lockf.3p (8769B)
- '\" et
- .TH LOCKF "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH PROLOG
- This manual page is part of the POSIX Programmer's Manual.
- The Linux implementation of this interface may differ (consult
- the corresponding Linux manual page for details of Linux behavior),
- or the interface may not be implemented on Linux.
- .\"
- .SH NAME
- lockf
- \(em record locking on files
- .SH SYNOPSIS
- .LP
- .nf
- #include <unistd.h>
- .P
- int lockf(int \fIfildes\fP, int \fIfunction\fP, off_t \fIsize\fP);
- .fi
- .SH DESCRIPTION
- The
- \fIlockf\fR()
- function shall lock sections of a file with advisory-mode locks. Calls
- to
- \fIlockf\fR()
- from threads in other processes which attempt to lock the locked file
- section shall either return an error value or block until the section
- becomes unlocked. All the locks for a process are removed when the
- process terminates. Record locking with
- \fIlockf\fR()
- shall be supported for regular files and may be supported for other
- files.
- .P
- The
- .IR fildes
- argument is an open file descriptor. To establish a lock with this
- function, the file descriptor shall be opened with write-only
- permission (O_WRONLY) or with read/write permission (O_RDWR).
- .P
- The
- .IR function
- argument is a control value which specifies the action to be taken. The
- permissible values for
- .IR function
- are defined in
- .IR <unistd.h>
- as follows:
- .TS
- box tab(!) center;
- cB | cB
- l | l.
- Function!Description
- _
- F_ULOCK!Unlock locked sections.
- F_LOCK!Lock a section for exclusive use.
- F_TLOCK!Test and lock a section for exclusive use.
- F_TEST!Test a section for locks by other processes.
- .TE
- .P
- F_TEST shall detect if a lock by another process is present on the
- specified section.
- .P
- F_LOCK and F_TLOCK shall both lock a section of a file if the section
- is available.
- .P
- F_ULOCK shall remove locks from a section of the file.
- .P
- The
- .IR size
- argument is the number of contiguous bytes to be locked or unlocked.
- The section to be locked or unlocked starts at the current offset in
- the file and extends forward for a positive size or backward for a
- negative size (the preceding bytes up to but not including the current
- offset). If
- .IR size
- is 0, the section from the current offset through the largest possible
- file offset shall be locked (that is, from the current offset through
- the present or any future end-of-file). An area need not be allocated
- to the file to be locked because locks may exist past the end-of-file.
- .P
- The sections locked with F_LOCK or F_TLOCK may, in whole or in part,
- contain or be contained by a previously locked section for the same
- process. When this occurs, or if adjacent locked sections would occur,
- the sections shall be combined into a single locked section. If the
- request would cause the number of locks to exceed a system-imposed
- limit, the request shall fail.
- .P
- F_LOCK and F_TLOCK requests differ only by the action taken if the
- section is not available. F_LOCK shall block the calling thread until
- the section is available. F_TLOCK shall cause the function to fail if
- the section is already locked by another process.
- .P
- File locks shall be released on first close by the locking process of
- any file descriptor for the file.
- .P
- F_ULOCK requests may release (wholly or in part) one or more locked
- sections controlled by the process. Locked sections shall be unlocked
- starting at the current file offset through
- .IR size
- bytes or to the end-of-file if
- .IR size
- is (\fBoff_t\fR)0. When all of a locked section is not released (that
- is, when the beginning or end of the area to be unlocked falls within a
- locked section), the remaining portions of that section shall remain
- locked by the process. Releasing the center portion of a locked section
- shall cause the remaining locked beginning and end portions to become two
- separate locked sections. If the request would cause the number of
- locks in the system to exceed a system-imposed limit, the request
- shall fail.
- .P
- A potential for deadlock occurs if the threads of a process controlling
- a locked section are blocked by accessing a locked section of
- another process. If the system detects that deadlock would occur,
- \fIlockf\fR()
- shall fail with an
- .BR [EDEADLK]
- error.
- .P
- The interaction between
- \fIfcntl\fR()
- and
- \fIlockf\fR()
- locks is unspecified.
- .P
- Blocking on a section shall be interrupted by any signal.
- .P
- An F_ULOCK request in which
- .IR size
- is non-zero and the offset of the last byte of the requested section is
- the maximum value for an object of type
- .BR off_t ,
- when the process has an existing lock in which
- .IR size
- is 0 and which includes the last byte of the requested section, shall be
- treated as a request to unlock from the start of the requested section
- with a size equal to 0. Otherwise, an F_ULOCK request shall attempt to
- unlock only the requested section.
- .P
- Attempting to lock a section of a file that is associated with a
- buffered stream produces unspecified results.
