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Diffstat (limited to 'lib/libc/sys/fcntl.2')
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diff --git a/lib/libc/sys/fcntl.2 b/lib/libc/sys/fcntl.2 new file mode 100644 index 0000000..a801b60 --- /dev/null +++ b/lib/libc/sys/fcntl.2 @@ -0,0 +1,639 @@ +.\" Copyright (c) 1983, 1993 +.\" The Regents of the University of California. All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" 4. Neither the name of the University nor the names of its contributors +.\" may be used to endorse or promote products derived from this software +.\" without specific prior written permission. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND +.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE +.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\" SUCH DAMAGE. +.\" +.\" @(#)fcntl.2 8.2 (Berkeley) 1/12/94 +.\" $FreeBSD$ +.\" +.Dd September 28, 2009 +.Dt FCNTL 2 +.Os +.Sh NAME +.Nm fcntl +.Nd file control +.Sh LIBRARY +.Lb libc +.Sh SYNOPSIS +.In fcntl.h +.Ft int +.Fn fcntl "int fd" "int cmd" "..." +.Sh DESCRIPTION +The +.Fn fcntl +system call provides for control over descriptors. +The argument +.Fa fd +is a descriptor to be operated on by +.Fa cmd +as described below. +Depending on the value of +.Fa cmd , +.Fn fcntl +can take an additional third argument +.Fa "int arg" . +.Bl -tag -width F_GETOWNX +.It Dv F_DUPFD +Return a new descriptor as follows: +.Pp +.Bl -bullet -compact -offset 4n +.It +Lowest numbered available descriptor greater than or equal to +.Fa arg . +.It +Same object references as the original descriptor. +.It +New descriptor shares the same file offset if the object +was a file. +.It +Same access mode (read, write or read/write). +.It +Same file status flags (i.e., both file descriptors +share the same file status flags). +.It +The close-on-exec flag associated with the new file descriptor +is set to remain open across +.Xr execve 2 +system calls. +.El +.It Dv F_DUP2FD +It is functionally equivalent to +.Bd -literal -offset indent +dup2(fd, arg) +.Ed +.Pp +The +.Dv F_DUP2FD +constant is not portable, so it should not be used if portability is needed. +Use +.Fn dup2 +instead. +.It Dv F_GETFD +Get the close-on-exec flag associated with the file descriptor +.Fa fd +as +.Dv FD_CLOEXEC . +If the returned value ANDed with +.Dv FD_CLOEXEC +is 0, +the file will remain open across +.Fn exec , +otherwise the file will be closed upon execution of +.Fn exec +.Fa ( arg +is ignored). +.It Dv F_SETFD +Set the close-on-exec flag associated with +.Fa fd +to +.Fa arg , +where +.Fa arg +is either 0 or +.Dv FD_CLOEXEC , +as described above. +.It Dv F_GETFL +Get descriptor status flags, as described below +.Fa ( arg +is ignored). +.It Dv F_SETFL +Set descriptor status flags to +.Fa arg . +.It Dv F_GETOWN +Get the process ID or process group +currently receiving +.Dv SIGIO +and +.Dv SIGURG +signals; process groups are returned +as negative values +.Fa ( arg +is ignored). +.It Dv F_SETOWN +Set the process or process group +to receive +.Dv SIGIO +and +.Dv SIGURG +signals; +process groups are specified by supplying +.Fa arg +as negative, otherwise +.Fa arg +is interpreted as a process ID. +.El +.Pp +The flags for the +.Dv F_GETFL +and +.Dv F_SETFL +flags are as follows: +.Bl -tag -width O_NONBLOCKX +.It Dv O_NONBLOCK +Non-blocking I/O; if no data is available to a +.Xr read 2 +system call, or if a +.Xr write 2 +operation would block, +the read or write call returns -1 with the error +.Er EAGAIN . +.It Dv O_APPEND +Force each write to append at the end of file; +corresponds to the +.Dv O_APPEND +flag of +.Xr open 2 . +.It Dv O_DIRECT +Minimize or eliminate the cache effects of reading and writing. +The system +will attempt to avoid caching the data you read or write. +If it cannot +avoid caching the data, it will minimize the impact the data has on the cache. +Use of this flag can drastically reduce performance if not used with care. +.It Dv O_ASYNC +Enable the +.Dv SIGIO +signal to be sent to the process group +when I/O is possible, e.g., +upon availability of data to be read. +.El +.Pp +Several commands are available for doing advisory file locking; +they all operate on the following structure: +.Bd -literal +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 */ + int l_sysid; /* remote system id or zero for local */ +}; +.Ed +The commands available for advisory record locking are as follows: +.Bl -tag -width F_SETLKWX +.It Dv F_GETLK +Get the first lock