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At some point hitherto, Jerry Feldman hath spake thusly:
> I do not recommend using flock(2). 

I concur with Jerry.  And I do so because W. Richard Stevens gives the
same advice.  Basically, implementations of flock are non-standard,
often buggy, and often just plain unavailable.  Locking with fcntl() is
standardised in POSIX.1 and available on virtually all modern Unix
systems.

> I'm not 100% sure of this in Linux, but flock(2) is an advisory lock, so 
> that anyone can modify the file regardless of the lock state. 

On Linux, flock is an advisory lock, but fcntl also is an advisory
lock (usually).  You *can* implement mandatory (i.e. kernel-enforced)
locking.  SVr3 introduced mandatory locking using fcntl by mucking
about with permission bits.  Linux implements this same mandatory
locking mechanism[1].   However, mandatory locking too is not with out
it's own problems[2]... 

> I strongly recommend that you use fcntl(2) as your lock mechanism as it is 
> semanticly and syntacticly standard across platforms. It locks across NFS. 
> You can also lock a portion of a file.

While locking using fcntl() is generally superior to locking via
flock() and some other methods, it is worth noting that it too is not
without its problems.  Most of these are with older, buggy
implementations of lockd and statd, which AFAIK are mostly fixed on
more recent releases of U*ixes.

Basically, locking sucks.  Locking with fcntl() just sucks the least
on most platforms.

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footnotes:

1.  For details of the implementation, see the documentation in the
kernel source tree in <source>/Documentation/mandatory.txt

2. See W. Richard Stevens, _Advanced Programming in the Unix
Environment_, pp 376-8.  [If you write C code on Unix systems, you
really should read this book cover to cover. :) ]

- -- 
Derek Martin               ddm@pizzashack.org    
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