File I/O in multi user environments.

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carydbAsked:
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jhanceConnect With a Mentor Commented:
If you are needing to write files un a multi-process or multi-user scenario, you must open the file and then use the flock() function on it before doing any write operation.  If you try and flock() a file that is already "flocked" by another process, your call will hang until the flock is removed by the other process.
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carydbAuthor Commented:
That's exactly what I was looking for. Only problem is that "flock()" is not documented in my C books. Could you provide a syntax: Thanks.
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jhanceCommented:

FLOCK(2)            Linux Programmer's Manual            FLOCK(2)

NAME
       flock - apply or remove an advisory lock on an open file

SYNOPSIS
       #include <sys/file.h>

       int flock(int fd, int operation)

DESCRIPTION
       Apply  or  remove  an  advisory lock on an open file.  The file is specified by fd.  Valid operations are given
       below:

              LOCK_SH   Shared lock.  More than one process may hold a shared lock for a given file at a given time.

              LOCK_EX   Exclusive lock.  Only one process may hold an exclusive lock for a given file at a given time.

              LOCK_UN   Unlock.

              LOCK_NB   Don't  block  when  locking.   May be specified (by or'ing) along with one of the other opera-
                        tions.

       A single file may not simultaneously have both shared and exclusive locks.

       A file is locked (i.e., the inode), not the file descriptor.  So, dup(2) and fork(2)  do  not  create  multiple
       instances of a lock.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       EWOULDBLOCK
              The file is locked an the LOCK_NB flag was selected.

NOTES
       Under Linux, flock is implemented as a call to fcntl.  Please see fcntl(2) for more details on errors.

SEE ALSO
       open(2), close(2), dup(2), execve(2), fcntl(2), fork(2),
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