Why an open descriptor is not closed on program exit?

Question

I have a small program below on 2.6.16-rc3, which uses busy box (on jffs2 file system). If I run the program multiple times, it starts to fail second time onwards . When the program quits, the descriptors shall be auto closed and next time it shall start fresh , right ?

Why I am getting -1 some times? (Note - On my Fedora Linux PC , it works fine)

root@badge 07:29:32 ~ >touch Hello.txt
root@badge 07:29:37 ~ >./a.out
FP = 3
root@badge 07:29:38 ~ >./a.out
FP = -1
root@badge 07:29:40 ~ >./a.out
FP = 3
root@badge 07:29:41 ~ >./a.out
FP = -1
root@badge 07:29:42 ~ >./a.out
FP = 3
root@badge 07:29:43 ~ >./a.out
FP = 3
root@badge 07:29:43 ~ >./a.out
FP = -1
root@badge 07:29:45 ~ >

Program:

#include <stdio.h>
int main()
{
        int fp;
        fp = open ("Hello.txt");
        printf("FP = %d\n", fp);
        return 0;  // No close() is used. On exit, it shall be closed.
}

Text File:

    -rw-r--r--    1 root     root            0 Sep 20 07:22 Hello.txt

Answer 1

You're not following the contract of the open() call. The man page (on Linux) states this:

SYNOPSIS
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>

       int open(const char *pathname, int flags);
       int open(const char *pathname, int flags, mode_t mode);

       int creat(const char *pathname, mode_t mode);

As you can see, you're forgetting to include the right headers, and open() also takes a flags parameter where you would state read/write rights, etc. As this is laid out you'll get a mystery argument passed to open(), this is whatever was on the stack or in the registers at the time.