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fcntl(2) System Calls Manual fcntl(2) NAME fcntl, dup, dup2 - Control open file descriptors SYNOPSIS #include <fcntl.h> #include <sys/types.h> #include <unistd.h> int fcntl ( int filedes, int request [ , int argument | struct flock *argument | advfs_opT *argument ] ); int dup( int filedes ); int dup2( int old, int new ); STANDARDS Interfaces documented on this reference page conform to industry stan- dards as follows: dup(), dup2(): XPG4 fcntl(): XPG4, XPG4-UNIX Refer to the standards(5) reference page for more information about in- dustry standards and associated tags. PARAMETERS Specifies an open file descriptor obtained from a successful open(), fcntl(), or pipe() function. Specifies the operation to be performed. Specifies a variable that depends on the value of the request parame- ter. Specifies an open file descriptor that is returned by the dup2() function. Specifies an open file descriptor that is returned by the dup2() function. DESCRIPTION The fcntl() function performs controlling operations on the open file specified by the filedes parameter. When the fcntl(), dup() and dup2() functions need to block, only the calling thread is suspended rather than all threads in the calling process. The following are values for the request parameter: Returns a new file descriptor as follows: Lowest numbered available file descriptor greater than or equal to the argument parameter, taken as type int. Same object references as the original file. Same file pointer as the original file. (That is, both file descriptors share one file pointer if the object is a file). Same access mode (read, write, or read- write). Same file status flags. (That is, both file descriptors share the same file status flags). The close-on-exec flag (FD_CLOEXEC bit) associated with the new file descriptor is cleared so that the file will remain open across exec functions. Gets the value of the close- on-exec flag associated with the file descriptor filedes. File de- scriptor flags are associated with a single file descriptor and do not affect other file descriptors that refer to the same file. The argu- ment parameter is ignored. Sets the close-on-exec flag associated with the filedes parameter to the value of the argument parameter, taken as type int. If the argument parameter is 0 (zero), the file remains open across the exec functions. If the argument parameter is FD_CLOEXEC, the file is closed on successful execution of the next exec function. Gets the file status flags and file access modes for the file referred to by the filedes parameter. The file access modes can be extracted by using the mask O_ACCMODE on the return value. File status flags and file access modes are associated with the file description and do not affect other file descriptors that refer to the same file with differ- ent open file descriptions. The argument parameter is ignored. Sets the file status flags to the argument parameter, taken as type int, for the file to which the filedes parameter refers. The file access mode is not changed. [XPG4-UNIX] If filedes refers to a socket, gets the process or process group ID currently receiving SIGURG signals when out-of-band data is available. Positive values indicate a process ID; negative values, other than -1, indicate a process group ID. [XPG4-UNIX] If filedes refers to a socket, sets the process or process group ID to receive SIGURG signals when out-of-band data is available, using the value of the argument parameter, taken as type int. Positive values indicate a process ID; negative values, other than -1, indicate a process group ID. [DIGITAL] Is used by the network lock daemon (rpc.lockd(8)) to communicate with the NFS server kernel to handle locks on the NFS files. [DIGITAL] Is used by the network lock daemon (rpc.lockd(8)) to communicate with the NFS server kernel to handle locks on the NFS files. [DIGITAL] Is used by the network lock daemon (rpc.lockd(8)) to communicate with the NFS server kernel to handle locks on the NFS files. [DIGITAL] Is used by the network lock daemon (rpc.lockd(8)) to communicate with the NFS server kernel to handle locks on the NFS files. [DIGITAL] Performs AdvFS-specific operations on AdvFS files. The value expects that the argument parameter is a pointer to a ad- vfs_opT structure as described in the <fcntl.h> header file. The following values for the request parameter are available for record locking: Gets the first lock that blocks the lock description pointed to by the argument parameter, taken as a pointer to type struct flock. The information retrieved overwrites the information passed to the fc- ntl() function in the flock structure. If no lock is found that would prevent this lock from being created, then the structure is left un- changed except for the lock type, which is set to F_UNLCK. Sets or clears a file segment lock according to the lock description pointed to by argument, taken as a pointer to type struct flock. F_SETLK is used to establish shared locks (F_RDLCK), or exclusive locks (F_WRLCK), as well as remove either type of lock (F_UNLCK). If a shared (read) or exclusive (write) lock cannot be set, the fcntl() function returns im- mediately with a value of -1. Same as F_SETLK except that if a shared or exclusive lock is blocked by other locks, the process will wait un- til it is unblocked. If a signal is received while fcntl() is waiting for a region, the function is interrupted, -1 is returned, and errno is set to [EINTR]. [DIGITAL] The O_NDELAY and O_NONBLOCK requests affect only operations against file descriptors derived from the same open() function. In BSD, these apply to all file descriptors that refer to the object. When a shared lock is set on a segment of a file, other processes are able to 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 descriptor was not opened with write access. The flock structure describes the type (l_type), starting offset (l_whence), relative offset (l_start), size (l_len) and process ID (l_pid) of the segment of the file to be affected. The value of l_whence is set to SEEK_SET, SEEK_CUR or SEEK_END, to in- dicate that the relative offset l_start bytes is measured from the start of the file, from the current position, or from the end of the file, respectively. The value of l_len is the number of consecutive bytes to be locked. The l_len value may be negative (where the defini- tion of off_t permits negative values of l_len). 