- .SH "RETURN VALUE"
- Upon successful completion,
- \fIlockf\fR()
- shall return 0. Otherwise, it shall return \-1, set
- .IR errno
- to indicate an error, and existing locks shall not be changed.
- .SH ERRORS
- The
- \fIlockf\fR()
- function shall fail if:
- .TP
- .BR EBADF
- The
- .IR fildes
- argument is not a valid open file descriptor; or
- .IR function
- is F_LOCK or F_TLOCK and
- .IR fildes
- is not a valid file descriptor open for writing.
- .TP
- .BR EACCES " or " EAGAIN
- .br
- The
- .IR function
- argument is F_TLOCK or F_TEST and the section is already locked by
- another process.
- .TP
- .BR EDEADLK
- The
- .IR function
- argument is F_LOCK and a deadlock is detected.
- .TP
- .BR EINTR
- A signal was caught during execution of the function.
- .TP
- .BR EINVAL
- The
- .IR function
- argument is not one of F_LOCK, F_TLOCK, F_TEST, or F_ULOCK; or
- .IR size
- plus the current file offset is less than 0.
- .TP
- .BR EOVERFLOW
- The offset of the first, or if
- .IR size
- is not 0 then the last, byte in the requested section cannot be
- represented correctly in an object of type
- .BR off_t .
- .P
- The
- \fIlockf\fR()
- function may fail if:
- .TP
- .BR EAGAIN
- The
- .IR function
- argument is F_LOCK or F_TLOCK and the file is mapped with
- \fImmap\fR().
- .TP
- .BR EDEADLK " or " ENOLCK
- .br
- The
- .IR function
- argument is F_LOCK, F_TLOCK, or F_ULOCK, and the request would cause
- the number of locks to exceed a system-imposed limit.
- .TP
- .BR EOPNOTSUPP " or " EINVAL
- .br
- The implementation does not support the locking of files of the type
- indicated by the
- .IR fildes
- argument.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- .SS "Locking a Portion of a File"
- .P
- In the following example, a file named
- .BR /home/cnd/mod1
- is being modified. Other processes that use locking are prevented from
- changing it during this process. Only the first 10\|000 bytes are
- locked, and the lock call fails if another process has any part of this
- area locked already.
- .sp
- .RS 4
- .nf
- #include <fcntl.h>
- #include <unistd.h>
- .P
- int fildes;
- int status;
- \&...
- fildes = open("/home/cnd/mod1", O_RDWR);
- status = lockf(fildes, F_TLOCK, (off_t)10000);
- .fi
- .P
- .RE
- .SH "APPLICATION USAGE"
- Record-locking should not be used in combination with the
- \fIfopen\fR(),
- \fIfread\fR(),
- \fIfwrite\fR(),
- and other
- .IR stdio
- functions. Instead, the more primitive, non-buffered functions (such as
- \fIopen\fR())
- should be used. Unexpected results may occur in processes that do
- buffering in the user address space. The process may later read/write
- data which is/was locked. The
- .IR stdio
- functions are the most common source of unexpected buffering.
- .P
- The
- \fIalarm\fR()
- function may be used to provide a timeout facility in applications
- requiring it.
- .SH RATIONALE
- None.
- .SH "FUTURE DIRECTIONS"
- None.
- .SH "SEE ALSO"
- .IR "\fIalarm\fR\^(\|)",
- .IR "\fIchmod\fR\^(\|)",
- .IR "\fIclose\fR\^(\|)",
- .IR "\fIcreat\fR\^(\|)",
- .IR "\fIfcntl\fR\^(\|)",
- .IR "\fIfopen\fR\^(\|)",
- .IR "\fImmap\fR\^(\|)",
- .IR "\fIopen\fR\^(\|)",
- .IR "\fIread\fR\^(\|)",
- .IR "\fIwrite\fR\^(\|)"
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "\fB<unistd.h>\fP"
- .\"
- .SH COPYRIGHT
- Portions of this text are reprinted and reproduced in electronic form
- from IEEE Std 1003.1-2017, Standard for Information Technology
- -- Portable Operating System Interface (POSIX), The Open Group Base
- Specifications Issue 7, 2018 Edition,
- Copyright (C) 2018 by the Institute of
- Electrical and Electronics Engineers, Inc and The Open Group.
- In the event of any discrepancy between this version and the original IEEE and
- The Open Group Standard, the original IEEE and The Open Group Standard
- is the referee document. The original Standard can be obtained online at
- http://www.opengroup.org/unix/online.html .
- .PP
- Any typographical or formatting errors that appear
- in this page are most likely
- to have been introduced during the conversion of the source files to
- man page format. To report such errors, see
- https://www.kernel.org/doc/man-pages/reporting_bugs.html .