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lockf(3) Library Functions Manual lockf(3) NAME lockf - Controls open file descriptors SYNOPSIS #include <unistd.h> int lockf( int filedes, int request, off_t size ); STANDARDS Interfaces documented on this reference page conform to industry stan- dards as follows: lockf(): XPG4-UNIX Refer to the standards(5) reference page for more information about in- dustry standards and associated tags. PARAMETERS Specifies the file to which the lock is to be applied or removed. The file descriptor is returned by a successful open() or fcntl() function. Specifies one of the following constants for the lockf() function: Un- locks a previously locked region in the file. Locks the region for ex- clusive use. This request causes the calling process to sleep if the region overlaps a locked region, and to resume when it is granted the lock. Same as F_LOCK, except that the request returns an error if the region overlaps a locked region. Tests to see if another process has already locked a region. The lockf() function returns 0 (zero) if the region is unlocked. If the region is locked, then -1 is returned and errno is set to [EACCES]. The number of bytes to be locked or unlocked for the lockf() function. The region starts at the current location in the open file and extends forward if size is positive and backward if size is negative. If the size parameter is 0 (zero), the region starts at the current location and extends forward to the maximum possible file size, including the unallocated space after the end of the file. DESCRIPTION The lockf() function locks and unlocks sections of an open file. Un- like the fcntl() function, however, its interface is limited to setting only write (exclusive) locks. Although the lockf() and fcntl() functions are different, the implemen- tations are fully integrated. Therefore, locks obtained from one func- tion are honored and enforced by the other lock function. Each lock is either an enforced lock or an advisory lock, and must also be either a read lock or a write lock. Locks on a file are advisory or enforced depending on the mode of the file (see the chmod() function.) A given file can have advisory or en- forced locks, but not both. See the sys/mode.h header file for a de- scription of file attributes. When a process holds an enforced exclusive lock on a section of a file, no other process can access that section of the file with the read() or write() functions. In addition, the open(), truncate(), and ftruncate() functions cannot truncate the locked section of the file. If another process attempts to read or modify the locked section of the file, it sleeps until the section is unlocked or returns with an error indica- tion. The file descriptor on which an exclusive lock is being placed must have been opened with write access. Some general rules about file locking include the following: Changing or unlocking part of a file in the middle of a locked section leaves two smaller sections locked at each end of the originally locked sec- tion. All locks associated with a file for a given process are removed when the process closes any file descriptor for that file. Locks are not inherited by a child process after running a fork() function. Locks can start and extend beyond the current end of a file, but cannot be negative relative to the beginning of the file. A lock can be set to extend to the end of the file by setting the l_len field to 0 (zero). If a lock is specified with the l_start field set to 0 and the l_whence field set to SEEK_SET, the whole file is locked. Advisory file region locking is supported over NFS, provided the lock- ing daemon (rpc.lockd) and status monitor daemon (rpc.statd) are run- ning. NOTES Buffered I/O does not work properly when used with file locking. Do not use the standard I/O package routines on files that will be locked. Deadlocks due to file locks in a distributed system are not always de- tected. When such deadlocks are possible, the programs requesting the locks should set time-out timers. RETURN VALUES Upon successful completion, a value of 0 (zero) is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error. ERRORS If the lockf() function fails, errno may be set to one of the following values: The file region is locked and F_TEST was specified; or the file region is locked and F_TLOCK was specified. The filedes parameter is not a valid open file descriptor; or the request parameter is F_LOCK or F_TLOCK and filedes is not a valid file descriptor open for writing. The lock is blocked by some lock from another process. Putting the calling process to sleep while waiting for that lock to become free would cause a deadlock. The request parameter is F_TLOCK and the lockf() function was interrupted by a signal which was caught. The re- quest parameter is not valid. The request parameter is F_LOCK or F_TLOCK and satisfying the lock or unlock request would exceed the con- figurable system limit of NLOCK_RECORD. [Digital] If using NFS, the server is out of resources or the file handle is stale. RELATED INFORMATION Functions: chmod(2), close(2), exec(2), fcntl(2), flock(2), fork(2), open(2), read(2), write(2) Commands: rpc.lockd(8), rpc.statd(8) Standards: standards(5) delim off lockf(3)