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 becomes SEEK_SET. If l_len is positive, the area affected starts at l_start and ends at l_start + l_len - 1. If l_len is negative, the area affected starts at l_start + l_len and ends at l_start - 1. Locks may start and extend beyond the current end of a file, but may not be negative relative to the beginning of the file. If l_len is set to 0 (zero), a lock may be set to always extend to the largest possible value of the file offset for that file. If such a lock also has l_start set to 0 (zero) and l_whence is set to SEEK_SET, the whole file is locked. Changing or unlocking a portion from the middle of a larger locked seg- ment leaves a smaller segment at either end. Locking a segment that is already locked by the calling process causes the old lock type to be removed and the new lock type to take effect. All locks associated with a file for a given process are removed when a file descriptor for that file is closed by that process or the process holding that file descriptor terminates. Locks are not inherited by a child process in a fork() function. [DIGITAL] The fcntl() record locks are implemented in the kernel for local locks and throughout the network by the network lock daemon (rpc.lockd(8)) for remote locks on NFS files. If the file server crashes and has to be rebooted, the lock daemon attempts to recover all locks that were associated with that server. If a lock cannot be re- claimed, the process that held the lock is issued a SIGLOST signal. [DIGITAL] In order to maintain consistency in the network case, data must not be cached on client machines. For this reason, file buffering for an NFS file is turned off when the first lock is attempted on the file. Buffering remains off as long as the file is open. Programs that do I/O buffering in the user address space, however, may have inconsis- tent results. The standard I/O package, for instance, is a common source of unexpected buffering. [DIGITAL] If a regular file has enforced record locking enabled, record locks on the file will affect calls to other calls, including creat(), open(), read(), write(), truncate(), and ftruncate(). A potential for deadlock occurs if a process controlling a locked re- gion is put to sleep by attempting to lock another process' locked re- gion. If the system detects that sleeping until a locked region is un- locked would cause a deadlock, the fcntl() function fails with an [EDEADLK] error. [DIGITAL] The F_ADVFS_OP request is used to perform operations on AdvFS files which do not have an analog on other file systems. The argument parameter is expected to be a pointer to an advfs_opT structure. The version field of the advfs_opT structure must be specified and set to ADVFS_OP_CURR_VERSION. It provides an in-memory check for the version of the structure. The operation field of the advfs_opT structure spec- ifies the general kind of operation. The action field of the advfs_opT structure refines the operation field in order to specify more exactly the operation to be performed. If the action specified is AD- VFS_GET_INFO, the info_buf and info_buf_size fields also must be used. The info_buf field is a pointer to the buffer that will contain the re- quested information. The info_buf_size field specifies the size of the buffer. See the <fcntl.h> header file for a description of the opera- tions, actions, and values returned by ADVFS_GET_INFO. To use the F_ADVFS_OP request on AdvFS files that are mounted across NFS, the NFS property list daemon, proplistd, must be running on the NFS client and the fileset must have been mounted on the client using the proplist option. The following code fragment shows how to activate atomic write data logging on an AdvFS file: . . . advfs_opT myop; int fd; . . . myop.version = ADVFS_OP_CURR_VERSION; myop.operation = AD- VFS_AW_DATA_LOGGING; myop.action = ADVFS_ACTIVATE; ret = fcntl(fd, F_ADVFS_OP, &myop); . . . The following code fragment shows how to query the current I/O mode for an AdvFS file: . . . advfs_opT myop; int fd; int io_mode; . . . myop.version = ADVFS_OP_CURR_VERSION; myop.operation = ad- vfs_aw_data_logging; myop.action = ADVFS_GET_INFO; myop.info_buf = &io_mode; myop.info_buf_size = sizeof(int): ret = fcntl(fd, F_ADVFS_OP, &myop): if (ret) { perror("fcntl failed"); } if (io_mode == ADVFS_ASYNC_IO) printf("I/O mode is asynchronous.\n"); else if (io_mode == AD- VFS_DATA_LOGGING_IO) printf("I/O mode is atomic write data logging.\n"); else if (io_mode == ADVFS_SYNC_IO) printf("I/O mode is forced synchronous writes.