4.4BSD/usr/share/man/cat2/fcntl.0
FCNTL(2) BSD Programmer's Manual FCNTL(2)
NNAAMMEE
ffccnnttll - file control
SSYYNNOOPPSSIISS
##iinncclluuddee <<ffccnnttll..hh>>
_i_n_t
ffccnnttll(_i_n_t _f_d, _i_n_t _c_m_d, _i_n_t _a_r_g);
DDEESSCCRRIIPPTTIIOONN
FFccnnttll() provides for control over descriptors. The argument _f_d is a de-
scriptor to be operated on by _c_m_d as follows:
F_DUPFD Return a new descriptor as follows:
++oo Lowest numbered available descriptor greater than or
equal to _a_r_g.
++oo Same object references as the original descriptor.
++oo New descriptor shares the same file offset if the ob-
ject was a file.
++oo Same access mode (read, write or read/write).
++oo Same file status flags (i.e., both file descriptors
share the same file status flags).
++oo The close-on-exec flag associated with the new file
descriptor is set to remain open across execv(2) sys-
tem calls.
F_GETFD Get the close-on-exec flag associated with the file descriptor
_f_d. If the low-order bit of the returned value is 0, the file
will remain open across eexxeecc(), otherwise the file will be
closed upon execution of eexxeecc() (_a_r_g is ignored).
F_SETFD Set the close-on-exec flag associated with _f_d to the low order
bit of _a_r_g (0 or 1 as above).
F_GETFL Get descriptor status flags, as described below (_a_r_g is ig-
nored).
F_SETFL Set descriptor status flags to _a_r_g.
F_GETOWN Get the process ID or process group currently receiving SIGIO
and SIGURG signals; process groups are returned as negative
values (_a_r_g is ignored).
F_SETOWN Set the process or process group to receive SIGIO and SIGURG
signals; process groups are specified by supplying _a_r_g as neg-
ative, otherwise _a_r_g is interpreted as a process ID.
The flags for the F_GETFL and F_SETFL flags are as follows:
O_NONBLOCK Non-blocking I/O; if no data is available to a read call, or
if a write operation would block, the read or write call re-
turns -1 with the error EAGAIN.
O_APPEND Force each write to append at the end of file; corresponds
to the O_APPEND flag of open(2).
O_ASYNC Enable the SIGIO signal to be sent to the process group when
I/O is possible, e.g., upon availability of data to be read.
Several commands are available for doing advisory file locking; they all
operate on the following structure:
struct flock {
off_t l_start; /* starting offset */
off_t l_len; /* len = 0 means until end of file */
pid_t l_pid; /* lock owner */
short l_type; /* lock type: read/write, etc. */
short l_whence; /* type of l_start */
};
The commands available for advisory record locking are as follows:
F_GETLK Get the first lock that blocks the lock description pointed to
by the third argument, _a_r_g, taken as a pointer to a _s_t_r_u_c_t
_f_l_o_c_k (see above). The information retrieved overwrites the
information passed to ffccnnttll in the _f_l_o_c_k structure. If no
lock is found that would prevent this lock from being created,
the structure is left unchanged by this function call except
for the lock type which is set to F_UNLCK.
F_SETLK Set or clear a file segment lock according to the lock de-
scription pointed to by the third argument, _a_r_g, taken as a
pointer to a _s_t_r_u_c_t _f_l_o_c_k (see above). F_SETLK is used to es-
tablish shared (or read) locks (F_RDLCK) or exclusive (or
write) locks, (F_WRLCK), as well as remove either type of lock
(F_UNLCK). If a shared or exclusive lock cannot be set, ffccnnttll
returns immediately with EACCES.
F_SETLKW This command is the same as F_SETLK except that if a shared or
exclusive lock is blocked by other locks, the process waits
until the request can be satisfied. If a signal that is to be
caught is received while ffccnnttll is waiting for a region, the
ffccnnttll will be interrupted if the signal handler has not speci-
fied the SA_RESTART (see sigaction(2)).
When a shared lock has been set on a segment of a file, other processes
can set shared locks on that segment or a portion of it. A shared lock
prevents any other process from setting an exclusive lock on any portion
of the protected area. A request for a shared lock fails if the file de-
scriptor was not opened with read access.
An exclusive lock prevents any other process from setting a shared lock
or an exclusive lock on any portion of the protected area. A request for
an exclusive lock fails if the file was not opened with write access.
The value of _l___w_h_e_n_c_e is SEEK_SET, SEEK_CUR, or SEEK_END to indicate that
the relative offset, _l___s_t_a_r_t bytes, will be measured from the start of
the file, current position, or end of the file, respectively. The value
of _l___l_e_n is the number of consecutive bytes to be locked. If _l___l_e_n is
negative, the result is undefined. The _l___p_i_d field is only used with
F_GETLK to return the process ID of the process holding a blocking lock.
After a successful F_GETLK request, the value of _l___w_h_e_n_c_e is SEEK_SET.
