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LOCKF(3) Linux Programmer's Manual LOCKF(3)

NAME

     lockf - apply, test or remove a POSIX lock on an open file

SYNOPSIS

     #include <unistd.h>

     int lockf(int fd, int cmd, off_t len);

 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

     lockf():
         _XOPEN_SOURCE >= 500
             || /* Glibc since 2.19: */ _DEFAULT_SOURCE
             || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION

     Apply,  test  or remove a POSIX lock on a section of an open file.  The
     file is specified by fd, a file descriptor open for writing, the action
     by  cmd,  and  the section consists of byte positions pos..pos+len-1 if
     len is positive, and pos-len..pos-1 if len is negative,  where  pos  is
     the current file position, and if len is zero, the section extends from
     the current file position to infinity,  encompassing  the  present  and
     future  end-of-file  positions.   In  all cases, the section may extend
     past current end-of-file.

     On Linux, lockf() is just an interface  on  top  of  fcntl(2)  locking.
     Many other systems implement lockf() in this way, but note that POSIX.1
     leaves the relationship between lockf() and fcntl(2) locks unspecified.
     A  portable  application  should  probably  avoid mixing calls to these
     interfaces.

     Valid operations are given below:

     F_LOCK Set an exclusive lock on the specified section of the file.   If
            (part  of) this section is already locked, the call blocks until
            the previous lock is released.  If this section overlaps an ear-
            lier  locked  section, both are merged.  File locks are released
            as soon as the  process  holding  the  locks  closes  some  file
            descriptor for the file.  A child process does not inherit these
            locks.

     F_TLOCK
            Same as F_LOCK but the call never blocks and  returns  an  error
            instead if the file is already locked.

     F_ULOCK
            Unlock  the  indicated  section  of  the file.  This may cause a
            locked section to be split into two locked sections.

     F_TEST Test the lock: return 0 if the specified section is unlocked  or
            locked  by  this process; return -1, set errno to EAGAIN (EACCES
            on some other systems), if another process holds a lock.

RETURN VALUE

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

ERRORS

     EACCES or EAGAIN
            The  file  is locked and F_TLOCK or F_TEST was specified, or the
            operation is prohibited because the file has been  memory-mapped
            by another process.

     EBADF  fd  is  not an open file descriptor; or cmd is F_LOCK or F_TLOCK
            and fd is not a writable file descriptor.

     EDEADLK
            The command was F_LOCK and this lock  operation  would  cause  a
            deadlock.

     EINVAL An invalid operation was specified in cmd.

     ENOLCK Too many segment locks open, lock table is full.

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).

     +----------+---------------+---------+
     |Interface | Attribute     | Value   |
     +----------+---------------+---------+
     |lockf()   | Thread safety | MT-Safe |
     +----------+---------------+---------+

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008, SVr4.