\n"); . . . See chfile(8) for information on the file's I/O mode. Note that the previous example is attempting to determine the I/O mode set- ting. The setting could also have been specified as ADVFS_SYNC_WRITE. NOTES The dup(filedes) function is equivalent to fcntl(filedes, F_DUPFD, 0). The dup2(oldfiledes, newfiledes) function has similar functionality to: close(newfiledes) fcntl(oldfiledes, F_DUPFD, newfiledes) The file locks set by the fcntl() and lockf() functions do not interact in any way with the file locks set by the flock()function. If a process sets an exclusive lock on a file using the fcntl() or lockf() function, the lock will not affect any process that is setting or clearing locks on the same file using the flock() function. It is therefore possible for an inconsistency to arise if a file is locked by different processes using flock() and fcntl(). (The fcntl() and lockf() functions use the same mechanism for record locking.) RETURN VALUES Upon successful completion, the value returned depends on the value of the request parameter as follows: Returns a new file descriptor. Re- turns FD_CLOEXEC or 0 (zero). Returns a value other than -1. Returns the value of file status flags and access modes. (The return value will not be negative.) Returns a value other than -1. Returns a value other than -1. Returns a value other than -1. [XPG4-UNIX] Returns the value of the socket owner process or process group; this will not be -1. [XPG4-UNIX] Returns a value other than -1. Returns a value other than -1. If the fcntl() function fails, a value of -1 is returned and errno is set to indicate the error. ERRORS The fcntl() function sets errno to the specified values for the follow- ing conditions: The request parameter is F_SETLK; the type of lock (l_type) is a shared (F_RDLCK) or exclusive (F_WRLCK) lock, 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. The filedes or old parameter is not a valid open file descriptor and the argument parameter file descriptor is negative or greater than or equal to the per-process limit. The request parameter is F_SETLK or F_SETLKW, the type of lock (l_type) is a shared lock (F_RDLCK), and filedes is not a valid file descriptor open for reading. The type of lock (l_type) is an exclusive lock (F_WRLCK), and filedes is not a valid file descriptor open for writing. The request parameter is F_SETLKW, the lock is blocked by some lock from another process and putting the calling process to sleep, and waiting for that lock to become free would cause a deadlock. The argument parameter is an invalid address. The request para- meter is F_DUPFD and the argument parameter is negative or greater than or equal to OPEN_MAX. [DIGITAL] Either the OPEN_MAX value or the per-process soft de- scriptor limit is checked. An illegal value was provided for the request parameter. The request parameter is F_GETLK, F_SETLK, or F_SETLKW and the data pointed to by argument is invalid, or filedes refers to a file that does not support locking. [DIGITAL] The F_ADVFS_OP request was performed and the fd re- ferred to a socket; or the action was ADVFS_GET_INFO and the info_buf_size was zero; or the operation to be performed was un- defined; or the action to be taken was undefined. The request parameter is F_DUPFD and too many or OPEN_MAX file descriptors are currently open in the calling process, or no file descrip- tors greater than or equal to argument are available. [DIGITAL] Either the OPEN_MAX value or the per-process soft descriptor limit is checked. [DIGITAL] The value of the re- quest parameter is F_SETOWN and the process ID given as argument is not in use. The request parameter is F_SETLKW and the fc- ntl() function was interrupted by a signal which was caught. The request parameter is F_SETLK or F_SETLKW and satisfying the lock or unlock request would exceed the configurable system limit of NLOCK_RECORD. [DIGITAL] The file is an NFS file, and either the client or server system is not running rpc.lockd, which is the NFS lock manager. [DIGITAL] The system was unable to allocate kernel memory for the requested file descriptor. [DIGITAL] The re- quest parameter is F_SETOWN and the calling process does not have a controlling terminal, the file is not the controlling terminal, or the controlling terminal is no longer associated with the calling process' session. [DIGITAL] The request para- meter is F_SETOWN and the argument specified by the pgrp_id is valid, but matches a process ID or process group ID of a process in another session. The dup() and dup2() functions set errno to the specified values for the following conditions: The request parameter is F_SETLK; the type of lock (l_type) is a read (F_RDLCK) lock, and the segment of a file to be locked is already write-locked by another process; or the type is a write (F_WRLCK) lock and the segment of a file to be locked is already read- or write-locked by another process. The filedes or old parameter is not a valid open file descriptor or the new parameter file descrip- tor is negative or greater than OPEN_MAX. [DIGITAL] Either the OPEN_MAX value or the per-process soft de- scriptor limit is checked. The dup2() function was interrupted by a signal which was caught. The number of file descriptors exceeds OPEN_MAX or the per-process limit, or there is no file descriptor above the value of the new parameter. [DIGITAL] The file descriptor specified by filedes is on a remote machine and the link to that machine is no longer active. [DIGITAL] The system was unable to allocate kernel memory for the requested file descriptor. Because in the future the variable errno is set to EAGAIN rather than EACCES when a section of a file is already locked by another process, portable application programs should expect and test for either value. RELATED INFORMATION Functions: close(2), exec(2), lockf(3), open(2), read(2), truncate(2), write(2) creat(2), dup(2), fork(2), pipe(2), getdtablesize(2) Commands: rpc.lockd(8), rpc.statd(8) Standards: standards(5) delim off fcntl(2)