---
name: "pthread_rwlock_rdlock"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd", "macos"]
url: "https://new.carta.tech/pthread_rwlock_rdlock"
---

# pthread\_rwlock\_rdlock(3)

acquire a read/write lock for reading

## NAME

**pthread\_rwlock\_rdlock**, **pthread\_rwlock\_tryrdlock** – acquire a read/write lock for reading

## LIBRARY

POSIX Threads Library (libpthread, -lpthread)

## SYNOPSIS

**#include \<pthread.h\>**

*int*\

**pthread\_rwlock\_rdlock**(*pthread\_rwlock\_t \*lock*);

*int*\

**pthread\_rwlock\_tryrdlock**(*pthread\_rwlock\_t \*lock*);

## DESCRIPTION

The **pthread\_rwlock\_rdlock**() function acquires a read lock on *lock* provided that *lock* is not presently held for writing and no writer threads are presently blocked on the lock. If the read lock cannot be immediately acquired, the calling thread blocks until it can acquire the lock.

The **pthread\_rwlock\_tryrdlock**() function performs the same action, but does not block if the lock cannot be immediately obtained (i.e., the lock is held for writing or there are waiting writers).

A thread may hold multiple concurrent read locks. If so, **pthread\_rwlock\_unlock**() must be called once for each lock obtained.

The results of acquiring a read lock while the calling thread holds a write lock are undefined.

## IMPLEMENTATION NOTES

To prevent writer starvation, writers are favored over readers.

## RETURN VALUES

If successful, the **pthread\_rwlock\_rdlock**() and **pthread\_rwlock\_tryrdlock**() functions will return zero. Otherwise an error number will be returned to indicate the error.

## ERRORS

The **pthread\_rwlock\_tryrdlock**() function will fail if:

- \[EBUSY\]

  The lock could not be acquired because a writer holds the lock or was blocked on it.

The **pthread\_rwlock\_rdlock**() and **pthread\_rwlock\_tryrdlock**() functions may fail if:

- \[EAGAIN\]

  The lock could not be acquired because the maximum number of read locks against *lock* has been exceeded.

- \[EDEADLK\]

  The current thread already owns *lock* for writing.

- \[EINVAL\]

  The value specified by *lock* is invalid.

- \[ENOMEM\]

  Insufficient memory exists to initialize the lock (applies to statically initialized locks only).

## SEE ALSO

[pthread\_rwlock\_init(3)](/pthread_rwlock_init/3.md), [pthread\_rwlock\_trywrlock(3)](/pthread_rwlock_trywrlock/3.md), [pthread\_rwlock\_unlock(3)](/pthread_rwlock_unlock/3.md), [pthread\_rwlock\_wrlock(3)](/pthread_rwlock_wrlock/3.md)

## STANDARDS

The **pthread\_rwlock\_rdlock**() and **pthread\_rwlock\_tryrdlock**() functions are expected to conform to Version 2 of the Single UNIX Specification (“SUSv2”).

## HISTORY

The **pthread\_rwlock\_rdlock**() function first appeared in FreeBSD 3.0.