umask not working in c code

Question

I have made a c code intended to create a file at root directory.

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>

const char *path="/";
int main(){
    FILE *fp;
    umask(0);
    chdir(path);

    fp=fopen("test.txt","w+");
    fclose(fp);

    return 0;
}

the compilation gives no errors but when I execute the file, the following error appears:

kwagjj@kwagjj-Inspiron-3420:~$ gcc -Wall a2.c -o a2
kwagjj@kwagjj-Inspiron-3420:~$ ./a2
Segmentation fault (core dumped)

how am I using the umask function wrong?

How do you know the segmentation fault is caused by `umask`? — Yu Hao, Sep 06 '14 at 13:01
@YuHao uh, your right... I guess the error is basically not because of `umask` but I was assuming that the problem is eventually related to `umask` because I intended `umask` to give the proper permission to fopen and fclose — kwagjj, Sep 06 '14 at 13:08
You/your program (probably) does not have permission to create files in the "/" (root) directory. So, what are the permission for "/" and is your program being executed via 'sudo'? — user3629249, Sep 07 '14 at 18:48

score 2 · Accepted Answer · edited May 23 '17 at 12:21

2

My money is on fopen() failing (probably due to insufficient permissions) and returning NULL, and fclose(NULL) causing the segfault.

edited May 23 '17 at 12:21

Community

1
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answered Sep 06 '14 at 13:03

NPE

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then how can I get the proper permission? I used the `umask` to get the permission but it doesn't seem to work – kwagjj Sep 06 '14 at 13:06
1

@kwagjj `umask` doesn't affect whether your program has the permission to do something. It affects the default permission bits when your program creates files. – Gilles 'SO- stop being evil' Sep 06 '14 at 13:19
@NPE thanks I understand what you are trying to say... I've moved on and am trying another way to get the job done – kwagjj Sep 06 '14 at 13:57

umask not working in c code

1 Answers1