chmod can't give all permissions

Question

I am trying to change file permissions using chmod function in C program

chmod("/tmp/toBoard", S_IRWXU | S_IRWXG | S_IRWXO);

But after program run I check permissions and get only

-rwxr-xr-x 1 root root

I run this program on Linux embedded board. toBoard is a file copied inside the program from other file from /var directory, source file has all permissions(set manually from terminal). When I tried to copy it manually and set permissions it worked, but when I copy the file and try give it all permissions - it fails without errors

copy("/var/www/defaults.dat", "/tmp/toBoard");
int err;
if(err = chmod("/tmp/toBoard", S_IRWXU | S_IRWXG | S_IRWXO)){
    perror("chmod");
}
struct stat buffer;
int status = stat("/tmp/toBoard", &buffer);

How can I set all permissions to green light?

You have to run it as the owner, in this case root. But you shouldn't use root unless you really have to, and certainly not for experimenting like this. — Kevin, Jan 15 '14 at 14:50
this works as given for me. Have to ``#include `` to get the S_IRWXU etc constants — Vorsprung, Jan 15 '14 at 14:59
Running as root. I need this to be able to rewrite this file from webpage, but it has no permissions. Vorsprung, contains all constants needed, thoug I tried your advice and it didn't help either — PaulPonomarev, Jan 15 '14 at 15:13

score 8 · Answer 1 · answered Jan 15 '14 at 14:51

8

As it stands, your call is correct but it is probably failing, and you're not checking the returned code. You could try:

if (chmod("/tmp/toBoard", S_IRWXU | S_IRWXG | S_IRWXO)) {
    perror("chmod");
    /* more error handling. */
}

In this instance it is likely a case of "Permission denied".

answered Jan 15 '14 at 14:51

cnicutar

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@PaulPonomarev Is `/tmp/` mounted in some funny way ? What happens if you try a different file ? – cnicutar Jan 15 '14 at 16:05
I have edited the main question to answer your question. /tmp is a folder on the board, that is emptied after reboot – PaulPonomarev Jan 16 '14 at 10:32
@PaulPonomarev So if you go to a terminal and use the straight `chmod 777 /tmp/toBoard`, then what happens ? – cnicutar Jan 16 '14 at 10:35
All permissions `-rwxrwxrwx` – PaulPonomarev Jan 16 '14 at 10:55
@PaulPonomarev Okay, do you have `strace` ? If so do a `strace chmod ... ` and see how it calls the `chmod` syscall. – cnicutar Jan 16 '14 at 10:57
If I understood you right you were asking for this: `execve("/bin/chmod", ["chmod", "..."], [/* 19 vars */]) = 0` – PaulPonomarev Jan 16 '14 at 11:05
@PaulPonomarev Great! Look further down, it needs to do `chmod` or `fchmod` at some point. That's the line we're looking for. – cnicutar Jan 16 '14 at 11:06
no chmod or fchmod functions found, only open, close, write, read access, mmap2, mprotect, munmap, stat64, fstat64 and set_tls – PaulPonomarev Jan 16 '14 at 11:12
@PaulPonomarev That's very odd. What system is this ? – cnicutar Jan 16 '14 at 11:33
(Embedded) Linux version 3.2.0-PD13.1.0 – PaulPonomarev Jan 16 '14 at 11:37
Whatever your chmod utility is, it's source is probably available. – Chris Stratton Jan 16 '14 at 15:38

chmod can't give all permissions

1 Answers1