Tools - GCC和动静态库

• Tools - GCC和动静态库
• 参考
• 概述
• 编译过程
  • 语法
  • 选项
  • 示例
    • 阶段编译
  • 使用静态库
  • 使用共享库
    • 动态库修改使用
  • 动态库和静态库比较
    • 静态库
    • 动态库

参考

• gcc 命令详解及最佳实践 (opens new window)

概述

GCC（英文全拼：GNU Compiler Collection）是 GNU 工具链的主要组成部分，是一套以 GPL 和 LGPL 许可证发布的程序语言编译器自由软件，由 Richard Stallman 于 1985 年开始开发。

GCC 原名为 GNU C语言编译器，因为它原本只能处理 C 语言，但如今的 GCC 不仅可以编译 C、C++ 和 Objective-C，还可以通过不同的前端模块支持各种语言，包括 Java、Fortran、Ada、Pascal、Go 和 D 语言等等。

编译过程

GCC 的编译过程可以划分为四个阶段：预处理（Pre-Processing）、编译（Compiling）、汇编（Assembling）以及链接（Linking）。

Linux 程序员可以根据自己的需要控制 GCC 的编译阶段，以便检查或使用编译器在该阶段的输出信息，帮助调试和优化程序。以 C 语言为例，从源文件的编译到可执行文件的运行，整个过程大致如下。

语法

gcc [options] file...

选项

可以通过 gcc --help 查看具体的选项内容。

[root@VM-16-6-centos test]# gcc --help
Usage: gcc [options] file...
Options:
  -pass-exit-codes         Exit with highest error code from a phase
  --help                   Display this information
  --target-help            Display target specific command line options
  --help={common|optimizers|params|target|warnings|[^]{joined|separate|undocumented}}[,...]
                           Display specific types of command line options
  (Use '-v --help' to display command line options of sub-processes)
  --version                Display compiler version information
  -dumpspecs               Display all of the built in spec strings
  -dumpversion             Display the version of the compiler
  -dumpmachine             Display the compiler's target processor
  -print-search-dirs       Display the directories in the compiler's search path
  -print-libgcc-file-name  Display the name of the compiler's companion library
  -print-file-name=<lib>   Display the full path to library <lib>
  -print-prog-name=<prog>  Display the full path to compiler component <prog>
  -print-multiarch         Display the target's normalized GNU triplet, used as
                           a component in the library path
  -print-multi-directory   Display the root directory for versions of libgcc
  -print-multi-lib         Display the mapping between command line options and
                           multiple library search directories
  -print-multi-os-directory Display the relative path to OS libraries
  -print-sysroot           Display the target libraries directory
  -print-sysroot-headers-suffix Display the sysroot suffix used to find headers
  -Wa,<options>            Pass comma-separated <options> on to the assembler
  -Wp,<options>            Pass comma-separated <options> on to the preprocessor
  -Wl,<options>            Pass comma-separated <options> on to the linker
  -Xassembler <arg>        Pass <arg> on to the assembler
  -Xpreprocessor <arg>     Pass <arg> on to the preprocessor
  -Xlinker <arg>           Pass <arg> on to the linker
  -save-temps              Do not delete intermediate files
  -save-temps=<arg>        Do not delete intermediate files
  -no-canonical-prefixes   Do not canonicalize paths when building relative
                           prefixes to other gcc components
  -pipe                    Use pipes rather than intermediate files
  -time                    Time the execution of each subprocess
  -specs=<file>            Override built-in specs with the contents of <file>
  -std=<standard>          Assume that the input sources are for <standard>
  --sysroot=<directory>    Use <directory> as the root directory for headers
                           and libraries
  -B <directory>           Add <directory> to the compiler's search paths
  -v                       Display the programs invoked by the compiler
  -###                     Like -v but options quoted and commands not executed
  -E                       Preprocess only; do not compile, assemble or link
  -S                       Compile only; do not assemble or link
  -c                       Compile and assemble, but do not link
  -o <file>                Place the output into <file>
  -pie                     Create a position independent executable
  -shared                  Create a shared library
  -x <language>            Specify the language of the following input files
                           Permissible languages include: c c++ assembler none
                           'none' means revert to the default behavior of
                           guessing the language based on the file's extension

Options starting with -g, -f, -m, -O, -W, or --param are automatically passed on to the various sub-processes invoked by gcc.  In order to passother options on to these processes the -W<letter> options must be used.

