Question

I am writing Linux ANSI C cgi-bin server program with simultaneous access to files.

1. Is it possible to distinguish between file existence and file locking?
   I can’t find the answer with Google.
   I’d like to write a program which tries to open file for a few seconds if fd<0
   (thinking that the file is locked for a while).
   But if the file does not exist it’s fd also <0. So the program will waste time waiting.

2. Suppose a few threads try to append to the same file with no locking.
   One tries to add “AAAA”, another – “BBBB”.
   Can the result file be like “AABBAABB”?
   Or it will always be like AAAABBBB or BBBBAAAA?
   Or the result is unpredictable?

Answer 1

Am assuming IEEE Std 1003.1-2001 might defer to the ISO C standard…

In case the fopen fails i.e. fd < 0 then the system sets some error codes…
you can check those error codes. In case of file non-existent, the returned error would be

ENOENT
A component of filename does not name an existing file or filename is an empty string.

For more reference visit:
http://pubs.opengroup.org/onlinepubs/009695399/functions/fopen.html

For point 2: I have been doing logging of certain data in my system by more than 100 processes writing to single file simultaneously but have never seen a merger of records(file is always opened in append mode). i.e. its always like AAAABBBB