DESCRIPTION fcntl() provides for control over descriptors. The argument fildes is a descriptor to be operated on by cmd as follows:
F_DUPFD Return a new descriptor as follows:
o Lowest numbered available descriptor greater than or equal to arg. o Same object references as the original descriptor. o New descriptor shares the same file offset if the object was a file. o Same access mode (read, write or read/write). o Same file status flags (i.e., both file descriptors share the same file status flags). o The close-on-exec flag associated with the new file descriptor is cleared so that the descriptor remains open across an execv(2) system call.
F_DUPFD_CLOEXEC Like F_DUPFD, except that the close-on-exec flag associated with the new file descriptor is set.
F_GETFD Get the flags associated with the file descriptor fildes, as described below (arg is ignored).
F_SETFD Set the file descriptor flags to arg.
F_GETFL Get descriptor status flags, as described below (arg is ignored).
F_SETFL Set descriptor status flags to arg.
F_GETOWN Get the process ID or process group currently receiving SIGIO and SIGURG signals; process groups are returned as negative values (arg is ignored).
F_SETOWN Set the process or process group to receive SIGIO and SIGURG signals; process groups are specified by supplying arg as negative, otherwise arg is interpreted as a process ID.
F_GETPATH Get the path of the file descriptor Fildes. The argument must be a buffer of size MAXPATHLEN or greater.
F_PREALLOCATE Preallocate file storage space. Note: upon success, the space that is allocated can be the size requested, larger than the size requested, or (if the F_ALLOCATEALL flag is not provided) smaller than the space requested.
F_PUNCHHOLE Deallocate a region and replace it with a hole. Subsequent reads of the affected region will return bytes of zeros that are usually not backed by physical blocks. This will not change the actual file size. Holes must be aligned to file system block boundaries. This will fail on file systems that do not support this inter- face.
F_SETSIZE Truncate a file without zeroing space. The calling process must have root privi- leges.
F_RDADVISE Issue an advisory read async with no copy to user.
F_RDAHEAD Turn read ahead off/on. A zero value in arg disables read ahead. A non-zero value in arg turns read ahead on.
F_READBOOTSTRAP Read bootstrap from disk.
F_WRITEBOOTSTRAP Write bootstrap on disk. The calling process must have root privileges.
F_NOCACHE Turns data caching off/on. A non-zero value in arg turns data caching off. A value of zero in arg turns data caching on.
F_LOG2PHYS Get disk device information. Currently this only returns the disk device address that corresponds to the current file offset. Note that the system may return -1 as the disk device address if the file is not backed by physical blocks. This is sub- ject to change.
F_LOG2PHYS_EXT Variant of F_LOG2PHYS that uses the passed in file offset and length.
F_FULLFSYNC Does the same thing as fsync(2) then asks the drive to flush all buffered data to the permanent storage device (arg is ignored). This is currently implemented on HFS, MS-DOS (FAT), and Universal Disk Format (UDF) file systems. The operation may take quite a while to complete. Certain FireWire drives have also been known to ignore the request to flush their buffered data.
F_SETNOSIGPIPE Determines whether a SIGPIPE signal will be generated when a write fails on a pipe or socket for which there is no reader. If arg is non-zero, SIGPIPE generation is disabled for descriptor fildes, while an arg of zero enables it (the default).
F_GETNOSIGPIPE Returns whether a SIGPIPE signal will be generated when a write fails on a pipe or socket for which there is no reader. The semantics of the return value match those of the arg of F_SETNOSIGPIPE.
The flags for the F_GETFD and F_SETFD commands are as follows:
FD_CLOEXEC Close-on-exec; the given file descriptor will be automatically closed in the suc- cessor process image when one of the execv(2) or posix_spawn(2) family of system calls is invoked.
The flags for the F_GETFL and F_SETFL commands are as follows:
O_NONBLOCK Non-blocking I/O; if no data is available to a read call, or if a write operation would block, the read or write call returns -1 with the error EAGAIN.
O_APPEND Force each write to append at the end of file; corresponds to the O_APPEND flag of open(2).
O_ASYNC Enable the SIGIO signal to be sent to the process group when I/O is possible, e.g., upon availability of data to be read.
