I am trying to create a write only file in C on Linux (Ubuntu). This is my code:
int fd2 = open ("/tmp/test.svg", O_RDWR|O_CREAT);
if (fd2 != -1) {
//....
}
But why do the files I created have 'xr' mode? How can I create it so that I can open it myself at command prompt?
------xr-- 1 michael michael 55788 2010-03-06 21:57 test.txt*
------xr-- 1 michael michael 9703 2010-03-06 22:41 test.svg*
You need the three-argument form of open() when you specify O_CREAT. When you omit the third argument, open() uses whatever value happens to be on the stack where the third argument was expected; this is seldom a coherent set of permissions (in your example, it appears that decimal 12 = octal 014 was on the stack).
The third argument is the permissions on the file - which will be modified by the umask() value.
int fd2 = open("/tmp/test.svg", O_RDWR | O_CREAT, S_IRUSR | S_IRGRP | S_IROTH);
Note that you can create a file without write permissions (to anyone else, or any other process) while still being able to write to it from the current process. There is seldom a need to use execute bits on files created from a program - unless you are writing a compiler (and '.svg' files are not normally executables!).
The S_xxxx flags come from <sys/stat.h> and <fcntl.h> — you can use either header to get the information (but open() itself is declared in <fcntl.h>).
Note that the fixed file name and the absence of protective options such as O_EXCL make even the revised open() call somewhat unsafe.
open.
ln -s /etc/passwd /tmp/test.svg. Now one of two things is going to happen when your program runs: (1) you get an open error because you can't write to /etc/passwd (you aren't root), or (2) you get a copy of your SVG image in /etc/passwd because you are root. Neither is normally considered desirable. And Mr Malicious can also do: ln -s /usr/lib/security/libmalpam.so /tmp/test.svg...
fopen is a buffered C Library "high-level" function, versus the open which is an OS syscall (man 2 intro). See Secure Programming for Linux and Unix HOWTO dwheeler.com/secure-programs for details about secure programming.Give access permissions as the third parameter:
int fd2 = open("/tmp/test.svg", O_RDWR|O_CREAT, 0777); // Originally 777 (see comments)
if (fd2 != -1) {
// use file descriptor
close(fd2);
}
By doing this, all read, write and execute permissions will be given to user, group and others. Modify the 3rd parameter according to your use.
S_IRWXU | S_IRWXG | S_IRWXO that provides user, group and others with read, write and execute access on a file. So, the previous comment that "0x777 does happen to be the same" is erroneous. It corresponds to: r-xrwSrwt (SGID bit set, SVTX bit set, plus 0567 permissions, and I don't have the patience to write that out symbolically — it requires 7 terms or'd together).
mkstemp()' or a relative to create the file name. And the list goes on, no doubt.