示例

阶段编译

假设有文件 hello.c，内容如下：

#include <stdio.h>

int main(void)
{
    printf("Hello, World!\n");
    return 0;
}

编译 hello.c，默认输出 a.out

gcc hello.c

编译 hello.c 并指定输出文件为 hello

gcc hello.c -o hello

只执行预处理，输出 hello.i 源文件

gcc -E hello.c -o hello.i

只执行预处理和编译，输出 hello.s 汇编文件

gcc -S hello.c

也可以由 hello.i 文件生成 hello.s 汇编文件

gcc -S hello.i -o hello.s

只执行预处理、编译和汇编，输出 hello.o 目标文件

gcc -c hello.c

也可以由 hello.i 或 hello.s 生成目标文件 hello.o

gcc -c hello.i -o hello.o
gcc -c hello.s -o hello.o

由 hello.o 目标文件链接成可执行文件 hello

gcc hello.o -o hello

hello.i 和 hello.s 文件内容如下

DETAILS

# 1 "hello.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "hello.c"
# 1 "/usr/include/stdio.h" 1 3 4
# 27 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/features.h" 1 3 4
# 375 "/usr/include/features.h" 3 4
# 1 "/usr/include/sys/cdefs.h" 1 3 4
# 392 "/usr/include/sys/cdefs.h" 3 4
# 1 "/usr/include/bits/wordsize.h" 1 3 4
# 393 "/usr/include/sys/cdefs.h" 2 3 4
# 376 "/usr/include/features.h" 2 3 4
# 399 "/usr/include/features.h" 3 4
# 1 "/usr/include/gnu/stubs.h" 1 3 4
# 10 "/usr/include/gnu/stubs.h" 3 4
# 1 "/usr/include/gnu/stubs-64.h" 1 3 4
# 11 "/usr/include/gnu/stubs.h" 2 3 4
# 400 "/usr/include/features.h" 2 3 4
# 28 "/usr/include/stdio.h" 2 3 4





# 1 "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include/stddef.h" 1 3 4
# 212 "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include/stddef.h" 3 4
typedef long unsigned int size_t;
# 34 "/usr/include/stdio.h" 2 3 4

# 1 "/usr/include/bits/types.h" 1 3 4
# 27 "/usr/include/bits/types.h" 3 4
# 1 "/usr/include/bits/wordsize.h" 1 3 4
# 28 "/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;

typedef signed long int __int64_t;
typedef unsigned long int __uint64_t;







typedef long int __quad_t;
typedef unsigned long int __u_quad_t;
# 130 "/usr/include/bits/types.h" 3 4
# 1 "/usr/include/bits/typesizes.h" 1 3 4
# 131 "/usr/include/bits/types.h" 2 3 4


typedef unsigned long int __dev_t;
typedef unsigned int __uid_t;
typedef unsigned int __gid_t;
typedef unsigned long int __ino_t;
typedef unsigned long int __ino64_t;
typedef unsigned int __mode_t;
typedef unsigned long int __nlink_t;
typedef long int __off_t;
typedef long int __off64_t;
typedef int __pid_t;
typedef struct { int __val[2]; } __fsid_t;
typedef long int __clock_t;
typedef unsigned long int __rlim_t;
typedef unsigned long int __rlim64_t;
typedef unsigned int __id_t;
typedef long int __time_t;
typedef unsigned int __useconds_t;
typedef long int __suseconds_t;

typedef int __daddr_t;
typedef int __key_t;


typedef int __clockid_t;


typedef void * __timer_t;


typedef long int __blksize_t;




typedef long int __blkcnt_t;
typedef long int __blkcnt64_t;


