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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>fcntl</title></head><body bgcolor=white><center><font size=2>The Single UNIX ® Specification, Version 2<br>Copyright © 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_000_004_152"> </a>NAME</h4><blockquote>fcntl - file control</blockquote><h4><a name = "tag_000_004_153"> </a>SYNOPSIS</h4><blockquote><pre><code>#include <<a href="systypes.h.html">sys/types.h</a>>#include <<a href="unistd.h.html">unistd.h</a>>#include <<a href="fcntl.h.html">fcntl.h</a>>int fcntl(int <i>fildes</i>, int <i>cmd</i>, ...);</code></pre></blockquote><h4><a name = "tag_000_004_154"> </a>DESCRIPTION</h4><blockquote>The<i>fcntl()</i>function provides for control over open files. The<i>fildes</i>argument is a file descriptor.<p>The available values for<i>cmd</i>are defined in the header<i><a href="fcntl.h.html"><fcntl.h></a></i>,which include:<dl compact><dt>F_DUPFD<dd>Return a new file descriptor which is the lowestnumbered available (that is, not already open) filedescriptor greater than or equal to the thirdargument,<i>arg</i>,taken as an integer of type<b>int</b>.The new file descriptor refers to the same open file descriptionas the original file descriptor, and shares any locks.The FD_CLOEXEC flag associated with the new filedescriptor is cleared to keep the file open acrosscalls to one of the<i>exec</i>functions.<dt>F_GETFD<dd>Get the file descriptor flags defined in<i><a href="fcntl.h.html"><fcntl.h></a></i>that are associated with the file descriptor<i>fildes</i>.File descriptor flags are associated with a single filedescriptor and do not affect other file descriptorsthat refer to the same file.<dt>F_SETFD<dd>Set the file descriptor flags defined in<i><a href="fcntl.h.html"><fcntl.h></a></i>,that are associated with<i>fildes</i>,to the third argument,<i>arg</i>,taken as type<b>int</b>.If the FD_CLOEXEC flag in the third argumentis 0, the file will remain open across the<i>exec</i>functions; otherwise the file will be closed uponsuccessful execution of one of the<i>exec</i>functions.<dt>F_GETFL<dd>Get the file status flags and file access modes, defined in<i><a href="fcntl.h.html"><fcntl.h></a></i>,for the file description associated with<i>fildes</i>.The file access modes can be extracted from thereturn value using the mask O_ACCMODE, which is defined in<i><a href="fcntl.h.html"><fcntl.h></a></i>.File status flags and file access modes are associatedwith the file description and do not affect otherfile descriptors that refer to the same file withdifferent open file descriptions.<dt>F_SETFL<dd>Set the file status flags, defined in<i><a href="fcntl.h.html"><fcntl.h></a></i>,for the file description associated with<i>fildes</i>from the corresponding bits in the third argument,<i>arg</i>,taken as type<b>int</b>.Bits corresponding to the file access mode and the<i>oflag</i>values that are set in<i>arg</i>are ignored. If any bits in<i>arg</i>other than those mentioned here are changed by the application, theresult is unspecified.</dl><p>The following values for<i>cmd</i>are available for advisory record locking.Record locking is supported for regular files, andmay be supported for other files.<dl compact><dt>F_GETLK<dd>Get the first lock which blocks the lock descriptionpointed to by the third argument,<i>arg</i>,taken as a pointer to type<b>struct flock</b>,defined in<i><a href="fcntl.h.html"><fcntl.h></a></i>.The information retrieved overwrites the information passed to<i>fcntl()</i>in the structure<b>flock</b>.If no lock is found that would prevent this lockfrom being created, then the structure will be leftunchanged except for the lock type which willbe set to F_UNLCK.<dt>F_SETLK<dd>Set or clear a file segment lock according to the lock descriptionpointed to by the third argument,<i>arg</i>,taken as a pointer to type<b>struct flock</b>,defined in<i><a href="fcntl.h.html"><fcntl.h></a></i>.F_SETLK is used to establish shared (or read) locks (F_RDLCK)or exclusive (or write) locks (F_WRLCK),as well as to remove either type of lock (F_UNLCK).F_RDLCK, F_WRLCK and F_UNLCK are defined in<i><a href="fcntl.h.html"><fcntl.h></a></i>.If a shared or exclusive lock cannot be set,<i>fcntl()</i>will return immediately with a return value of -1.