Locks may start and extend beyond the current end of a file, but may not
start or extend before the beginning of the file. A lock is set to ex-
tend to the largest possible value of the file offset for that file if
_l___l_e_n is set to zero. If _l___w_h_e_n_c_e and _l___s_t_a_r_t point to the beginning of
the file, and _l___l_e_n is zero, the entire file is locked. If an applica-
tion wishes only to do entire file locking, the flock(2) system call is
much more efficient.
There is at most one type of lock set for each byte in the file. Before
a successful return from an F_SETLK or an F_SETLKW request when the call-
ing process has previously existing locks on bytes in the region speci-
fied by the request, the previous lock type for each byte in the speci-
fied region is replaced by the new lock type. As specified above under
the descriptions of shared locks and exclusive locks, an F_SETLK or an
F_SETLKW request fails or blocks respectively when another process has
existing locks on bytes in the specified region and the type of any of
those locks conflicts with the type specified in the request.
This interface follows the completely stupid semantics of System V and
IEEE Std1003.1-1988 (``POSIX'') that require that all locks associated
with a file for a given process are removed when _a_n_y file descriptor for
that file is closed by that process. This semantic means that applica-
tions must be aware of any files that a subroutine library may access.
For example if an application for updating the password file locks the
password file database while making the update, and then calls getpw-
name(3) to retrieve a record, the lock will be lost because getpwname(3)
opens, reads, and closes the password database. The database close will
release all locks that the process has associated with the database, even
if the library routine never requested a lock on the database. Another
minor semantic problem with this interface is that locks are not inherit-
ed by a child process created using the fork(2) function. The flock(2)
interface has much more rational last close semantics and allows locks to
be inherited by child processes. Flock(2) is recommended for applica-
tions that want to ensure the integrity of their locks when using library
routines or wish to pass locks to their children. Note that flock(2) and
fcntl(2) locks may be safely used concurrently.
All locks associated with a file for a given process are removed when the
process terminates.
A potential for deadlock occurs if a process controlling a locked region
is put to sleep by attempting to lock the locked region of another pro-
cess. This implementation detects that sleeping until a locked region is
unlocked would cause a deadlock and fails with an EDEADLK error.
RREETTUURRNN VVAALLUUEESS
Upon successful completion, the value returned depends on _c_m_d as follows:
F_DUPFD A new file descriptor.
F_GETFD Value of flag (only the low-order bit is defined).
F_GETFL Value of flags.
F_GETOWN Value of file descriptor owner.
other Value other than -1.
Otherwise, a value of -1 is returned and _e_r_r_n_o is set to indicate the er-
ror.
EERRRROORRSS
FFccnnttll() will fail if:
[EACCES] The argument _a_r_g is F_SETLK, the type of lock _(_l___t_y_p_e_) is a
shared lock (F_RDLCK) or exclusive lock (F_WRLCK), and the
segment of a file to be locked is already exclusive-locked
by another process; or the type is an exclusive lock and
some portion of the segment of a file to be locked is al-
ready shared-locked or exclusive-locked by another process.
[EBADF] _F_i_l_d_e_s is not a valid open file descriptor.
The argument _c_m_d is F_SETLK or F_SETLKW, the type of lock
_(_l___t_y_p_e_) is a shared lock (F_RDLCK), and _f_i_l_d_e_s is not a
valid file descriptor open for reading.
The argument _c_m_d is F_SETLK or F_SETLKW, the type of lock
_(_l___t_y_p_e_) is an exclusive lock (F_WRLCK), and _f_i_l_d_e_s is not
a valid file descriptor open for writing.
[EMFILE] _C_m_d is F_DUPFD and the maximum allowed number of file de-
scriptors are currently open.
[EDEADLK] The argument _c_m_d is F_SETLKW, and a deadlock condition was
detected.
[EINTR] The argument _c_m_d is F_SETLKW, and the function was inter-
rupted by a signal.
[EINVAL] _C_m_d is F_DUPFD and _a_r_g is negative or greater than the max-
imum allowable number (see getdtablesize(2)).
The argument _c_m_d is F_GETLK, F_SETLK, or F_SETLKW and the
data to which _a_r_g points is not valid, or _f_i_l_d_e_s refers to
a file that does not support locking.
[EMFILE] The argument _c_m_d is F_DUPED and the maximum number of file
descriptors permitted for the process are already in use,
or no file descriptors greater than or equal to _a_r_g are
available.
[ENOLCK] The argument _c_m_d is F_SETLK or F_SETLKW, and satisfying the
lock or unlock request would result in the number of locked
regions in the system exceeding a system-imposed limit.
[ESRCH] _C_m_d is F_SETOWN and the process ID given as argument is not
in use.
SSEEEE AALLSSOO
close(2), execve(2), flock(2), getdtablesize(2), open(2), sigvec(2)
HHIISSTTOORRYY
The ffccnnttll function call appeared in 4.2BSD.
4.2 Berkeley Distribution July 11, 1993 4