typedef unsigned long int __fsblkcnt_t;
typedef unsigned long int __fsblkcnt64_t;


typedef unsigned long int __fsfilcnt_t;
typedef unsigned long int __fsfilcnt64_t;


typedef long int __fsword_t;

typedef long int __ssize_t;


typedef long int __syscall_slong_t;

typedef unsigned long int __syscall_ulong_t;



typedef __off64_t __loff_t;
typedef __quad_t *__qaddr_t;
typedef char *__caddr_t;


typedef long int __intptr_t;


typedef unsigned int __socklen_t;
# 36 "/usr/include/stdio.h" 2 3 4
# 44 "/usr/include/stdio.h" 3 4
struct _IO_FILE;



typedef struct _IO_FILE FILE;





# 64 "/usr/include/stdio.h" 3 4
typedef struct _IO_FILE __FILE;
# 74 "/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/x86_64-redhat-linux/4.8.5/include/stddef.h" 1 3 4
# 16 "/usr/include/_G_config.h" 2 3 4




# 1 "/usr/include/wchar.h" 1 3 4
# 82 "/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;
# 33 "/usr/include/libio.h" 2 3 4
# 50 "/usr/include/libio.h" 3 4
# 1 "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include/stdarg.h" 1 3 4
# 40 "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include/stdarg.h" 3 4
typedef __builtin_va_list __gnuc_va_list;
# 51 "/usr/include/libio.h" 2 3 4
# 145 "/usr/include/libio.h" 3 4
struct _IO_jump_t; struct _IO_FILE;
# 155 "/usr/include/libio.h" 3 4
typedef void _IO_lock_t;





struct _IO_marker {
  struct _IO_marker *_next;
  struct _IO_FILE *_sbuf;



  int _pos;
# 178 "/usr/include/libio.h" 3 4
};


enum __codecvt_result
{
  __codecvt_ok,
  __codecvt_partial,
  __codecvt_error,
  __codecvt_noconv
};
# 246 "/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;
# 294 "/usr/include/libio.h" 3 4
  __off64_t _offset;
# 303 "/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_;
# 339 "/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);
# 391 "/usr/include/libio.h" 3 4
extern int __underflow (_IO_FILE *);
extern int __uflow (_IO_FILE *);
extern int __overflow (_IO_FILE *, int);
# 435 "/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__ , __leaf__));
extern int _IO_ferror (_IO_FILE *__fp) __attribute__ ((__nothrow__ , __leaf__));

extern int _IO_peekc_locked (_IO_FILE *__fp);





extern void _IO_flockfile (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__));
extern void _IO_funlockfile (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__));
extern int _IO_ftrylockfile (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__));
# 465 "/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__ , __leaf__));
# 75 "/usr/include/stdio.h" 2 3 4




typedef __gnuc_va_list va_list;
# 90 "/usr/include/stdio.h" 3 4
typedef __off_t off_t;
# 102 "/usr/include/stdio.h" 3 4
typedef __ssize_t ssize_t;







typedef _G_fpos_t fpos_t;




# 164 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/bits/stdio_lim.h" 1 3 4
# 165 "/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__ , __leaf__));

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




extern int renameat (int __oldfd, const char *__old, int __newfd,
       const char *__new) __attribute__ ((__nothrow__ , __leaf__));








extern FILE *tmpfile (void) ;
# 209 "/usr/include/stdio.h" 3 4
extern char *tmpnam (char *__s) __attribute__ ((__nothrow__ , __leaf__)) ;





extern char *tmpnam_r (char *__s) __attribute__ ((__nothrow__ , __leaf__)) ;
# 227 "/usr/include/stdio.h" 3 4
extern char *tempnam (const char *__dir, const char *__pfx)
     __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__malloc__)) ;








extern int fclose (FILE *__stream);




extern int fflush (FILE *__stream);

# 252 "/usr/include/stdio.h" 3 4
extern int fflush_unlocked (FILE *__stream);
# 266 "/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) ;
# 295 "/usr/include/stdio.h" 3 4