that blocks the lock description pointed to by the +third argument, +.Fa arg , +taken as a pointer to a +.Fa "struct flock" +(see above). +The information retrieved overwrites the information passed to +.Fn fcntl +in the +.Fa flock +structure. +If no lock is found that would prevent this lock from being created, +the structure is left unchanged by this system call except for the +lock type which is set to +.Dv F_UNLCK . +.It Dv F_SETLK +Set or clear a file segment lock according to the lock description +pointed to by the third argument, +.Fa arg , +taken as a pointer to a +.Fa "struct flock" +(see above). +.Dv F_SETLK +is used to establish shared (or read) locks +.Pq Dv F_RDLCK +or exclusive (or write) locks, +.Pq Dv F_WRLCK , +as well as remove either type of lock +.Pq Dv F_UNLCK . +If a shared or exclusive lock cannot be set, +.Fn fcntl +returns immediately with +.Er EAGAIN . +.It Dv F_SETLKW +This command is the same as +.Dv 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 +.Fn fcntl +is waiting for a region, the +.Fn fcntl +will be interrupted if the signal handler has not specified the +.Dv SA_RESTART +(see +.Xr sigaction 2 ) . +.It Dv F_READAHEAD +Set or clear the read ahead amount for sequential access to the third +argument, +.Fa arg , +which is rounded up to the nearest block size. +A zero value in +.Fa arg +turns off read ahead. +.It Dv F_RDAHEAD +Equivalent to Darwin counterpart which sets read ahead amount of 128KB +when the third argument, +.Fa arg +is non-zero. +A zero value in +.Fa arg +turns off read ahead. +.El +.Pp +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 descriptor was not +opened with read access. +.Pp +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. +.Pp +The value of +.Fa l_whence +is +.Dv SEEK_SET , +.Dv SEEK_CUR , +or +.Dv SEEK_END +to indicate that the relative offset, +.Fa l_start +bytes, will be measured from the start of the file, +current position, or end of the file, respectively. +The value of +.Fa l_len +is the number of consecutive bytes to be locked. +If +.Fa l_len +is negative, +.Fa l_start +means end edge of the region. +The +.Fa l_pid +and +.Fa l_sysid +fields are only used with +.Dv F_GETLK +to return the process ID of the process holding a blocking lock and +the system ID of the system that owns that process. +Locks created by the local system will have a system ID of zero. +After a successful +.Dv F_GETLK +request, the value of +.Fa l_whence +is +.Dv SEEK_SET . +.Pp +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 extend to the largest possible value of the +file offset for that file if +.Fa l_len +is set to zero. +If +.Fa l_whence +and +.Fa l_start +point to the beginning of the file, and +.Fa l_len +is zero, the entire file is locked. +If an application wishes only to do entire file locking, the +.Xr flock 2 +system call is much more efficient. +.Pp +There is at most one type of lock set for each byte in the file. +Before a successful return from an +.Dv F_SETLK +or an +.Dv F_SETLKW +request when the calling process has previously existing locks +on bytes in the region specified by the request, +the previous lock type for each byte in the specified +region is replaced by the new lock type. +As specified above under the descriptions +of shared locks and exclusive locks, an +.Dv F_SETLK +or an +.Dv 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. +.Pp +This interface follows the completely stupid semantics of System V and +.St -p1003.1-88 +that require that all locks associated with a file for a given process are +removed when +.Em any +file descriptor for that file is closed by that process. +This semantic means that applications 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 +.Xr getpwnam 3 +to retrieve a record, +the lock will be lost because +.Xr getpwnam 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 inherited by a child process created using the +.Xr fork 2 +system call. +The +.Xr flock 2 +interface has much more rational last close semantics and +allows locks to be inherited by child processes. +The +.Xr flock 2 +system call is recommended for applications that want to ensure the integrity +of their locks when using library routines or wish to pass locks +to their children. +.Pp +The +.Fn fcntl , +.Xr flock 2 , +and +.Xr lockf 3 +locks are compatible. +Processes using different locking interfaces can cooperate +over the same file safely. +However, only one of such interfaces should be used within +the same