在Unix中“创建”系统调用

Question

I am using creat system call to creat a file. The following is the program to creat a file

#include<stdio.h>
#include<fcntl.h>
void main()
{
    int fd=creat("a.txt",S_IRWXU|S_IWUSR|S_IRGRP|S_IROTH);
    printf("fd  = %d\n",fd);
}

So, At first time, the program creates a file named a.txt with appropriate permission. If I execute a.out one more time, the new a.txt will be created. But, the inode of the file remains same. How, it will be.

$ ./a.out
fd  = 3
$ ls -li a.txt
2444 -rw-r--r-- 1 mohanraj mohanraj 0 Aug 27 15:02 a.txt
$ cat>a.txt
this is file a.txt
$ ./a.out
fd  = 3
$ cat a.txt
$ls -li a.txt
2444 -rw-r--r-- 1 mohanraj mohanraj 0 Aug 27 15:02 a.txt
$

In the above output, a.txt have the content as "This is file a.txt". Once I execute the a.out, the new a.txt created. But, the inode 2444 remains same. So, how creat system call works?

Thanks in advance.

Answer 1

creat only creates a file if it doesn't exist. If it already exists, it's just truncated.

creat(filename, mode);

is equivalent to

open(filename, O_WRONLY|O_CREAT|O_TRUNC, mode);

And as specified in the open(2) documentation:

O_CREAT
If the file does not exist it will be created.