程序员的自我修养 - 编译

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#include <stdio.h>

int main(){
    printf("Hello World!\n");
}

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$ gcc -E main.c -o main.i

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


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

# 3 "main.c" 2


# 4 "main.c"
int main(){
    printf("Hello World!\n");
}

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$ gcc -S main.i -o main.s

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    .file       "main.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: (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609"
    .section    .note.GNU-stack,"",@progbits

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$ gcc -c main.s -o main.o

$ nm main.o
0000000000000000 T main
                 U puts

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$ gcc -o main main.o

$ nm main
···
000000000040052d T main
                 U puts@@GLIBC_2.2.5
···

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$ ./main
Hello World!

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// addvec.h
void addvec(int* x, int* y, int*z, int n);

// addvec.cpp
#include "addvec.h"
void addvec(int* x, int* y, int*z, int n)
{
    int i=0;
    for(; i< n;++i)
        z[i] = x[i] + y[i];
}

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// mulvec.h
void multvec(int*x, int* y, int*  z, int n);

// mulvec.cpp
#include "mulvec.h"
void multvec(int*x, int* y, int*  z, int n)
{
    int i = 0;
    for(; i < n; ++i)
        z[i] = x[i] * y[i];
}

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// main.cpp
#include <iostream>
#include "addvec.h"

int x[2] = {1, 2};
int y[2] = {3, 4};
int z[2] = {0};

int main() {
    addvec(x, y, z, 2);
    std::cout << "z = [" << z[0] << " " << z[1] << "]\n" << std::endl;
    return 0;
}

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$ gcc -c addvec.cpp mulvec.cpp            # 产生.o重定位文件
$ ar r libstatic.a addvec.o  mulvec.o     # 生成静态库文件libstatic.a

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$ ar tv libstatic.a
rw-r--r-- 0/0   1344 Apr 19 09:29 2018 addvec.o
rw-r--r-- 0/0   1344 Apr 19 09:29 2018 mulvec.o

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$ ar d libstatic.a mulvec.o
$ ar tv libstatic.a
rw-r--r-- 0/0   1344 Apr 19 09:29 2018 addvec.o

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$ g++ -o main_static main.cpp ./libstatic.a
$ ./main_static
z = [4 6]

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$ g++ -o main_static main.cpp -L./lib -lstatic  # 从本地的lib目录下链接libstatic.a静态库
$ ./main_static
z = [4 6]

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$ g++ -o main_static main.cpp -L./lib -static -lstatic

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$ g++ -fPIC -Wall -c addvec.cpp mulvec.cpp
$ g++ -g -shared -o libdynamic.so addvec.o mulvec.o

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# 使用soname选项编译
$ g++ -g -shared -Wl,-soname,libdy.so -o libdynamic.so addvec.o mulvec.o

# 使用readelf查看动态库后，发现多了一个SONAME标签
$ readelf -d libdynamic.so

Dynamic section at offset 0xdd8 contains 28 entries:
  标记        类型                         名称/值
 0x0000000000000001 (NEEDED)             共享库:[libstdc++.so.6]
 0x0000000000000001 (NEEDED)             共享库:[libm.so.6]
 0x0000000000000001 (NEEDED)             共享库:[libgcc_s.so.1]
 0x0000000000000001 (NEEDED)             共享库:[libc.so.6]
 0x000000000000000e (SONAME)             Library soname: [libdy.so]
 ···

# 编译可执行程序，仍使用动态库的真是名称
$ g++ -g -Wall -o main_dynamic main.cpp ./lib/libdynamic.so

# 使用readelf查看可执行程序，依赖的共享库为libdy.so
$ readelf -d main_dynamic

Dynamic section at offset 0xde8 contains 28 entries:
  标记        类型                         名称/值
 0x0000000000000001 (NEEDED)             共享库:[libdy.so]
 0x0000000000000001 (NEEDED)             共享库:[libstdc++.so.6]
 0x0000000000000001 (NEEDED)             共享库:[libm.so.6]
 0x0000000000000001 (NEEDED)             共享库:[libgcc_s.so.1]
 0x0000000000000001 (NEEDED)             共享库:[libc.so.6]
 ···
  