Several commands are available for doing advisory file locking; they all operate on the following structure:
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 */ };
The commands available for advisory record locking are as follows:
F_GETLK Get the first lock that blocks the lock description pointed to by the third argument, arg, taken as a pointer to a struct flock (see above). The information retrieved overwrites the information passed to fcntl in the flock structure. If no lock is found that would prevent this lock from being created, the structure is left unchanged by this function call except for the lock type which is set to F_UNLCK.
F_SETLK Set or clear a file segment lock according to the lock description pointed to by the third argument, arg, taken as a pointer to a struct flock (see above). F_SETLK is used to establish shared (or read) locks (F_RDLCK) or exclusive (or write) locks, (F_WRLCK), as well as remove either type of lock (F_UNLCK). If a shared or exclusive lock cannot be set, fcntl returns immediately with EAGAIN.
F_SETLKW This command is the same as 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 fcntl is waiting for a region, the fcntl will be inter- rupted if the signal handler has not specified the SA_RESTART (see sigaction(2)).
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.
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.
The value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END to indicate that the relative offset, l_start bytes, will be measured from the start of the file, current position, or end of the file, respectively. The value of l_len is the number of consecutive bytes to be locked. If l_len is nega- tive, the result is undefined. The l_pid field is only used with F_GETLK to return the process ID of the process holding a blocking lock. After a successful F_GETLK request, the value of l_whence is SEEK_SET.
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 l_len is set to zero. If l_whence and l_start point to the beginning of the file, and l_len is zero, the entire file is locked. If an application wishes only to do entire file locking, the flock(2) system call is much more efficient.
There is 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 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 exclu- sive locks, an F_SETLK or an 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.
This interface follows the completely stupid semantics of System V and IEEE Std 1003.1-1988 (``POSIX.1'') that require that all locks associated with a file for a given process are removed when 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 getpwname(3) to retrieve a record, the lock will be lost because getpwname(3) opens, reads, and closes the password database. The database close will release all locks that the process has associ- ated 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 fork(2) function. The flock(2) interface has much more rational last close semantics and allows locks to be inherited by child processes. Flock(2) 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. Note that flock(2) and fcntl(2) locks may be safely used concurrently.
All locks associated with a file for a given process are removed when the process terminates.
A potential for deadlock occurs if a process controlling a locked region is put to sleep by attempt- ing 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 EDEADLK error.
The F_PREALLOCATE command operates on the following structure:
typedef struct fstore { u_int32_t fst_flags; /* IN: flags word */ int fst_posmode; /* IN: indicates offset field */ off_t fst_offset; /* IN: start of the region */ off_t fst_length; /* IN: size of the region */ off_t fst_bytesalloc; /* OUT: number of bytes allocated */ } fstore_t;
The flags (fst_flags) for the F_PREALLOCATE command are as follows:
F_ALLOCATECONTIG Allocate contiguous space.
F_ALLOCATEALL Allocate all requested space or no space at all.
The position modes (fst_posmode) for the F_PREALLOCATE command indicate how to use the offset field. The modes are as follows:
F_PEOFPOSMODE Allocate from the physical end of file.
F_VOLPOSMODE Allocate from the volume offset.
The F_PUNCHHOLE command operates on the following structure:
typedef struct fpunchhole { u_int32_t fp_flags; /* unused */ u_int32_t reserved; /* (to maintain 8-byte alignment) */ off_t fp_offset; /* IN: start of the region */ off_t fp_length; /* IN: size of the region */ } fpunchhole_t;
The F_RDADVISE command operates on the following structure which holds information passed from the user to the system:
struct radvisory { off_t ra_offset; /* offset into the file */ int ra_count; /* size of the read */ };
The F_READBOOTSTRAP and F_WRITEBOOTSTRAP commands operate on the following structure.
typedef struct fbootstraptransfer { off_t fbt_offset; /* IN: offset to start read/write */ size_t fbt_length; /* IN: number of bytes to transfer */ void *fbt_buffer; /* IN: buffer to be read/written */ } fbootstraptransfer_t;
The F_LOG2PHYS command operates on the following structure:
struct log2phys { u_int32_t l2p_flags; /* unused so far */ off_t l2p_contigbytes; /* unused so far */ off_t l2p_devoffset; /* bytes into device */ };
The F_LOG2PHYS_EXT command operates on the same structure as F_LOG2PHYS but treats it as an in/out:
struct log2phys { u_int32_t l2p_flags; /* unused so far */ off_t l2p_contigbytes; /* IN: number of bytes to be queried; OUT: number of contiguous bytes allocated at this position */ off_t l2p_devoffset; /* IN: bytes into file; OUT: bytes into device */ };
If fildes is a socket, then the F_SETNOSIGPIPE and F_GETNOSIGPIPE commands are directly analogous, and fully interoperate with the SO_NOSIGPIPE option of setsockopt(2) and getsockopt(2) respectively.
