Standard C Library Functions pthreadmutexlock(3C)
NAME
pthreadmutexlock, pthreadmutextrylock,
pthreadmutexunlock - lock or unlock a mutex
SYNOPSIS
cc -mt [ flag... ] file... -lpthread [ library... ]
#include
int pthreadmutexlock(pthreadmutext *mutex);
int pthreadmutextrylock(pthreadmutext *mutex);
int pthreadmutexunlock(pthreadmutext *mutex);
DESCRIPTION
The mutex object referenced by mutex is locked by calling
pthreadmutexlock(). If the mutex is already locked, the
calling thread blocks until the mutex becomes available.
This operation returns with the mutex object referenced by
mutex in the locked state with the calling thread as its
owner.
If the mutex type is PTHREADMUTEXNORMAL, deadlock detec-
tion is not provided. Attempting to relock the mutex causes
deadlock. If a thread attempts to unlock a mutex that it has
not locked or a mutex that is unlocked, undefined behavior
results.
If the mutex type is PTHREADMUTEXERORCHECK, then error
checking is provided. If a thread attempts to relock a mutex
that it has already locked, an error will be returned. If a
thread attempts to unlock a mutex that it has not locked or
a mutex which is unlocked, an error will be returned.
If the mutex type is PTHREADMUTEXRECURSIVE, then the
mutex maintains the concept of a lock count. When a thread
successfully acquires a mutex for the first time, the lock
count is set to 1. Every time a thread relocks this mutex,
the lock count is incremented by one. Each time the thread
unlocks the mutex, the lock count is decremented by one.
When the lock count reaches 0, the mutex becomes available
for other threads to acquire. If a thread attempts to
unlock a mutex that it has not locked or a mutex that is
unlocked, an error will be returned.
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Standard C Library Functions pthreadmutexlock(3C)
If the mutex type is PTHREADMUTEXDEFAULT, attempting to
recursively lock the mutex results in undefined behavior.
Attempting to unlock the mutex if it was not locked by the
calling thread results in undefined behavior. Attempting to
unlock the mutex if it is not locked results in undefined
behavior.
The pthreadmutextrylock() function is identical to
pthreadmutexlock() except that if the mutex object refer-
enced by mutex is currently locked (by any thread, including
the current thread), the call fails immediately with EBUSY.
The pthreadmutexunlock() function releases the mutex
object referenced by mutex. The manner in which a mutex is
released is dependent upon the mutex's type attribute. If
there are threads blocked on the mutex object referenced by
mutex when pthreadmutexunlock() is called, resulting in
the mutex becoming available, the scheduling policy is used
to determine which thread will acquire the mutex. (In the
case of PTHREADMUTEXRECURSIVE mutexes, the mutex becomes
available when the count reaches 0 and the calling thread
no longer has any locks on this mutex.)
If a signal is delivered to a thread waiting for a mutex,
upon return from the signal handler the thread resumes wait-
ing for the mutex as if it was not interrupted.
RETURN VALUES
If successful, the pthreadmutexlock() and
pthreadmutexunlock() functions return 0. Otherwise, an
error number is returned to indicate the error.
The pthreadmutextrylock() function returns 0 if a lock
on the mutex object referenced by mutex is acquired. Other-
wise, an error number is returned to indicate the error.
ERORS
The pthreadmutexlock() and pthreadmutextrylock() func-
tions will fail if:
EAGAIN The mutex could not be acquired because the max-
imum number of recursive locks for mutex has been
exceeded.
EINVAL The mutex was created with the protocol attribute
having the value PTHREADPRIOPROTECT and the cal-
ling thread's priority is higher than the mutex's
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Standard C Library Functions pthreadmutexlock(3C)
current priority ceiling.
EPERM The mutex was created with the protocol attribute
having the value PTHREADPRIOPROTECT and the cal-
ling thread is not in the real-time class
(SCHEDR or SCHEDFIFO scheduling class).
The pthreadmutextrylock() function will fail if:
EBUSY The mutex could not be acquired because it was
already locked.
The pthreadmutexlock(), pthreadmutextrylock() and
pthreadmutexunlock() functions may fail if:
EINVAL The value specified by mutex does not refer to an
initialized mutex object.
The pthreadmutexlock() function may fail if:
EDEADLK The current thread already owns the mutex.
ENOMEM The limit on the number of simultaneously held
mutexes has been exceeded.
The pthreadmutexunlock() function will fail if:
EPERM The mutex type is PTHREADMUTEXERORCHECK or the
mutex is a robust mutex, and the current thread
does not own the mutex.
When a thread makes a call to pthreadmutexlock() or
pthreadmutextrylock(), if the mutex is initialized with
the robustness attribute having the value
PTHREADMUTEXROBUST (see pthreadmutexattrgetrobust(3C)),
the call will return these error values if:
EOWNERDEAD The last owner of this mutex died while
holding the mutex, or the process con-
taining the owner of the mutex unmapped
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Standard C Library Functions pthreadmutexlock(3C)
the memory containing the mutex or per-
formed one of the exec(2) functions. This
mutex is now owned by the caller. The
caller must now attempt to make the state
protected by the mutex consistent. If it
is able to clean up the state, then it
should call pthreadmutexconsistent()
for the mutex and unlock the mutex. Sub-
sequent calls to pthreadmutexlock() and
pthreadmutextrylock() will behave nor-
mally, as before. If the caller is not
able to clean up the state,
pthreadmutexconsistent() should not be
called for the mutex, but the mutex
should be unlocked. Subsequent calls to
pthreadmutexlock() and
pthreadmutextrylock() will fail to
acquire the mutex with the error value
ENOTRECOVERABLE. If the owner who
acquired the lock with EOWNERDEAD dies,
the next owner will acquire the lock with
EOWNERDEAD.
ENOTRECOVERABLE The mutex trying to be acquired was pro-
tecting the state that has been left
irrecoverable by the mutex's last owner.
The mutex has not been acquired. This
condition can occur when the lock was
previously acquired with EOWNERDEAD, and
the owner was not able to clean up the
state and unlocked the mutex without cal-
ling pthreadmutexconsistent().
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
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Standard C Library Functions pthreadmutexlock(3C)
ATRIBUTE TYPE ATRIBUTE VALUE
Interface Stability Committed
MT-Level MT-Safe
Standard See standards(5).
SEE ALSO
pthreadmutexconsistent(3C), pthreadmutexinit(3C),
pthreadmutexattrsetprotocol(3C),
pthreadmutexattrsetrobust(3C),
pthreadmutexattrsettype(3C), attributes(5), standards(5)
NOTES
In the current implementation of threads,
pthreadmutexlock(), pthreadmutexunlock(), mutexlock(),
mutexunlock(), pthreadmutextrylock(), and mutextrylock()
do not validate the mutex type. Therefore, an uninitialized
mutex or a mutex with an invalid type does not return EIN-
VAL. Interfaces for mutexes with an invalid type have
unspecified behavior.
Uninitialized mutexes that are allocated locally may contain
junk data. Such mutexes need to be initialized using
pthreadmutexinit() or mutexinit().
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