# 调用动态库时，提示错误:libdy.so未找到
$ LD_LIBRARY_PATH=./lib/ ./main_dynamic 
./main_dynamic: error while loading shared libraries: libdy.so: cannot open shared object file: No such file or directory

# 需要软链接连接libdy.so至libdynamic.so
$ ln -s libdynamic.so libdy.so

# 再次运行
$ LD_LIBRARY_PATH=./lib/ ./main_dynamic
z = [4 6]

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$ g++ -g -Wall -o main_dynamic main.cpp libdynamic.so

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$ readelf -d  main_dynamic

Dynamic section at offset 0xde8 contains 28 entries:
  标记        类型                         名称/值
 0x0000000000000001 (NEEDED)             共享库:[libdynamic.so]
 0x0000000000000001 (NEEDED)             共享库:[libstdc++.so.6]
 0x0000000000000001 (NEEDED)             共享库:[libm.so.6]
 0x0000000000000001 (NEEDED)             共享库:[libgcc_s.so.1]
 0x0000000000000001 (NEEDED)             共享库:[libc.so.6]
 ···

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$ ./main_dynamic
./main_dynamic: error while loading shared libraries: libdynamic.so: cannot open shared object file: No such file or directory

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$ LD_LIBRARY_PATH=./lib ./main_dynamic 
z = [4 6]

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# 指定main_dynamic在执行时搜索./lib目录下的libdynamic.so
$ g++ -g -Wall -Wl,-rpath=./lib/ -o main_dynamic main.cpp ./lib/libdynamic.so

# 通过readelf命令可以发现，多了一个RPATH标签
$ readelf -d main_dynamic

Dynamic section at offset 0xdd8 contains 29 entries:
  标记        类型                         名称/值
 0x0000000000000001 (NEEDED)             共享库:[libdynamic.so]
 0x0000000000000001 (NEEDED)             共享库:[libstdc++.so.6]
 0x0000000000000001 (NEEDED)             共享库:[libm.so.6]
 0x0000000000000001 (NEEDED)             共享库:[libgcc_s.so.1]
 0x0000000000000001 (NEEDED)             共享库:[libc.so.6]
 0x000000000000000f (RPATH)              Library rpath: [./lib/]
···

# 再次调用main_dynamic，只要./lib目录下含有对应的动态库文件，则能够成功执行
$ ./main_dynamic
z = [4 6]

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#include <dlfcn.h>

void *dlopen(const char *libfilename, int mode);

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#include <dlfcn.h>

void *dlsym(void *handle, char *symbol);

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int *ip

ip = (int *)dlsym(handle, symbol);
if (ip != NULL)
    printf("Value is %d\n", *ip);

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int (*funcp)(int);  # 定义函数指针

*(void **)(&funcp) = dlsym(handle, symbol);    # 获取函数
res = (*funcp)(arg);        # 函数使用

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#include <dlfcn.h>

int dlclose(void *handle);

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#include <dlfcn.h>

const char *dlerror();

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# 使用nm命令查看动态库文件时，显示函数名附加了别的内容，所以dlsym函数无法通过"addvec"找到对应函数
$ nm libdynamic.so
···
0000000000000a50 T _Z6addvecPiS_S_i
0000000000000ac0 T _Z7multvecPiS_S_i
···

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// addvec.h
extern "C"
{
    void addvec(int *x, int *y, int *z, int n);
}

// addvec.cpp
#include "addvec.h"


extern "C" void addvec(int *x, int *y, int *z, int n) {
    int i = 0;
    for (; i < n; ++i)
        z[i] = x[i] + y[i];
}