process. +If a file is locked by a process through +.Xr flock 2 , +any record within the file will be seen as locked +from the viewpoint of another process using +.Fn fcntl +or +.Xr lockf 3 , +and vice versa. +Note that +.Fn fcntl F_GETLK +returns \-1 in +.Fa l_pid +if the process holding a blocking lock previously locked the +file descriptor by +.Xr flock 2 . +.Pp +All locks associated with a file for a given process are +removed when the process terminates. +.Pp +All locks obtained before a call to +.Xr execve 2 +remain in effect until the new program releases them. +If the new program does not know about the locks, they will not be +released until the program exits. +.Pp +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 process. +This implementation detects that sleeping until a locked region is unlocked +would cause a deadlock and fails with an +.Er EDEADLK +error. +.Sh RETURN VALUES +Upon successful completion, the value returned depends on +.Fa cmd +as follows: +.Bl -tag -width F_GETOWNX -offset indent +.It Dv F_DUPFD +A new file descriptor. +.It Dv F_DUP2FD +A file descriptor equal to +.Fa arg . +.It Dv F_GETFD +Value of flag (only the low-order bit is defined). +.It Dv F_GETFL +Value of flags. +.It Dv F_GETOWN +Value of file descriptor owner. +.It other +Value other than -1. +.El +.Pp +Otherwise, a value of -1 is returned and +.Va errno +is set to indicate the error. +.Sh ERRORS +The +.Fn fcntl +system call will fail if: +.Bl -tag -width Er +.It Bq Er EAGAIN +The argument +.Fa cmd +is +.Dv F_SETLK , +the type of lock +.Pq Fa l_type +is a shared lock +.Pq Dv F_RDLCK +or exclusive lock +.Pq Dv 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 already shared-locked or +exclusive-locked by another process. +.It Bq Er EBADF +The +.Fa fd +argument +is not a valid open file descriptor. +.Pp +The argument +.Fa cmd +is +.Dv F_DUP2FD , +and +.Fa arg +is not a valid file descriptor. +.Pp +The argument +.Fa cmd +is +.Dv F_SETLK +or +.Dv F_SETLKW , +the type of lock +.Pq Fa l_type +is a shared lock +.Pq Dv F_RDLCK , +and +.Fa fd +is not a valid file descriptor open for reading. +.Pp +The argument +.Fa cmd +is +.Dv F_SETLK +or +.Dv F_SETLKW , +the type of lock +.Pq Fa l_type +is an exclusive lock +.Pq Dv F_WRLCK , +and +.Fa fd +is not a valid file descriptor open for writing. +.It Bq Er EDEADLK +The argument +.Fa cmd +is +.Dv F_SETLKW , +and a deadlock condition was detected. +.It Bq Er EINTR +The argument +.Fa cmd +is +.Dv F_SETLKW , +and the system call was interrupted by a signal. +.It Bq Er EINVAL +The +.Fa cmd +argument +is +.Dv F_DUPFD +and +.Fa arg +is negative or greater than the maximum allowable number +(see +.Xr getdtablesize 2 ) . +.Pp +The argument +.Fa cmd +is +.Dv F_GETLK , +.Dv F_SETLK +or +.Dv F_SETLKW +and the data to which +.Fa arg +points is not valid. +.It Bq Er EMFILE +The argument +.Fa cmd +is +.Dv F_DUPFD +or +.Dv F_DUP2FD +and the maximum number of file descriptors permitted for the +process are already in use, +or no file descriptors greater than or equal to +.Fa arg +are available. +.It Bq Er ENOLCK +The argument +.Fa cmd +is +.Dv F_SETLK +or +.Dv 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. +.It Bq Er EOPNOTSUPP +The argument +.Fa cmd +is +.Dv F_GETLK , +.Dv F_SETLK +or +.Dv F_SETLKW +and +.Fa fd +refers to a file for which locking is not supported. +.It Bq Er EOVERFLOW +The argument +.Fa cmd +is +.Dv F_GETLK , +.Dv F_SETLK +or +.Dv F_SETLKW +and an +.Fa off_t +calculation overflowed. +.It Bq Er EPERM +The +.Fa cmd +argument +is +.Dv F_SETOWN +and +the process ID or process group given as an argument is in a +different session than the caller. +.It Bq Er ESRCH +The +.Fa cmd +argument +is +.Dv F_SETOWN +and +the process ID given as argument is not in use. +.El +.Pp +In addition, if +.Fa fd +refers to a descriptor open on a terminal device (as opposed to a +descriptor open on a socket), a +.Fa cmd +of +.Dv F_SETOWN +can fail for the same reasons as in +.Xr tcsetpgrp 3 , +and a +.Fa cmd +of +.Dv F_GETOWN +for the reasons as stated in +.Xr tcgetpgrp 3 . +.Sh SEE ALSO +.Xr close 2 , +.Xr dup2 2 , +.Xr execve 2 , +.Xr flock 2 , +.Xr getdtablesize 2 , +.Xr open 2 , +.Xr sigaction 2 , +.Xr lockf 3 , +.Xr tcgetpgrp 3 , +.Xr tcsetpgrp 3 +.Sh STANDARDS +The +.Dv F_DUP2FD +constant is non portable. +It is provided for compatibility with AIX and Solaris. +.Sh HISTORY +The +.Fn fcntl +system call appeared in +.Bx 4.2 . +.Pp +The +.Dv F_DUP2FD +constant first appeared in +.Fx 7.1 . |
