This page describes some programming hints that might help you to write more portable C code.

File Handling

File locking

BSD-style unixes uses flock(2), which uses "advisory" locks. Ie, a process with sufficient read or write permission can ignore the lock and read/write a file. SysV-style unixes use either advisory locks or "mandatory" locks (if enabled in the filesystem), and access them by the fcntl(2) command and a struct flock object.

A more portable way (POSIX 1003.1-2001) is to use the lockf(3) function from unistd.h, which will do the correct type of locking for the unix it is compiled on. (In Linux/GNU libc, this function is a wrapper around fcntl(2)).

printf types

64bitisms and fixed sized types

If you have a fixed sized type (eg, uint64_t) and you want to use printf to display it, you need to know the real type of the integer. eg:

 uint64_t x;
 printf("%llu",x);

This however will fail on 64bit machines as uint64_t is "long" not "long long".

POSIX defines some macros to use in this case, normally found in inttypes.h. Eg, for an unsigned 64 bit value, use the macro PRIu64. This will be substituted for whatever is appropriate on your host - either lu or llu, depending on if you are on a 32bit or 64bit host.eg:

 uint64_t x;
 printf("%"PRIu64,x);

Other pages:

• HP has a set of C portability notes online, although it targets HP-UX.
• Intel has a similar set of notes online (for porting to ia64 and amd64)

Part of CategoryProgramming