选项。

加上-g的调试信息与否的文件大小：

-I dirname:将dirname所指向的目录作为编译器寻找头文件的标准路径。

C程序中的头文件包含两种情况：

#include <a.h>

#include "b.h"

对于<>,gcc在系统预设的头文件目录/usr/include中寻找相应的文件；而对于""，gcc在当前目录中搜索头文件。-I选项的作用是告诉gcc，如果在当前目录中没有找到需要的文件，就到指定的dirname目录中去寻找。

例如：gcc hello.c –I /home/include –o hello

编译错误与编译警告都是我们的程序设计中经常遇到的问题。编译错误一般是代码中出现语法错误，程序无法编译执行，必须修改。编译错误是编译器发现程序中不合理地方（比如类型不匹配），建议最后修改，但不修改也可以编译执行。

-Wall：生成所有警告信息

-w：不生成任何警告信息。

-Dmydefi：定义mydefi宏，等效于在程序中使用#define mydefi.

如果在文件里没有定义需要的宏，可以在编译的时候用-D参数指定宏：