【Linux系统编程】（二十七）手撕动静态库原理与实战：从底层逻辑到代码落地

# 查看C标准动态库
ls -l /lib/x86_64-linux-gnu/libc-2.31.so
# 输出：-rwxr-xr-x 1 root root 2029592 May 1 02:20 /lib/x86_64-linux-gnu/libc-2.31.so

# 查看C标准静态库
ls -l /lib/x86_64-linux-gnu/libc.a
# 输出：-rw-r--r-- 1 root root 5747594 May 1 02:20 /lib/x86_64-linux-gnu/libc.a

ls -l /usr/lib/gcc/x86_64-linux-gnu/9/libstdc++.so
# 输出：lrwxrwxrwx 1 root root 40 Oct 24 2022 /usr/lib/gcc/x86_64-linux-gnu/9/libstdc++.so -> ../../../x86_64-linux-gnu/libstdc++.so.6

#pragma once

#define SIZE 1024
#define FLUSH_NONE 0
#define FLUSH_LINE 1
#define FLUSH_FULL 2

// 模拟标准库的FILE结构体
struct IO_FILE
{
    int flag;      // 刷新模式
    int fileno;    // 文件描述符
    char outbuffer[SIZE]; // 输出缓冲区
    int cap;       // 缓冲区容量
    int size;      // 缓冲区已使用大小
};

typedef struct IO_FILE mFILE;

// 函数声明
mFILE *mfopen(const char *filename, const char *mode);
int mfwrite(const void *ptr, int num, mFILE *stream);
void mfflush(mFILE *stream);
void mfclose(mFILE *stream);

#include "my_stdio.h"
#include <string.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>

// 打开文件，模拟fopen
mFILE *mfopen(const char *filename, const char *mode)
{
    int fd = -1;
    if (strcmp(mode, "r") == 0)
    {
        fd = open(filename, O_RDONLY);
    }
    else if (strcmp(mode, "w") == 0)
    {
        fd = open(filename, O_CREAT | O_WRONLY | O_TRUNC, 0666);
    }
    else if (strcmp(mode, "a") == 0)
    {
        fd = open(filename, O_CREAT | O_WRONLY | O_APPEND, 0666);
    }

    if (fd < 0) return NULL;

    mFILE *mf = (mFILE *)malloc(sizeof(mFILE));
    if (!mf)
    {
        close(fd);
        return NULL;
    }

    mf->fileno = fd;
    mf->flag = FLUSH_LINE;
    mf->size = 0;
    mf->cap = SIZE;

    return mf;
}

// 刷新缓冲区，模拟fflush
void mfflush(mFILE *stream)
{
    if (stream->size > 0)
    {
        write(stream->fileno, stream->outbuffer, stream->size);
        fsync(stream->fileno); // 强制写入磁盘
        stream->size = 0;
    }
}

// 写入数据，模拟fwrite
int mfwrite(const void *ptr, int num, mFILE *stream)
{
    // 数据拷贝到缓冲区
    memcpy(stream->outbuffer + stream->size, ptr, num);
    stream->size += num;

    // 行缓冲：遇到换行符刷新
    if (stream->flag == FLUSH_LINE && stream->size > 0 && stream->outbuffer[stream->size - 1] == '\n')
    {
        mfflush(stream);
    }

    return num;
}

// 关闭文件，模拟fclose
void mfclose(mFILE *stream)
{
    if (stream->size > 0)
    {
        mfflush(stream);
    }
    close(stream->fileno);
    free(stream);
}

#include "my_string.h"

// 计算字符串长度
int my_strlen(const char *s)
{
    const char *end = s;
    while (*end != '\0') end++;
    return end - s;
}

#pragma once

int my_strlen(const char *s);

# 目标：静态库libmystdio.a
libmystdio.a: my_stdio.o my_string.o
	@ar -rc $@ $^
	@echo "build $^ to $@ ... done"

# 编译生成.o文件（-c表示只编译不链接）
%.o: %.c
	@gcc -c $<
	@echo "compling $< to $@ ... done"

# 清理目标文件和库文件
.PHONY: clean
clean:
	@rm -rf *.a *.o stdc*
	@echo "clean ... done"

# 打包头文件和库文件，方便分发
.PHONY: output
output:
	@mkdir -p stdc/include
	@mkdir -p stdc/lib
	@cp -f *.h stdc/include
	@cp -f *.a stdc/lib
	@tar -czf stdc.tgz stdc
	@echo "output stdc ... done"

make
# 输出：
# compling my_stdio.c to my_stdio.o ... done
# compling my_string.c to my_string.o ... done
# build my_stdio.o my_string.o to libmystdio.a ... done

#include "my_stdio.h"
#include "my_string.h"
#include <stdio.h>

int main()
{
    const char *s = "hello static library!\n";
    // 测试my_strlen
    printf("%s: length = %d\n", s, my_strlen(s));

    // 测试文件写入
    mFILE *fp = mfopen("./log.txt", "a");
    if (fp == NULL) return 1;

    mfwrite(s, my_strlen(s), fp);
    mfwrite(s, my_strlen(s), fp);
    mfwrite(s, my_strlen(s), fp);

    mfclose(fp);
    return 0;
}

gcc main.c -lmystdio -o main

gcc main.c -L. -lmystdio -o main

gcc main.c -I./include -L./lib -lmystdio -o main

rm -f libmystdio.a
./main
# 输出：
# hello static library!
# : length = 22
cat log.txt
# 输出3行hello static library!

# 目标：动态库libmystdio.so
libmystdio.so: my_stdio.o my_string.o
	gcc -o $@ $^ -shared
	@echo "build $^ to $@ ... done"

# 编译生成位置无关的.o文件
%.o: %.c
	gcc -fPIC -c $<
	@echo "compling $< to $@ ... done"

# 清理
.PHONY: clean
clean:
	@rm -rf *.so *.o stdc*
	@echo "clean ... done"

# 打包分发
.PHONY: output
output:
	@mkdir -p stdc/include
	@mkdir -p stdc/lib
	@cp -f *.h stdc/include
	@cp -f *.so stdc/lib
	@tar -czf stdc.tgz stdc
	@echo "output stdc ... done"

make
# 输出：
# compling my_stdio.c to my_stdio.o ... done
# compling my_string.c to my_string.o ... done
# build my_stdio.o my_string.o to libmystdio.so ... done

gcc main.c -I. -L. -lmystdio -o main

./main
# 输出：
# ./main: error while loading shared libraries: libmystdio.so: cannot open shared object file: No such file or directory

sudo cp libmystdio.so /usr/lib
./main
# 正常运行

export LD_LIBRARY_PATH=./:$LD_LIBRARY_PATH
./main
# 正常运行

sudo ln -s $(pwd)/libmystdio.so /usr/lib/libmystdio.so

sudo echo $(pwd) > /etc/ld.so.conf.d/mystdio.conf
sudo ldconfig # 更新缓存

ldd main
# 输出：
# linux-vdso.so.1 =>  (0x00007fffacbbf000)
# libmystdio.so => ./libmystdio.so (0x00007f8917335000)
# libc.so.6 => /lib64/libc.so.6 (0x00007f8916f67000)
# /lib64/ld-linux-x86-64.so.2 (0x00007f8917905000)

# CentOS
sudo yum install -y ncurses-devel
# Ubuntu
sudo apt install -y libncurses-dev