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



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

SYNOPSIS
     #include <sys/file.h>

     #define   LOCK_SH	 1    /* shared lock */
     #define   LOCK_EX	 2    /* exclusive lock */
     #define   LOCK_NB	 4    /* don't block when locking */
     #define   LOCK_UN	 8    /* unlock */

     flock(fd, operation)
     int fd, operation;

DESCRIPTION
     _F_l_o_c_k applies or removes an _a_d_v_i_s_o_r_y lock on the file asso-
     ciated with the file descriptor _f_d.  A lock is applied by
     specifying an _o_p_e_r_a_t_i_o_n parameter that is the inclusive or
     of LOCK_SH or LOCK_EX and, possibly, LOCK_NB.  To unlock an
     existing lock _o_p_e_r_a_t_i_o_n should be LOCK_UN.

     Advisory locks allow cooperating processes to perform con-
     sistent operations on files, but do not guarantee con-
     sistency (i.e., processes may still access files without
     using advisory locks possibly resulting in inconsistencies).

     The locking mechanism allows two types of locks: _s_h_a_r_e_d
     locks and _e_x_c_l_u_s_i_v_e locks.  At any time multiple shared
     locks may be applied to a file, but at no time are multiple
     exclusive, or both shared and exclusive, locks allowed
     simultaneously on a file.

     A shared lock may be _u_p_g_r_a_d_e_d to an exclusive lock, and vice
     versa, simply by specifying the appropriate lock type; this
     results in the previous lock being released and the new lock
     applied (possibly after other processes have gained and
     released the lock).

     Requesting a lock on an object that is already locked nor-
     mally causes the caller to be blocked until the lock may be
     acquired.	If LOCK_NB is included in _o_p_e_r_a_t_i_o_n, then this
     will not happen; instead the call will fail and the error
     EWOULDBLOCK will be returned.

NOTES
     Locks are on files, not file descriptors.	That is, file
     descriptors duplicated through _d_u_p(2) or _f_o_r_k(2) do not
     result in multiple instances of a lock, but rather multiple
     references to a single lock.  If a process holding a lock on
     a file forks and the child explicitly unlocks the file, the
     parent will lose its lock.




Printed 11/26/99	  May 22, 1986				1






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



     Processes blocked awaiting a lock may be awakened by sig-
     nals.

RETURN VALUE
     Zero is returned if the operation was successful; on an
     error a -1 is returned and an error code is left in the glo-
     bal location _e_r_r_n_o.

ERRORS
     The _f_l_o_c_k call fails if:

     [EWOULDBLOCK]	 The file is locked and the LOCK_NB
			 option was specified.

     [EBADF]		 The argument _f_d is an invalid descrip-
			 tor.

     [EINVAL]		 The argument _f_d refers to an object
			 other than a file.

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

































Printed 11/26/99	  May 22, 1986				2