FLOCK(2)                                                              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 associated 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 consistent opera‐
       tions  on  files, but do not guarantee consistency (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 exclu‐
       sive, 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 normally 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.

       Processes blocked awaiting a lock may be awakened by signals.

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 global 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 spec‐
                           ified.

       [EBADF]             The argument _f_d is an invalid descriptor.

       [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)



4.2 Berkeley Distribution        May 22, 1986                         FLOCK(2)