Question

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*

Answer 1

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.