fcntl函数

http://man7.org/linux/man-pages/man2/fcntl.2.html
int fcntl(int fd, int cmd, ... /* arg */ );

fcntl函数是一个复杂功能强大的函数，关于文件锁的用法如下：

Advisory record locking

       Linux implements traditional ("process-associated") UNIX record       locks, as standardized by POSIX.  For a Linux-specific alternative       with better semantics, see the discussion of open file description       locks below.       F_SETLK, F_SETLKW, and F_GETLK are used to acquire, release, and test       for the existence of record locks (also known as byte-range, file-       segment, or file-region locks).  The third argument, lock, is a       pointer to a structure that has at least the following fields (in       unspecified order).           struct flock {               ...               short l_type;    /* Type of lock: F_RDLCK,                                   F_WRLCK, F_UNLCK */               short l_whence;  /* How to interpret l_start:                                   SEEK_SET, SEEK_CUR, SEEK_END */               off_t l_start;   /* Starting offset for lock */               off_t l_len;     /* Number of bytes to lock */               pid_t l_pid;     /* PID of process blocking our lock                                   (set by F_GETLK and F_OFD_GETLK) */               ...           };       The l_whence, l_start, and l_len fields of this structure specify the       range of bytes we wish to lock.  Bytes past the end of the file may       be locked, but not bytes before the start of the file.       l_start is the starting offset for the lock, and is interpreted       relative to either: the start of the file (if l_whence is SEEK_SET);       the current file offset (if l_whence is SEEK_CUR); or the end of the       file (if l_whence is SEEK_END).  In the final two cases, l_start can       be a negative number provided the offset does not lie before the       start of the file.       l_len specifies the number of bytes to be locked.  If l_len is       positive, then the range to be locked covers bytes l_start up to and       including l_start+l_len-1.  Specifying 0 for l_len has the special       meaning: lock all bytes starting at the location specified by       l_whence and l_start through to the end of file, no matter how large       the file grows.       POSIX.1-2001 allows (but does not require) an implementation to       support a negative l_len value; if l_len is negative, the interval       described by lock covers bytes l_start+l_len up to and including       l_start-1.  This is supported by Linux since kernel versions 2.4.21       and 2.5.49.       The l_type field can be used to place a read (F_RDLCK) or a write       (F_WRLCK) lock on a file.  Any number of processes may hold a read       lock (shared lock) on a file region, but only one process may hold a       write lock (exclusive lock).  An exclusive lock excludes all other       locks, both shared and exclusive.  A single process can hold only one       type of lock on a file region; if a new lock is applied to an       already-locked region, then the existing lock is converted to the new       lock type.  (Such conversions may involve splitting, shrinking, or       coalescing with an existing lock if the byte range specified by the       new lock does not precisely coincide with the range of the existing       lock.)       F_SETLK (struct flock *)              Acquire a lock (when l_type is F_RDLCK or F_WRLCK) or release              a lock (when l_type is F_UNLCK) on the bytes specified by the              l_whence, l_start, and l_len fields of lock.  If a conflicting              lock is held by another process, this call returns -1 and sets              errno to EACCES or EAGAIN.  (The error returned in this case              differs across implementations, so POSIX requires a portable              application to check for both errors.)       F_SETLKW (struct flock *)              As for F_SETLK, but if a conflicting lock is held on the file,              then wait for that lock to be released.  If a signal is caught              while waiting, then the call is interrupted and (after the              signal handler has returned) returns immediately (with return              value -1 and errno set to EINTR; see signal(7)).       F_GETLK (struct flock *)              On input to this call, lock describes a lock we would like to              place on the file.  If the lock could be placed, fcntl() does              not actually place it, but returns F_UNLCK in the l_type field              of lock and leaves the other fields of the structure              unchanged.              If one or more incompatible locks would prevent this lock              being placed, then fcntl() returns details about one of those              locks in the l_type, l_whence, l_start, and l_len fields of              lock.  If the conflicting lock is a traditional (process-              associated) record lock, then the l_pid field is set to the              PID of the process holding that lock.  If the conflicting lock              is an open file description lock, then l_pid is set to -1.              Note that the returned information may already be out of date              by the time the caller inspects it.       In order to place a read lock, fd must be open for reading.  In order       to place a write lock, fd must be open for writing.  To place both       types of lock, open a file read-write.       When placing locks with F_SETLKW, the kernel detects deadlocks,       whereby two or more processes have their lock requests mutually       blocked by locks held by the other processes.  For example, suppose       process A holds a write lock on byte 100 of a file, and process B       holds a write lock on byte 200.  If each process then attempts to       lock the byte already locked by the other process using F_SETLKW,       then, without deadlock detection, both processes would remain blocked       indefinitely.  When the kernel detects such deadlocks, it causes one       of the blocking lock requests to immediately fail with the error       EDEADLK; an application that encounters such an error should release       some of its locks to allow other applications to proceed before       attempting regain the locks that it requires.  Circular deadlocks       involving more than two processes are also detected.  Note, however,       that there are limitations to the kernel's deadlock-detection       algorithm; see BUGS.       As well as being removed by an explicit F_UNLCK, record locks are       automatically released when the process terminates.       Record locks are not inherited by a child created via fork(2), but       are preserved across an execve(2).       