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$ nm libdynamic.so
···
00000000000006c5 T addvec           # addvec的symbol为函数名
···
0000000000000735 T _Z7multvecPiS_S_i    # mulvec的symbol仍为之前
···

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// main.cpp
#include <dlfcn.h>
#include <string>
#include <iostream>

int x[2] = {1, 2};
int y[2] = {3, 4};
int z[2] = {0};

int main(int argc, char *argv[]) {

    void *dl_handle;
    float (*func)(int *, int *, int *, int);
    char *error;

    // 打开动态库文件，获取句柄
    dl_handle = dlopen("../libdynamic.so", RTLD_NOW);
    if (!dl_handle) {
        std::cout << "!!! " << dlerror() << std::endl;
        return 0;
    }

    // 根据函数名查找函数地址，C++需要注意name mangling的问题
    *(void **) (&func) = dlsym(dl_handle, "addvec");
    error = dlerror();
    if (error != nullptr) {
        std::cout << "!!! " << dlerror() << std::endl;
        return 0;
    }

    // 执行获取的函数
    (*func)(x, y, z, 2);
    std::cout << "z = [" << z[0] << " " << z[1] << "]\n" << std::endl;

    // 结束句柄
    dlclose(dl_handle);

    return 0;
}

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# 在addvec.h中添加初始化和终止函数声明
extern "C"
{
    void addvec(int *x, int *y, int *z, int n);
    void __attribute__((constructor)) x_init(void);
    void __attribute__((destructor)) x_fini(void);
}

# 在addvec.cpp中添加初始化和终止函数实现
#include "addvec.h"
#include <iostream>

extern "C" void addvec(int *x, int *y, int *z, int n) {
    int i = 0;
    for (; i < n; ++i)
        z[i] = x[i] + y[i];
}

extern "C" void __attribute__((constructor)) x_init(void) {
    std::cout << "void __attribute__((constructor)) x_init(void) execute!!" << std::endl;
}

extern "C" void __attribute__((destructor)) x_fini(void) {
    std::cout << "void __attribute__((constructor)) x_fini(void) execute!!" << std::endl;

}

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$ nm libdynamic.so
···
0000000000000a15 T addvec
0000000000000ae1 T x_fini
0000000000000a85 T x_init
0000000000000b9a T _Z7multvecPiS_S_i
···

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$ ./main_dynamic
void __attribute__((constructor)) x_init(void) execute!!
z = [4 6]

void __attribute__((constructor)) x_fini(void) execute!!

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$ldd libdynamic.so
linux-vdso.so.1 =>  (0x00007fff9b57b000)
libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007f9ec959a000)
libm.so.6 => /lib64/libm.so.6 (0x00007f9ec9298000)
libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007f9ec9081000)
libc.so.6 => /lib64/libc.so.6 (0x00007f9ec8cbe000)
/lib64/ld-linux-x86-64.so.2 (0x0000558c095f8000)

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# 显示可执行程序的头部信息
$ readelf -h main_dynamic
ELF 头:
  Magic:  7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (可执行文件)      # 根据不同类型的文件会显示不同的type
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  入口点地址:                        0x4009c6
  程序头起点:                        64 (bytes into file)
  Start of section headers:          27448 (bytes into file)
  标志:                              0x0
  本头的大小:                        64 (字节)
  程序头大小:                        56 (字节)
  Number of program headers:         9
  节头大小:                          64 (字节)
  节头数量:                          37
  字符串表索引节头:                   34

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# 显示程序头信息
$ readelf -l main_dynamic