RETURN VALUES Upon successful completion, the value returned depends on cmd as follows:
F_DUPFD A new file descriptor.
F_GETFD Value of flag (only the low-order bit is defined).
F_GETFL Value of flags.
F_GETOWN Value of file descriptor owner.
other Value other than -1.
Otherwise, a value of -1 is returned and errno is set to indicate the error.
ERRORS The fcntl() system call will fail if:
[EAGAIN] The argument cmd is F_SETLK, the type of lock (l_type) is a shared lock (F_RDLCK) or exclusive lock (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 exclu- sive-locked by another process.
[EACCESS] The argument cmd is either F_SETSIZE or F_WRITEBOOTSTRAP and the calling process does not have root privileges.
[EBADF] Fildes is not a valid open file descriptor.
The argument cmd is F_SETLK or F_SETLKW, the type of lock (l_type) is a shared lock (F_RDLCK), and fildes is not a valid file descriptor open for reading.
The argument cmd is F_SETLK or F_SETLKW, the type of lock (l_type) is an exclusive lock (F_WRLCK), and fildes is not a valid file descriptor open for writing.
The argument cmd is F_PREALLOCATE and the calling process does not have file write permission.
The argument cmd is F_LOG2PHYS or F_LOG2PHYS_EXT and fildes is not a valid file descriptor open for reading.
[EDEADLK] The argument cmd is F_SETLKW, and a deadlock condition was detected.
[EINTR] The argument cmd is F_SETLKW, and the function was interrupted by a signal.
[EINVAL] Cmd is F_DUPFD and arg is negative or greater than the maximum allowable number (see getdtablesize(2)).
The argument cmd is F_GETLK, F_SETLK, or F_SETLKW and the data to which arg points is not valid, or fildes refers to a file that does not support locking.
The argument cmd is F_PREALLOCATE and the fst_posmode is not a valid mode, or when F_PEOFPOSMODE is set and fst_offset is a non-zero value, or when F_VOLPOSMODE is set and fst_offset is a negative or zero value.
The argument cmd is F_PUNCHHOLE and either fp_offset or fp_length are negative, or both fp_offset and fp_length are not multiples of the file system block size.
The argument cmd is either F_READBOOTSTRAP or F_WRITEBOOTSTRAP and the operation was attempted on a non-HFS disk type.
[EMFILE] Cmd is F_DUPFD and the maximum allowed number of file descriptors are currently open.
[EMFILE] The argument cmd is F_DUPED and the maximum number of file descriptors permitted for the process are already in use, or no file descriptors greater than or equal to arg are available.
[ENOLCK] The argument cmd is F_SETLK or 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.
[ENOSPC] The argument cmd is F_PREALLOCATE and either there is no space available on the volume containing fildes or fst_flags contains F_ALLOCATEALL and there is not enough space available on the volume containing fildes to satisfy the entire request.
The argument cmd is F_PUNCHHOLE and there is not enough space available on the volume containing fildes to satisfy the request. As an example, a filesystem that supports cloned files may return this error if punching a hole requires the cre- ation of a clone and there is not enough space available to do so.
[EOVERFLOW] A return value would overflow its representation. For example, cmd is F_GETLK, F_SETLK, or F_SETLKW and the smallest (or, if l_len is non-zero, the largest) off- set of a byte in the requested segment will not fit in an object of type off_t.
[EPERM] The argument cmd is F_PUNCHHOLE and the calling process does not have file write permission.
[ESRCH] Cmd is F_SETOWN and the process ID given as argument is not in use.
SEE ALSO close(2), execve(2), flock(2), getdtablesize(2), open(2), pipe(2), socket(2), setsockopt(2), sigaction(3)
HISTORY The fcntl() function call appeared in 4.2BSD.
