CODEWITHSUNDEEP
clinuxunixchmod
mohangraj
mohangraj

Reputation: 11044

fchmod function in C

Program:

#include<stdio.h>
#include<sys/stat.h>
#include<sys/types.h>
#include<fcntl.h>
void main()
{
    int fd=open("b.txt",O_RDONLY);
    fchmod(fd,S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH);
}

Output:

$ ls -l b.txt
----r----- 1 mohanraj mohanraj 0 Sep 12 15:09 b.txt
$ ./a.out
$ ls -l b.txt
----r----- 1 mohanraj mohanraj 0 Sep 12 15:09 b.txt  
$

For the above program, My expected output is to set the permission for b.txt as "rw_rw_r__". But, It still remains in the old permission. Why it will be like this. Is this code have any bug ?

Upvotes: 3

Views: 3017

Answers (4)

KunMing Xie
KunMing Xie

Reputation: 1667

$ ls -l b.txt ----r----- 1 mohanraj mohanraj 0 Sep 12 15:09 b.txt

----r-----, it means owner of b.txt has no Permission to read or write 

$chmod 644 b.txt //add read and write Permission, -rw-r--r--

also, change file permission need O_RDWR flag

make sure function success before continue

void main()
{
    int fd=open("b.txt",O_RDWR);
    if (fd > 0) {
       //do some thing
    }
}

Upvotes: 0

mohangraj
mohangraj

Reputation: 11044

For file b.txt I didn't set the read permission for owner. So, While we calling the open function, it does not have permission to open b.txt. So, it returns bad file descriptor error. So, it will be like this.

Program:

#include<stdio.h>
#include<sys/stat.h>
#include<sys/types.h>
#include<fcntl.h>
void main()
{
      int fd=open("b.txt",O_RDONLY);
      perror("open");
      fchmod(fd,S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH);
      perror("fchmod");
}

Output:

$ ./a.out 
open: Permission denied
fchmod: Bad file descriptor
$

Upvotes: 1

Krystian Sakowski
Krystian Sakowski

Reputation: 1653

First of all you should check if fd returned by open syscall is fine and you should check fchmod syscall status as well.

Secondly, I tested your example code and in my case it works as follows.

Before running your program:

pi@raspberrypi ~ $ ls -l hej.txt 
-rw-r--r-- 1 pi pi 0 Sep 12 11:53 hej.txt

After running your program:

pi@raspberrypi ~ $ ls -l hej.txt 
-rwxrwxrw- 1 pi pi 0 Sep 12 11:53 hej.txt

Your program might missing permission to this file.

Upvotes: 1

ouah
ouah

Reputation: 145829

You don't have permission to modify the file, call your program with sudo to have it succeed.

Also always check the return value of functions like open and fchmod and handle the errors .

Upvotes: 1

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