九、文件IO——案例构建标准库

例子如下：

　　mystdio.h

#ifndef __MYSTDIO_H__
#define __MYSTDIO_H__

#include <sys/types.h>

#define MYEOF -1

enum mode{READ, WRITE, APPEND};

typedef struct {
    int     _fd;
    char     *_buff;
    char     *_nextc;
    int     _mode;
    off_t     _left;
}MYFILE;

extern MYFILE * myfopen(const char * const pathname, const char * const mode);

extern int myfclose(MYFILE *fp);

extern void myfflush(MYFILE *fp);

extern MYFILE* myfdopen(int fd, const char * const mode);

extern int myfgetc(MYFILE *fp);

extern int myfputc(int character, MYFILE *fp);

#endif

　　mystdio.c

#include "mystdio.h"
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <assert.h>
#include <malloc.h>
#include <memory.h>

#define BUFFER_LEN         4096

MYFILE * myfopen(const char * const pathname, const char * const mode)
{
    int fd;
    if(!strcmp(mode, "r")) {
        fd = open(pathname, O_RDONLY);
    } else if(!strcmp(mode, "w")) {
        fd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC, 0777);
    } else if(!strcmp(mode, "a")) {
        fd = open(pathname, O_WRONLY | O_CREAT | O_APPEND, 0777);
    } else { 
        return NULL;
    }

    if(fd < 0)
        return NULL;

    return myfdopen(fd, mode);
}

int myfclose(MYFILE *fp)
{
    myfflush(fp);
    int ret = close(fp->_fd);
    free(fp->_buff);
    free(fp);
    return ret;
}

void myfflush(MYFILE *fp)
{
    if(fp->_mode == READ) {
        fp->_nextc = fp->_buff;//指向缓存开始地址
        fp->_left = 0;
    } else {// write or append
        write(fp->_fd, fp->_buff,(BUFFER_LEN - fp->_left));
        fp->_nextc = fp->_buff;
        fp->_left = BUFFER_LEN;
    }
}

MYFILE* myfdopen(int fd, const char * const mode)
{
    MYFILE *fp = (MYFILE *)malloc(sizeof(MYFILE));//堆中创建fp
    assert(fp != NULL);//断言，即保证fp不能为空，为空就在这里跳出

    fp->_buff = (char *)malloc(BUFFER_LEN);
    assert(fp->_buff != NULL);
    fp->_fd = fd;
    fp->_nextc = fp->_buff;//nextc 指向缓存中第一个字节
    if(!strcmp(mode, "r")) {
        fp->_mode = READ;
        fp->_left = 0;//缓存没有任何数据可读
    }

    if(!strcmp(mode, "w")) {
        fp->_mode = WRITE;
        fp->_left = BUFFER_LEN;//缓存的大小长度
    }

    if(!strcmp(mode, "w")) {
        fp->_mode = APPEND;
        fp->_left = BUFFER_LEN;
    }

    return fp;
}

int myfgetc(MYFILE *fp)
{
    assert(fp->_mode == READ);

    //当缓存中的数据已经读取完毕，再从文件中读取一批新的数据放入到缓存当中
    if(fp->_left == 0) {
        ssize_t size = read(fp->_fd, fp->_buff, BUFFER_LEN);
        assert(size >= 0);
        if(size == 0) return MYEOF;
        fp->_nextc = fp->_buff;
        fp->_left = size;
    }

    char c = *(fp->_nextc);
    fp->_nextc++;
    fp->_left--;

    return c;
}

int myfputc(int character, MYFILE *fp)
{
    assert(fp->_mode == WRITE || fp->_mode == APPEND);

    //若缓存满，将缓存中的数据写入到文件中
    if(fp->_left == 0) {
        if(write(fp->_fd, fp->_buff, BUFFER_LEN) != BUFFER_LEN) {
            return 0;
        }

        fp->_nextc = fp->_buff;
        fp->_left = BUFFER_LEN;
    }

    // 将字符写入到缓存制定的位置
    *(fp->_nextc) = (char)character;
    fp->_nextc++;
    fp->_left--;

    return 1;
}

　　mystdio_test.c

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <assert.h>
#include "mystdio.h"

int main(int argc, const char *argv[])
{
    MYFILE *fp1 = myfopen("/etc/passwd", "r");
    assert(fp1 != NULL);
    MYFILE *fp2 = myfopen("mypasswd", "w");
    assert(fp2 != NULL);

    char c;
    while((c = myfgetc(fp1)) != MYEOF)
    {
        myfputc(c, fp2);
    }

    myfclose(fp1);
    myfclose(fp2);
    return 0;
}

　　编译调试：