| flock(2) | System Calls Manual | flock(2) |
flock - impone o elimina un bloqueo de recomendación en un archivo abierto.
Biblioteca Estándar C (libc, -lc)
#include <sys/file.h>
int flock(int fd, int operation);
Impone o elimina un bloqueo de recomendación (advisory lock) sobre el fichero abierto especificado por fd. El parámetro operation puede tener uno de los siguientes valores:
Una llamada a flock() puede bloquearse si otro proceso mantiene un bloqueo incompatible. Para hacer una solicitud no bloqueante, incluya LOCK_NB (mediante operaciones OR) con cualquiera de las operaciones de arriba.
Un fichero no puede tener simultaneamente bloqueo compartido y exclusivo.
Locks created by flock() are associated with an open file description (see open(2)). This means that duplicate file descriptors (created by, for example, fork(2) or dup(2)) refer to the same lock, and this lock may be modified or released using any of these file descriptors. Furthermore, the lock is released either by an explicit LOCK_UN operation on any of these duplicate file descriptors, or when all such file descriptors have been closed.
If a process uses open(2) (or similar) to obtain more than one file descriptor for the same file, these file descriptors are treated independently by flock(). An attempt to lock the file using one of these file descriptors may be denied by a lock that the calling process has already placed via another file descriptor.
Un proceso puede tener solamente un tipo de bloqueo (compartido o exclusivo) sobre un fichero. Las llamadas posteriores a flock() sobre un fichero ya bloqueado convertirán un bloqueo existente al nuevo modo de bloqueo.
Los bloqueos creados con flock() se preservan tras un execve(2).
Un bloqueo compartido o exclusivo puede ponerse sobre un fichero sin importar el modo en el que fue abierto dicho fichero.
En caso de éxito se devuelve cero. En caso de error se devuelve -1, y errno se configura para indicar el error.
4.4BSD (la llamada al sistema flock() apareció por primera vez en 4.2BSD). Una versión de flock(), posiblemente implementada en términos de fcntl(2), aparece en la mayoría de UNIX's.
Since Linux 2.0, flock() is implemented as a system call in its own right rather than being emulated in the GNU C library as a call to fcntl(2). With this implementation, there is no interaction between the types of lock placed by flock() and fcntl(2), and flock() does not detect deadlock. (Note, however, that on some systems, such as the modern BSDs, flock() and fcntl(2) locks do interact with one another.)
flock() coloca bloqueos de recomendación solamente; con los permisos adecuados sobre un fichero, un proceso es libre de ignorar el uso de flock() y realizar E/S sobre el fichero.
flock() and fcntl(2) locks have different semantics with respect to forked processes and dup(2). On systems that implement flock() using fcntl(2), the semantics of flock() will be different from those described in this manual page.
Converting a lock (shared to exclusive, or vice versa) is not guaranteed to be atomic: the existing lock is first removed, and then a new lock is established. Between these two steps, a pending lock request by another process may be granted, with the result that the conversion either blocks, or fails if LOCK_NB was specified. (This is the original BSD behavior, and occurs on many other implementations.)
Up to Linux 2.6.11, flock() does not lock files over NFS (i.e., the scope of locks was limited to the local system). Instead, one could use fcntl(2) byte-range locking, which does work over NFS, given a sufficiently recent version of Linux and a server which supports locking.
Since Linux 2.6.12, NFS clients support flock() locks by emulating them as fcntl(2) byte-range locks on the entire file. This means that fcntl(2) and flock() locks do interact with one another over NFS. It also means that in order to place an exclusive lock, the file must be opened for writing.
Since Linux 2.6.37, the kernel supports a compatibility mode that allows flock() locks (and also fcntl(2) byte region locks) to be treated as local; see the discussion of the local_lock option in nfs(5).
A partir de la versión 5.4, flock() no se despliega sobre SMB. Un archivo con estos bloqueos, no aparecerá bloqueado para usuarios remotos.
Since Linux 5.5, flock() locks are emulated with SMB byte-range locks on the entire file. Similarly to NFS, this means that fcntl(2) and flock() locks interact with one another. Another important side-effect is that the locks are not advisory anymore: any IO on a locked file will always fail with EACCES when done from a separate file descriptor. This difference originates from the design of locks in the SMB protocol, which provides mandatory locking semantics.
Remote and mandatory locking semantics may vary with SMB protocol, mount options and server type. See mount.cifs(8) for additional information.
flock(1), close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3), lslocks(8)
Documentation/filesystems/locks.txt in the Linux kernel source tree (Documentation/locks.txt in older kernels)
La traducción al español de esta página del manual fue creada por Gerardo Aburruzaga García <gerardo.aburruzaga@uca.es>, Juan Piernas <piernas@ditec.um.es>, Miguel Pérez Ibars <mpi79470@alu.um.es> y Marcos Fouces <marcos@debian.org>
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| 4 Diciembre 2022 | Páginas de manual de Linux 6.03 |