4.2 Berkeley Distribution August 24, 2017 4.2 Berkeley Distribution
GETSOCKOPT(2) BSD System Calls Manual GETSOCKOPT(2)
NAME getsockopt, setsockopt -- get and set options on sockets
SYNOPSIS #include <sys/socket.h>
int getsockopt(int socket, int level, int option_name, void *restrict option_value, socklen_t *restrict option_len);
int setsockopt(int socket, int level, int option_name, const void *option_value, socklen_t option_len);
DESCRIPTION getsockopt() and setsockopt() manipulate the options associated with a socket. Options may exist at multiple protocol levels; they are always present at the uppermost ``socket'' level.
When manipulating socket options the level at which the option resides and the name of the option must be specified. To manipulate options at the socket level, level is specified as SOL_SOCKET. To manipulate options at any other level the protocol number of the appropriate protocol controlling the option is supplied. For example, to indicate that an option is to be interpreted by the TCP proto- col, level should be set to the protocol number of TCP; see getprotoent(3).
The parameters option_value and option_len are used to access option values for setsockopt(). For getsockopt() they identify a buffer in which the value for the requested option(s) are to be returned. For getsockopt(), option_len is a value-result parameter, initially containing the size of the buffer pointed to by option_value, and modified on return to indicate the actual size of the value returned. If no option value is to be supplied or returned, option_value may be NULL.
option_name and any specified options are passed uninterpreted to the appropriate protocol module for interpretation. The include file <sys/socket.h> contains definitions for socket level options, described below. Options at other protocol levels vary in format and name; consult the appropriate entries in section 4 of the manual.
Most socket-level options utilize an int parameter for option_value. For setsockopt(), the parameter should be non-zero to enable a boolean option, or zero if the option is to be disabled. SO_LINGER uses a struct linger parameter, defined in <sys/socket.h>, which specifies the desired state of the option and the linger interval (see below). SO_SNDTIMEO and SO_RCVTIMEO use a struct timeval parame- ter, defined in <sys/time.h>.
The following options are recognized at the socket level. Except as noted, each may be examined with getsockopt() and set with setsockopt().
SO_DEBUG enables recording of debugging information SO_REUSEADDR enables local address reuse SO_REUSEPORT enables duplicate address and port bindings SO_KEEPALIVE enables keep connections alive SO_DONTROUTE enables routing bypass for outgoing messages SO_LINGER linger on close if data present SO_BROADCAST enables permission to transmit broadcast messages SO_OOBINLINE enables reception of out-of-band data in band SO_SNDBUF set buffer size for output SO_RCVBUF set buffer size for input SO_SNDLOWAT set minimum count for output SO_RCVLOWAT set minimum count for input SO_SNDTIMEO set timeout value for output SO_RCVTIMEO set timeout value for input SO_TYPE get the type of the socket (get only) SO_ERROR get and clear error on the socket (get only) SO_NOSIGPIPE do not generate SIGPIPE, instead return EPIPE SO_NREAD number of bytes to be read (get only) SO_NWRITE number of bytes written not yet sent by the protocol (get only) SO_LINGER_SEC linger on close if data present with timeout in seconds
SO_DEBUG enables debugging in the underlying protocol modules.
SO_REUSEADDR indicates that the rules used in validating addresses supplied in a bind(2) call should allow reuse of local addresses.
SO_REUSEPORT allows completely duplicate bindings by multiple processes if they all set SO_REUSEPORT before binding the port. This option permits multiple instances of a program to each receive UDP/IP multicast or broadcast datagrams destined for the bound port.
SO_KEEPALIVE enables the periodic transmission of messages on a connected socket. Should the con- nected party fail to respond to these messages, the connection is considered broken and processes using the socket are notified via a SIGPIPE signal when attempting to send data.
SO_DONTROUTE indicates that outgoing messages should bypass the standard routing facilities. Instead, messages are directed to the appropriate network interface according to the network portion of the destination address.
SO_LINGER controls the action taken when unsent messages are queued on socket and a close(2) is per- formed. If the socket promises reliable delivery of data and SO_LINGER is set, the system will block the process on the close attempt until it is able to transmit the data or until it decides it is unable to deliver the information (a timeout period, termed the linger interval, is specified in the setsockopt() call when SO_LINGER is requested). If SO_LINGER is disabled and a close is issued, the system will process the close in a manner that allows the process to continue as quickly as possible.
SO_LINGER_SEC is the same option as SO_LINGER except the linger time is in seconds for SO_LINGER_SEC.
