"Syscall param open(mode) contains uninitialised byte(s)" in open sys call

Question

fd = open(pathname, O_WRONLY | O_LARGEFILE | O_APPEND | O_CREAT);

When running my tests I encounter a valgrind error:

==14280== Syscall param open(mode) contains uninitialised byte(s)
==14280==    at 0x4111084: open64 (open64.c:42)

And the return value of open is -2

Errno is set to 13 (Permission denied)

The pathname buffer contains:

0x68
0x72
0x32
0x2E
0x66
0x61
0x0 

Info about my system:

$ ls -la
drwxr-xr-x 7 rzetterberg zed 4096 Jul  4 13:56 .
$ id
uid=1000(rzetterberg) gid=1000(rzetterberg) groups=1000(rzetterberg), ... etc
$ sudo file -Ls /dev/sda1
/dev/sda1: sticky Linux rev 1.0 ext3 filesystem data, UUID=XXXX (needs journal recovery) (large files)
$ uname -a
Linux xxxx 3.2.0-4-686-pae #1 SMP Debian 3.2.46-1 i686 GNU/Linux

Headers included:

#include <stdio.h>
#include <stdint.h>
#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

I don't understand how mode can have unitialised bytes and why I'm getting a Permission denied. It seems very strange to me!

Answer 1

If you supply the O_CREAT flag to open(), you must supply a third argument. This is an integer which represents the permissions to apply to the created file (modified by the current umask).