在Linux系统下对文件的读函数

前面我们了解了write函数，这次我们了解下read函数

老样子，现在命令终端输入以下命令：

man 2 read

#include 

ssize_t read(int fd, void *buf, size_t count);

//说明；读多少个字节（读size_t count个）放到buf中

    

基于前面的写入函数的代码进行修改：

#include 
#include 
#include 
#include 
#include 
#include 
#include 
int main()
{
    int fd;
    char *buf = "I love China !";
    fd = open("./file1",O_RDWR);

    if(fd == -1)
    {
        printf("open file1 failedn");
        fd = open("./file1",O_RDWR|O_CREAT,0600);
        if(fd > 0 )
        {
            printf("creat file1 success !n");

        }
    }
    printf("fd = %d n",fd);

    int n_write = write(fd,buf,strlen(buf));
    if(n_write != -1)
    {
        printf("write %d to filen",n_write);
    }

    char *readBuf;
    readBuf =(char *)malloc(sizeof (char)*n_write + 1);
//  ssize_t read(int fd, void *buf, size_t count);
    int n_read = read(fd,readBuf,n_write);
    printf("read %d ,context %s n",n_read,readBuf);


    close(fd);



    return 0;
}

会看到read 0 ,context

为什么？

因为我们写完后光标在末尾，读的是光标末尾后的数据，所以看不到任何数据

那怎么办呢？我们写完后将文件关闭，然后再打开，这样，光标就在开头了。代码修改如下：

#include 
#include 
#include 
#include 
#include 
#include 
#include 
int main()
{
    int fd;
    char *buf = "I love China !";
    fd = open("./file1",O_RDWR);

    if(fd == -1)
    {
        printf("open file1 failedn");
        fd = open("./file1",O_RDWR|O_CREAT,0600);
        if(fd > 0 )
        {
            printf("creat file1 success !n");

        }
    }
    printf("fd = %d n",fd);

    int n_write = write(fd,buf,strlen(buf));
    if(n_write != -1)
    {
        printf("write %d to filen",n_write);
    }
    close(fd);//写完后关闭文件
    open("./file1",O_RDWR);关闭文件后重新打开，让光标在开头
    char *readBuf;
    readBuf =(char *)malloc(sizeof (char)*n_write + 1);
//  ssize_t read(int fd, void *buf, size_t count);
    int n_read = read(fd,readBuf,n_write);
    printf("read %d ,context %s n",n_read,readBuf);


    close(fd);


    return 0;
}