The option SO_BROADCAST requests permission to send broadcast datagrams on the socket. Broadcast was a privileged operation in earlier versions of the system.
With protocols that support out-of-band data, the SO_OOBINLINE option requests that out-of-band data be placed in the normal data input queue as received; it will then be accessible with recv or read calls without the MSG_OOB flag. Some protocols always behave as if this option is set.
SO_SNDBUF and SO_RCVBUF are options to adjust the normal buffer sizes allocated for output and input buffers, respectively. The buffer size may be increased for high-volume connections, or may be decreased to limit the possible backlog of incoming data. The system places an absolute limit on these values.
SO_SNDLOWAT is an option to set the minimum count for output operations. Most output operations process all of the data supplied by the call, delivering data to the protocol for transmission and blocking as necessary for flow control. Nonblocking output operations will process as much data as permitted (subject to flow control) without blocking, but will process no data if flow control does not allow the smaller of the low-water mark value or the entire request to be processed. A select(2) operation testing the ability to write to a socket will return true only if the low-water mark amount could be processed. The default value for SO_SNDLOWAT is set to a convenient size for network effi- ciency, often 2048.
SO_RCVLOWAT is an option to set the minimum count for input operations. In general, receive calls will block until any (non-zero) amount of data is received, then return with the smaller of the amount available or the amount requested. The default value for SO_RCVLOWAT is 1. If SO_RCVLOWAT is set to a larger value, blocking receive calls normally wait until they have received the smaller of the low-water mark value or the requested amount. Receive calls may still return less than the low- water mark if an error occurs, a signal is caught, or the type of data next in the receive queue is different than that returned.
SO_SNDTIMEO is an option to set a timeout value for output operations. It accepts a struct timeval parameter with the number of seconds and microseconds used to limit waits for output operations to complete. If a send operation has blocked for this much time, it returns with a partial count or with the error EWOULDBLOCK if no data were sent. In the current implementation, this timer is restarted each time additional data are delivered to the protocol, implying that the limit applies to output portions ranging in size from the low-water mark to the high-water mark for output.
SO_RCVTIMEO is an option to set a timeout value for input operations. It accepts a struct timeval parameter with the number of seconds and microseconds used to limit waits for input operations to complete. In the current implementation, this timer is restarted each time additional data are received by the protocol, and thus the limit is in effect an inactivity timer. If a receive opera- tion has been blocked for this much time without receiving additional data, it returns with a short count or with the error EWOULDBLOCK if no data were received. The struct timeval parameter must rep- resent a positive time interval; otherwise, setsockopt() returns with the error EDOM.
SO_NOSIGPIPE is an option that prevents SIGPIPE from being raised when a write fails on a socket to which there is no reader; instead, the write to the socket returns with the error EPIPE when there is no reader.
Finally, SO_TYPE, SO_ERROR, SO_NREAD, and SO_NWRITE are options used only with getsockopt().
SO_TYPE returns the type of the socket, such as SOCK_STREAM; it is useful for servers that inherit sockets on startup.
SO_ERROR returns any pending error on the socket and clears the error status. It may be used to check for asynchronous errors on connected datagram sockets or for other asynchronous errors.
SO_NREAD returns the amount of data in the input buffer that is available to be received. For data- gram oriented sockets, SO_NREAD returns the size of the first packet -- this differs from the ioctl() command FIONREAD that returns the total amount of data available.
SO_NWRITE returns the amount of data in the output buffer not yet sent by the protocol.
RETURN VALUES Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
ERRORS The getsockopt() and setsockopt() system calls will succeed unless:
[EBADF] The argument socket is not a valid file descriptor.
[EFAULT] The address pointed to by option_value is not in a valid part of the process address space. For getsockopt(), this error may also be returned if option_len is not in a valid part of the process address space.
[EINVAL] The option is invalid at the level indicated.
[ENOBUFS] Insufficient system resources available for the call to complete.
[ENOMEM] Insufficient memory available for the system call to complete.
[ENOPROTOOPT] The option is unknown at the level indicated.
[ENOTSOCK] The argument socket is not a socket (e.g., a plain file).
The setsockopt() system call will succeed unless:
[EDOM] The argument option_value is out of bounds.
[EISCONN] socket is already connected and a specified option cannot be set while this is the case.
