Linux应用编程之文件IO open 学习之O_TRUNC 标志_o_trnuc

O_TRUNC含义：

        当文件存在时，将文件内容长度制定为0，与文件写入模式一起使用，可以达到覆盖写入的作用！

一下面这段文件复制的C代码为例：

//copy.c 复制文件
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include<stdio.h>
#include <unistd.h>
int main(int argc,char** arg)
{
        int file_old;
        int file_new;
        char rbuff[1024];
        int len;
        //argument num isn't enougth
        if(argc<3)
        {
          printf("usage: %s <old_filepath>  <new_filepath>\n",arg[0]);
          return -1;
        }
        // step 1 open new file if successful will return a file ID("fd") else return -1
        file_old = open(arg[1],O_RDONLY);
        if(file_old<0)
        {
           printf("can't open file : %s\n",arg[1]);
           return -1;
        }
        //step 2 create or open new file
        //if want to write or read new file ,we need config its access mode
        file_new = open(arg[2],O_RDWR|O_CREAT|O_TRUNC,00664);//00 XXX与chmod xxx的权限类似
        if(file_new<0)
        {
          printf("can't create or open new file : %s\n",arg[2]);
        }
        //step 3 read old file and write to new file
        while((len=read(file_old,rbuff,1024))>0)//read old file
        {
          if(write(file_new,rbuff,len)!=len)
          {
             printf("can not write %s\n",arg[2]);
          } //write to new file
        }
        //step 4  close file ,use " open" must use "close"
        close(file_old);
        close(file_new);
        return 0;
 
}

将代码放到Linux系统下的文件目录，在终端通过gcc编译指令生成可执行应用文件copy：

gcc -o copy copy.c

随后执行：

./copy copy.c copy2.c

此时，copy.c的内容将被复制到copy2.c当中，copy2.c的内容如下：

//copy from copy.c
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include<stdio.h>
#include <unistd.h>
int main(int argc,char** arg)
{
        int file_old;
        int file_new;
        char rbuff[1024];
        int len;
        //argument num isn't enougth
        if(argc<3)
        {
          printf("usage: %s <old_filepath>  <new_filepath>\n",arg[0]);
          return -1;
        }
        // step 1 open new file if successful will return a file ID("fd") else return -1
        file_old = open(arg[1],O_RDONLY);
        if(file_old<0)
        {
           printf("can't open file : %s\n",arg[1]);
           return -1;
        }
        //step 2 create or open new file
        //if want to write or read new file ,we need config its access mode
        file_new = open(arg[2],O_RDWR|O_CREAT|O_TRUNC,00664);//00 XXX与chmod xxx的权限类似
        if(file_new<0)
        {
          printf("can't create or open new file : %s\n",arg[2]);
        }
        //step 3 read old file and write to new file
        while((len=read(file_old,rbuff,1024))>0)//read old file
        {
          if(write(file_new,rbuff,len)!=len)
          {
             printf("can not write %s\n",arg[2]);
          } //write to new file
        }
        //step 4  close file ,use " open" must use "close"
        close(file_old);
        close(file_new);
        return 0;
 
}

假设我们此时有一个1.txt文件，其内容为：

HELLO WORLD,I AM 1.txt

再次执行：

./copy 1.txt copy2.c

copy2.c的内容将被覆盖为

Hello World,I am 1.txt

 若无添加O_TRUNC选项，则只会覆盖一部分文件内容，此时，copy2.c的内容如下：

 总结：

        O_TRUNC可以在打开文件的时候，将文件的内容长度设置为0，并于文件写入一同使用，达到覆盖写入的效果。

题外：若要实现追加写入，则使用选项O_APPEND