<dt>F_SETLKW<dd>This command is the same as F_SETLK except thatif a shared or exclusive lock is blocked by other locks,the thread will wait until the request can be satisfied.If a signal that is to be caught is received while<i>fcntl()</i>is waiting for a region,<i>fcntl()</i>will be interrupted. Upon return from the signal handler,<i>fcntl()</i>will return -1 with<i>errno</i>set to [EINTR], and the lock operation will not be done.</dl><p>Additional implementation-dependent values for<i>cmd</i>may be defined in<i><a href="fcntl.h.html"><fcntl.h></a></i>.Their names will start with F_.<p>When a shared lock is set on a segment of a file, otherprocesses will be able to set shared locks on that segmentor a portion of it.A shared lock prevents any other process from setting anexclusive lock on any portion of the protected area.A request for a shared lock will fail if the file descriptorwas not opened with read access.<p>An exclusive lock will prevent any other process from settinga shared lock or an exclusive lock on any portion of theprotected area.A request for an exclusive lock will fail if the filedescriptor was not opened with write access.<p>The structure<b>flock</b>describes the type(<b>l_type</b>),starting offset(<b>l_whence</b>),relative offset(<b>l_start</b>),size(<b>l_len</b>)and process ID(<b>l_pid</b>)of the segment of the file to be affected.<p>The value of<b>l_whence</b>is SEEK_SET, SEEK_CUR or SEEK_END,to indicate that the relative offset<b>l_start</b>bytes will be measured fromthe start of the file,current position or end of the file,respectively.The value of<b>l_len</b>is the number of consecutive bytes to be locked.The value of<b>l_len</b>may be negative (where the definition of<b>off_t</b>permits negative values of<b>l_len</b>).The<b>l_pid</b>field is only used with F_GETLK to returnthe process ID of the process holdinga blocking lock. After a successful F_GETLK request,that is, one in which a lock was found, the value of<b>l_whence</b>will be SEEK_SET.<p>If <b>l_len</b> is positive, the area affected starts at<b>l_start</b> and ends at <b>l_start</b> +<b>l_len</b>-1.If <b>l_len</b> is negative, the area affected starts at<b>l_start</b> + <b>l_len</b> and ends at<b>l_start</b>-1.Locks may start and extend beyond the current end of a file,but must not be negative relative to the beginning of the file.A lock will be set to extend to the largest possible valueof the file offset for that file by setting<b>l_len</b>to 0.If such a lock also has<b>l_start</b>set to 0 and<b>l_whence</b>is set to SEEK_SET, the whole file will be locked.<p>There will be 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 thecalling process has previously existing locks on bytes in the regionspecified by the request, the previous lock type for each byte in thespecified region will be replaced by the new lock type.As specifiedabove under the descriptions of shared locks and exclusive locks, anF_SETLK or an F_SETLKW request will (respectively) fail or block whenanother process has existing locks on bytes in the specified region andthe type of any of those locks conflicts with the type specified inthe request.<p>All locks associated with a file for a given process are removedwhen a file descriptor for that file is closed by that processor the process holding that file descriptor terminates.Locks are not inherited by a child process created using<i><a href="fork.html">fork()</a></i>.<p>A potential for deadlock occurs if a process controllinga locked region is put to sleep by attempting to lockanother process' locked region.If the system detects that sleeping until a lockedregion is unlocked would cause a deadlock,<i>fcntl()</i>will fail with an [EDEADLK] error.<br><p>If _XOPEN_REALTIME is definedand has a value other than -1:<dl compact><dt> <dd>When the file descriptor<i>fildes</i>refers to a shared memory object, the behaviour of<i>fcntl()</i>is the same as for a regular fileexcept the effect of the following values for the argument<i>cmd</i>are unspecified: F_SETFL, F_GETLK, F_SETLK, and F_SETLKW.