Elf 文件类型为 EXEC (可执行文件)
入口点 0x4009c6
共有 9 个程序头，开始于偏移量64

程序头:
  Type           Offset             VirtAddr           PhysAddr           FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000400040 0x0000000000400040 0x00000000000001f8 0x00000000000001f8  R E    8
  INTERP         0x0000000000000238 0x0000000000400238 0x0000000000400238 0x000000000000001c 0x000000000000001c  R      1
  [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000 0x0000000000000cbc 0x0000000000000cbc  R E    200000
  LOAD           0x0000000000000db8 0x0000000000600db8 0x0000000000600db8 0x00000000000002d4 0x0000000000000408  RW     200000
  DYNAMIC        0x0000000000000dd8 0x0000000000600dd8 0x0000000000600dd8 0x0000000000000220 0x0000000000000220  RW     8
  NOTE           0x0000000000000254 0x0000000000400254 0x0000000000400254 0x0000000000000044 0x0000000000000044  R      4
  GNU_EH_FRAME   0x0000000000000b64 0x0000000000400b64 0x0000000000400b64 0x000000000000003c 0x000000000000003c  R      4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000  RW     10
  GNU_RELRO      0x0000000000000db8 0x0000000000600db8 0x0000000000600db8 0x0000000000000248 0x0000000000000248  R      1

 Section to Segment mapping:
  段节...
   00
   01     .interp
   02     .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame
   03     .init_array .fini_array .jcr .dynamic .got .got.plt .data .bss
   04     .dynamic
   05     .note.ABI-tag .note.gnu.build-id
   06     .eh_frame_hdr
   07
   08     .init_array .fini_array .jcr .dynamic .got

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# 显示段头信息
$ readelf -S main_dynamic
共有 37 个节头，从偏移量 0x6b38 开始:

节头:
  [Nr] Name              Type             Address           Offset       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000     0000000000000000  0000000000000000           0     0     0
  [ 1] .interp           PROGBITS         0000000000400238  00000238     000000000000001c  0000000000000000   A       0     0     1
  [ 2] .note.ABI-tag     NOTE             0000000000400254  00000254     0000000000000020  0000000000000000   A       0     0     4
  [ 3] .note.gnu.build-i NOTE             0000000000400274  00000274     0000000000000024  0000000000000000   A       0     0     4
  [ 4] .gnu.hash         GNU_HASH         0000000000400298  00000298     0000000000000028  0000000000000000   A       5     0     8
  [ 5] .dynsym           DYNSYM           00000000004002c0  000002c0     0000000000000198  0000000000000018   A       6     1     8
  [ 6] .dynstr           STRTAB           0000000000400458  00000458     00000000000001a4  0000000000000000   A       0     0     1
  [ 7] .gnu.version      VERSYM           00000000004005fc  000005fc     0000000000000022  0000000000000002   A       5     0     2
  ···

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$ nm main_dynamic
000000000060108c B __bss_start
00000000006011b0 b completed.6354
                 U __cxa_atexit@@GLIBC_2.2.5
0000000000601078 D __data_start
0000000000601078 W data_start
00000000004009f0 t deregister_tm_clones
                 U dlclose@@GLIBC_2.2.5
                 U dlerror@@GLIBC_2.2.5
                 U dlopen@@GLIBC_2.2.5
                 U dlsym@@GLIBC_2.2.5
0000000000400a60 t __do_global_dtors_aux 
···

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# ldconfig参数说明
$ ldconfig --help
用法: ldconfig [选项...]
Configure Dynamic Linker Run Time Bindings.

  -c, --format=FORMAT        采用的格式:新、旧或兼容(默认)
  -C CACHE                   将 CACHE 用作缓冲区文件
  -f CONF                    将 CONF 用作配置文件
  -i, --ignore-aux-cache     忽略辅助缓存文件
  -l                         手工连接独立的库。
  -n
                             只在命令行中给出了进程目录。未创建缓冲区。
  -N                         不要创建缓冲区
  -p, --print-cache          打印缓冲区
  -r ROOT                    进入 ROOT 目录并将其作为根目录
  -v, --verbose              生成详细消息
  -X                         不要生成连接
  -?, --help                 给出该系统求助列表
      --usage                给出简要的用法信息
  -V, --version              打印程序版本号

长选项的强制或可选参数对对应的短选项也是强制或可选的。