(libc.info.gz) File Locks
Info Catalog
(libc.info.gz) File Status Flags
(libc.info.gz) Low-Level I/O
(libc.info.gz) Interrupt Input
13.15 File Locks
================
The remaining 'fcntl' commands are used to support "record locking",
which permits multiple cooperating programs to prevent each other from
simultaneously accessing parts of a file in error-prone ways.
An "exclusive" or "write" lock gives a process exclusive access for
writing to the specified part of the file. While a write lock is in
place, no other process can lock that part of the file.
A "shared" or "read" lock prohibits any other process from requesting
a write lock on the specified part of the file. However, other
processes can request read locks.
The 'read' and 'write' functions do not actually check to see whether
there are any locks in place. If you want to implement a locking
protocol for a file shared by multiple processes, your application must
do explicit 'fcntl' calls to request and clear locks at the appropriate
points.
Locks are associated with processes. A process can only have one
kind of lock set for each byte of a given file. When any file
descriptor for that file is closed by the process, all of the locks that
process holds on that file are released, even if the locks were made
using other descriptors that remain open. Likewise, locks are released
when a process exits, and are not inherited by child processes created
using 'fork' ( Creating a Process).
When making a lock, use a 'struct flock' to specify what kind of lock
and where. This data type and the associated macros for the 'fcntl'
function are declared in the header file 'fcntl.h'.
-- Data Type: struct flock
This structure is used with the 'fcntl' function to describe a file
lock. It has these members:
'short int l_type'
Specifies the type of the lock; one of 'F_RDLCK', 'F_WRLCK',
or 'F_UNLCK'.
'short int l_whence'
This corresponds to the WHENCE argument to 'fseek' or 'lseek',
and specifies what the offset is relative to. Its value can
be one of 'SEEK_SET', 'SEEK_CUR', or 'SEEK_END'.
'off_t l_start'
This specifies the offset of the start of the region to which
the lock applies, and is given in bytes relative to the point
specified by 'l_whence' member.
'off_t l_len'
This specifies the length of the region to be locked. A value
of '0' is treated specially; it means the region extends to
the end of the file.
'pid_t l_pid'
This field is the process ID ( Process Creation
Concepts) of the process holding the lock. It is filled in
by calling 'fcntl' with the 'F_GETLK' command, but is ignored
when making a lock.
-- Macro: int F_GETLK
This macro is used as the COMMAND argument to 'fcntl', to specify
that it should get information about a lock. This command requires
a third argument of type 'struct flock *' to be passed to 'fcntl',
so that the form of the call is:
fcntl (FILEDES, F_GETLK, LOCKP)
If there is a lock already in place that would block the lock
described by the LOCKP argument, information about that lock
overwrites '*LOCKP'. Existing locks are not reported if they are
compatible with making a new lock as specified. Thus, you should
specify a lock type of 'F_WRLCK' if you want to find out about both
read and write locks, or 'F_RDLCK' if you want to find out about
write locks only.
There might be more than one lock affecting the region specified by
the LOCKP argument, but 'fcntl' only returns information about one
of them. The 'l_whence' member of the LOCKP structure is set to
'SEEK_SET' and the 'l_start' and 'l_len' fields set to identify the
locked region.
If no lock applies, the only change to the LOCKP structure is to
update the 'l_type' to a value of 'F_UNLCK'.
The normal return value from 'fcntl' with this command is an
unspecified value other than -1, which is reserved to indicate an
error. The following 'errno' error conditions are defined for this
command:
'EBADF'
The FILEDES argument is invalid.
'EINVAL'
Either the LOCKP argument doesn't specify valid lock
information, or the file associated with FILEDES doesn't
support locks.
-- Macro: int F_SETLK
This macro is used as the COMMAND argument to 'fcntl', to specify
that it should set or clear a lock. This command requires a third
argument of type 'struct flock *' to be passed to 'fcntl', so that
the form of the call is:
fcntl (FILEDES, F_SETLK, LOCKP)
If the process already has a lock on any part of the region, the
old lock on that part is replaced with the new lock. You can
remove a lock by specifying a lock type of 'F_UNLCK'.
If the lock cannot be set, 'fcntl' returns immediately with a value
of -1. This function does not block waiting for other processes to
release locks. If 'fcntl' succeeds, it return a value other than
-1.
The following 'errno' error conditions are defined for this
function:
'EAGAIN'
'EACCES'
The lock cannot be set because it is blocked by an existing
lock on the file. Some systems use 'EAGAIN' in this case, and
other systems use 'EACCES'; your program should treat them
alike, after 'F_SETLK'. (GNU/Linux and GNU/Hurd systems
always use 'EAGAIN'.)
'EBADF'
Either: the FILEDES argument is invalid; you requested a read
lock but the FILEDES is not open for read access; or, you
requested a write lock but the FILEDES is not open for write
access.
'EINVAL'
Either the LOCKP argument doesn't specify valid lock
information, or the file associated with FILEDES doesn't
support locks.
'ENOLCK'
The system has run out of file lock resources; there are
already too many file locks in place.
Well-designed file systems never report this error, because
they have no limitation on the number of locks. However, you
must still take account of the possibility of this error, as
it could result from network access to a file system on
another machine.
-- Macro: int F_SETLKW
This macro is used as the COMMAND argument to 'fcntl', to specify
that it should set or clear a lock. It is just like the 'F_SETLK'
command, but causes the process to block (or wait) until the
request can be specified.
This command requires a third argument of type 'struct flock *', as
for the 'F_SETLK' command.
The 'fcntl' return values and errors are the same as for the
'F_SETLK' command, but these additional 'errno' error conditions
are defined for this command:
'EINTR'
The function was interrupted by a signal while it was waiting.
Interrupted Primitives.
'EDEADLK'
The specified region is being locked by another process. But
that process is waiting to lock a region which the current
process has locked, so waiting for the lock would result in
deadlock. The system does not guarantee that it will detect
all such conditions, but it lets you know if it notices one.
The following macros are defined for use as values for the 'l_type'
member of the 'flock' structure. The values are integer constants.
'F_RDLCK'
This macro is used to specify a read (or shared) lock.
'F_WRLCK'
This macro is used to specify a write (or exclusive) lock.
'F_UNLCK'
This macro is used to specify that the region is unlocked.
As an example of a situation where file locking is useful, consider a
program that can be run simultaneously by several different users, that
logs status information to a common file. One example of such a program
might be a game that uses a file to keep track of high scores. Another
example might be a program that records usage or accounting information
for billing purposes.
Having multiple copies of the program simultaneously writing to the
file could cause the contents of the file to become mixed up. But you
can prevent this kind of problem by setting a write lock on the file
before actually writing to the file.
If the program also needs to read the file and wants to make sure
that the contents of the file are in a consistent state, then it can
also use a read lock. While the read lock is set, no other process can
lock that part of the file for writing.
Remember that file locks are only a _voluntary_ protocol for
controlling access to a file. There is still potential for access to
the file by programs that don't use the lock protocol.
Info Catalog
(libc.info.gz) File Status Flags
(libc.info.gz) Low-Level I/O
(libc.info.gz) Interrupt Input
automatically generated by
info2html