</dl><p><p>An unlock (F_UNLCK) request in which <i>l_len</i>is non-zero and the offset of the last byte of the requested segment isthe maximum value for an object of type<b>off_t</b>,when the process has an existing lock in which<i>l_len</i>is 0 and which includes the last byte of the requested segment, will betreated as a request to unlock from the start of the requested segmentwith an<i>l_len</i>equal to 0. Otherwise an unlock (F_UNLCK) request will attempt tounlock only the requested segment.</blockquote><h4><a name = "tag_000_004_155"> </a>RETURN VALUE</h4><blockquote>Upon successful completion,the value returned depends on<i>cmd</i>as follows:<dl compact><dt>F_DUPFD<dd>A new file descriptor.<dt>F_GETFD<dd>Value of flags defined in<i><a href="fcntl.h.html"><fcntl.h></a></i>.The return value will not be negative.<dt>F_SETFD<dd>Value other than -1.<dt>F_GETFL<dd>Value of file status flags and access modes.The return value will not be negative.<dt>F_SETFL<dd>Value other than -1.<dt>F_GETLK<dd>Value other than -1.<dt>F_SETLK<dd>Value other than -1.<dt>F_SETLKW<dd>Value other than -1.</dl><p>Otherwise, -1 is returned and<i>errno</i>is set to indicate the error.<br></blockquote><h4><a name = "tag_000_004_156"> </a>ERRORS</h4><blockquote>The<i>fcntl()</i>function will fail if:<dl compact><dt>[EACCES] or [EAGAIN]<dd>The<i>cmd</i>argument is F_SETLK;the type of lock(<b>l_type</b>)is a shared (F_RDLCK) or exclusive (F_WRLCK)lock and the segment of a file to be locked is already exclusive-lockedby another process,or the type is an exclusive lock and some portion of thesegment of a file to be locked is already shared-lockedor exclusive-locked by another process.<dt>[EBADF]<dd>The<i>fildes</i>argumentis not a valid openfile descriptor, or the argument<i>cmd</i>is F_SETLK or F_SETLKW, the type of lock,<b>l_type</b>,is a shared lock (F_RDLCK), and<i>fildes</i>is not a valid file descriptor open for reading, orthe type of lock<b>l_type</b>,is an exclusive lock (F_WRLCK), and<i>fildes</i>is not a valid file descriptor open for writing.<dt>[EINTR]<dd>The<i>cmd</i>argumentis F_SETLKW and the function was interrupted by a signal.<dt>[EINVAL]<dd>The<i>cmd</i>argument is invalid,or the<i>cmd</i>argument is F_DUPFD and<i>arg</i>is negative or greater than or equal to {OPEN_MAX}, or the<i>cmd</i>argument is F_GETLK, F_SETLK or F_SETLKWand the data pointed to by<i>arg</i>is not valid, or <i>fildes</i> refersto a file that does not support locking.<dt>[EMFILE]<dd>The argument<i>cmd</i>is F_DUPFD and {OPEN_MAX}file descriptors are currently openin the calling process, or no file descriptors greater than or equal to<i>arg</i>are available.<dt>[ENOLCK]<dd>The argument<i>cmd</i>is F_SETLK or F_SETLKWand satisfying the lock or unlock request would resultin the number of locked regions in the system exceedinga system-imposed limit.<dt>[EOVERFLOW]<dd>One of the values to be returned cannot be represented correctly.<dt>[EOVERFLOW]<dd>The<i>cmd</i>argument is F_GETLK, F_SETLK or F_SETLKW and the smallest or, if<i>l_len</i>is non-zero, the largest offset of any byte in the requested segmentcannot be represented correctly in an object of type<b>off_t</b>.</dl><p>The<i>fcntl()</i>function may fail if:<dl compact><dt>[EDEADLK]<dd>The<i>cmd</i>argument is F_SETLKW,the lock is blocked by some lock from another process and puttingthe calling process to sleep,waiting for that lock to become freewould cause a deadlock.</dl></blockquote><h4><a name = "tag_000_004_157"> </a>EXAMPLES</h4><blockquote>None.</blockquote><h4><a name = "tag_000_004_158"> </a>APPLICATION USAGE</h4><blockquote>None.</blockquote><h4><a name = "tag_000_004_159"> </a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_004_160"> </a>SEE ALSO</h4><blockquote><i><a href="close.html">close()</a></i>,<i><a href="exec.html">exec</a></i>,<i><a href="open.html">open()</a></i>,<i><a href="sigaction.html">sigaction()</a></i>,<i><a href="fcntl.h.html"><fcntl.h></a></i>,<i><a href="signal.h.html"><signal.h></a></i>,<i><a href="systypes.h.html"><sys/types.h></a></i>,<i><a href="unistd.h.html"><unistd.h></a></i>.</blockquote><h4>DERIVATION</h4><blockquote>Derived from Issue 1 of the SVID.</blockquote><hr size=2 noshade><center><font size=2>UNIX ® is a registered Trademark of The Open Group.<br>Copyright © 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>⌨️ 快捷键说明
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