# 306 "/usr/include/stdio.h" 3 4
extern FILE *fdopen (int __fd, const char *__modes) __attribute__ ((__nothrow__ , __leaf__)) ;
# 319 "/usr/include/stdio.h" 3 4
extern FILE *fmemopen (void *__s, size_t __len, const char *__modes)
  __attribute__ ((__nothrow__ , __leaf__)) ;




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






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



extern int setvbuf (FILE *__restrict __stream, char *__restrict __buf,
      int __modes, size_t __n) __attribute__ ((__nothrow__ , __leaf__));





extern void setbuffer (FILE *__restrict __stream, char *__restrict __buf,
         size_t __size) __attribute__ ((__nothrow__ , __leaf__));


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








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)));

# 412 "/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__ , __leaf__));
# 443 "/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__ , __leaf__))

                      ;
# 463 "/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__ , __leaf__)) __attribute__ ((__format__ (__scanf__, 2, 0)));
# 494 "/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__ , __leaf__))



     __attribute__ ((__format__ (__scanf__, 2, 0)));
# 522 "/usr/include/stdio.h" 3 4









extern int fgetc (FILE *__stream);
extern int getc (FILE *__stream);





extern int getchar (void);

# 550 "/usr/include/stdio.h" 3 4
extern int getc_unlocked (FILE *__stream);
extern int getchar_unlocked (void);
# 561 "/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);

# 594 "/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)
     ;
# 638 "/usr/include/stdio.h" 3 4
extern char *gets (char *__s) __attribute__ ((__deprecated__));


# 665 "/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);

# 737 "/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);

# 773 "/usr/include/stdio.h" 3 4
extern int fseeko (FILE *__stream, __off_t __off, int __whence);




extern __off_t ftello (FILE *__stream) ;
# 792 "/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);
# 815 "/usr/include/stdio.h" 3 4

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


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

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

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




extern void clearerr_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));
extern int feof_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
extern int ferror_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;








extern void perror (const char *__s);






# 1 "/usr/include/bits/sys_errlist.h" 1 3 4
# 26 "/usr/include/bits/sys_errlist.h" 3 4
extern int sys_nerr;
extern const char *const sys_errlist[];
# 854 "/usr/include/stdio.h" 2 3 4




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




extern int fileno_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
# 873 "/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__ , __leaf__));
# 913 "/usr/include/stdio.h" 3 4
extern void flockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));



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


extern void funlockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));
# 943 "/usr/include/stdio.h" 3 4

# 2 "hello.c" 2

int main(void)
{
    printf("Hello, World!\n");
    return 0;
}

	.file	"hello.c"
	.section	.rodata
.LC0:
	.string	"Hello, World!"
	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register 6
	movl	$.LC0, %edi
	call	puts
	movl	$0, %eax
	popq	%rbp
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.ident	"GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-44)"
	.section	.note.GNU-stack,"",@progbits

使用静态库

创建一个 fun.c 文件，内容如下：

#include <stdio.h>

void fun(void)
{
    printf("Here is a static library!\n");
}

将 fun.c 编译成静态库 libfun.a

gcc -c fun.c
ar rcs libfun.a fun.o

查看文件描述 TODO

$ file *
fun.c:    C source, ASCII text
fun.o:    ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
libfun.a: current ar archive

修改 hello1.c 文件，调用 fun 函数

#include <stdio.h>

void fun(void);

int main(void)
{
    printf("Hello, World!\n");
    fun();
    return 0;
}

编译 hello1.c 并链接静态库 libfun.a（加上 -static 选项）

gcc hello1.c -static libfun.a -o hello1

也可以使用 -L 指定库的搜索路径，并使用 -l 指定库名

gcc hello1.c -L. -static -lfun -o hello1

运行结果

[root@VM-16-6-centos test]# ./hello1
Hello, World!
Here is a static library!