Because of the buffering performed by the stdio(3) library, the use       of record locking with routines in that package should be avoided;       use read(2) and write(2) instead.       The record locks described above are associated with the process       (unlike the open file description locks described below).  This has       some unfortunate consequences:       *  If a process closes any file descriptor referring to a file, then          all of the process's locks on that file are released, regardless          of the file descriptor(s) on which the locks were obtained.  This          is bad: it means that a process can lose its locks on a file such          as /etc/passwd or /etc/mtab when for some reason a library          function decides to open, read, and close the same file.       *  The threads in a process share locks.  In other words, a          multithreaded program can't use record locking to ensure that          threads don't simultaneously access the same region of a file.       Open file description locks solve both of these problems.   Open file description locks (non-POSIX)       Open file description locks are advisory byte-range locks whose       operation is in most respects identical to the traditional record       locks described above.  This lock type is Linux-specific, and       available since Linux 3.15.  For an explanation of open file       descriptions, see open(2).       The principal difference between the two lock types is that whereas       traditional record locks are associated with a process, open file       description locks are associated with the open file description on       which they are acquired, much like locks acquired with flock(2).       Consequently (and unlike traditional advisory record locks), open       file description locks are inherited across fork(2) (and clone(2)       with CLONE_FILES), and are only automatically released on the last       close of the open file description, instead of being released on any       close of the file.       Open file description locks always conflict with traditional record       locks, even when they are acquired by the same process on the same       file descriptor.       Open file description locks placed via the same open file description       (i.e., via the same file descriptor, or via a duplicate of the file       descriptor created by fork(2), dup(2), fcntl(2) F_DUPFD, and so on)       are always compatible: if a new lock is placed on an already locked       region, then the existing lock is converted to the new lock type.       (Such conversions may result in splitting, shrinking, or coalescing       with an existing lock as discussed above.)       On the other hand, open file description locks may conflict with each       other when they are acquired via different open file descriptions.       Thus, the threads in a multithreaded program can use open file       description locks to synchronize access to a file region by having       each thread perform its own open(2) on the file and applying locks       via the resulting file descriptor.       As with traditional advisory locks, the third argument to fcntl(),       lock, is a pointer to an flock structure.  By contrast with       traditional record locks, the l_pid field of that structure must be       set to zero when using the commands described below.       The commands for working with open file description locks are       analogous to those used with traditional locks:       F_OFD_SETLK (struct flock *)              Acquire an open file description lock (when l_type is F_RDLCK              or F_WRLCK) or release an open file description lock (when              l_type is F_UNLCK) on the bytes specified by the l_whence,              l_start, and l_len fields of lock.  If a conflicting lock is              held by another process, this call returns -1 and sets errno              to EAGAIN.       F_OFD_SETLKW (struct flock *)              As for F_OFD_SETLK, but if a conflicting lock is held on the              file, then wait for that lock to be released.  If a signal is              caught while waiting, then the call is interrupted and (after              the signal handler has returned) returns immediately (with              return value -1 and errno set to EINTR; see signal(7)).       F_OFD_GETLK (struct flock *)              On input to this call, lock describes an open file description              lock we would like to place on the file.  If the lock could be              placed, fcntl() does not actually place it, but returns              F_UNLCK in the l_type field of lock and leaves the other              fields of the structure unchanged.  If one or more              incompatible locks would prevent this lock being placed, then              details about one of these locks are returned via lock, as              described above for F_GETLK.       In the current implementation, no deadlock detection is performed for       open file description locks.  (This contrasts with process-associated       record locks, for which the kernel does perform deadlock detection.)   Mandatory locking       Warning: the Linux implementation of mandatory locking is unreliable.       See BUGS below.       By default, both traditional (process-associated) and open file       description record locks are advisory.  Advisory locks are not       enforced and are useful only between cooperating processes.       Both lock types can also be mandatory.  Mandatory locks are enforced       for all processes.  If a process tries to perform an incompatible       access (e.g., read(2) or write(2)) on a file region that has an       incompatible mandatory lock, then the result depends upon whether the       O_NONBLOCK flag is enabled for its open file description.  If the       O_NONBLOCK flag is not enabled, then the system call is blocked until       the lock is removed or converted to a mode that is compatible with       the access.  If the O_NONBLOCK flag is enabled, then the system call       fails with the error EAGAIN.       To make use of mandatory locks, mandatory locking must be enabled       both on the filesystem that contains the file to be locked, and on       the file itself.  Mandatory locking is enabled on a filesystem using       the "-o mand" option to mount(8), or the MS_MANDLOCK flag for       mount(2).  Mandatory locking is enabled on a file by disabling group       execute permission on the file and enabling the set-group-ID       permission bit (see chmod(1) and chmod(2)).       Mandatory locking is not specified by POSIX.  Some other systems also       support mandatory locking, although the details of how to enable it       vary across systems.