查看 hello1 文件描述

[root@VM-16-6-centos test]# file hello1
hello1: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, for GNU/Linux 2.6.32, BuildID[sha1]=0fc4821ab804afd1d4a1d486616964714e9a7f6b, not stripped

使用共享库

创建一个 shareFun.c 文件，内容如下：

#include <stdio.h>

void fun(void)
{
    printf("Here is a shared library!\n");
}

编译 hello1.c 并链接共享库 libshareFun.so

gcc shareFun.c -shared -fPIC -o libshareFun.so

也可以使用 -L 和 -l 选项指定库的路径和名称

gcc hello1.c -L. -lshareFun -o hello1

运行结果

[root@VM-16-6-centos test]# export LD_LIBRARY_PATH=$(pwd)
[root@VM-16-6-centos test]# ./hello1
Hello, World!
Here is a shared library!

动态库修改使用

# 修改动态库并重新生成
[root@VM-16-6-centos test]# vi shareFun.c
[root@VM-16-6-centos test]# gcc shareFun.c -shared -fPIC -o libshareFun.so

# 不需要重新编译 hello1，新增内容 “Test”，可以直接看到，运行时重新加载so
[root@VM-16-6-centos test]# ./hello1
Hello, World!
Test Here is a shared library!

查看链接内容：

[root@VM-16-6-centos test]# ldd hello1
        linux-vdso.so.1 =>  (0x00007fff03bbd000)
        /$LIB/libonion.so => /lib64/libonion.so (0x00007fd57b8b1000)
        libshareFun.so => /home/klc/test/libshareFun.so (0x00007fd57b596000)
        libc.so.6 => /lib64/libc.so.6 (0x00007fd57b1c8000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007fd57afc4000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fd57b798000)

修改完后，新增内容 “111”，查看链接信息的确变化了。

[root@VM-16-6-centos test]# vi shareFun.c
[root@VM-16-6-centos test]# gcc shareFun.c -shared -fPIC -o libshareFun.so
[root@VM-16-6-centos test]# ./hello1
Hello, World!
Test111 Here is a shared library!
[root@VM-16-6-centos test]# ldd hello1
        linux-vdso.so.1 =>  (0x00007ffe2e427000)
        /$LIB/libonion.so => /lib64/libonion.so (0x00007f11fdad2000)
        libshareFun.so => /home/klc/test/libshareFun.so (0x00007f11fd7b7000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f11fd3e9000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007f11fd1e5000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f11fd9b9000)

动态库和静态库比较

静态库

静态库是程序在编译链接时把库的代码复制到可执行文件中，生成的可执行程序在运行时将不再需要静态库，因此使用静态库生成的可执行程序的大小一般比较大。

• 优点
  • 使用静态库生成可执行程序后，该可执行程序就可以独自运行，不再需要库了。
• 缺点
  • 使用静态库生成可执行程序会占用大量空间，特别是当有多个静态程序同时加载而这些静态程序使用的都是相同的库，这时在内存当中就会存在大量的重复代码。

动态库

动态库是程序在运行时才去链接相应的动态库代码的，多个程序共享使用库的代码。一个与动态库链接的可执行文件仅仅包含它用到的函数入口地址的一个表，而不是外部函数所在目标文件的整个机器码。

在可执行文件开始运行前，外部函数的机器码由操作系统从磁盘上的该动态库中复制到内存中，这个过程称为动态链接。动态库在多个程序间共享，节省了磁盘空间，操作系统采用虚拟内存机制允许物理内存中的一份动态库被要用到该库的所有进程共用，节省了内存和磁盘空间。

• 优点
  • 节省磁盘空间，且多个用到相同动态库的程序同时运行时，库文件会通过进程地址空间进行共享，内存当中不会存在重复代码。
  • 加载器在加载动态库时，操作系统会先检查动态库是否因为其它程序已经将这个动态库信息加载到了内存中。如果没有加载到内存中，操作系统会将动态库载入内存，并将它的引用计数设置为1;如果已经加载到内存，仅将动态库的引用计数加1。
• 缺点
  • 必须依赖动态库，否则无法运行。