pthread_rwlock_rdlock()¶

Purpose¶

pthread_rwlock_rdlock() acquires a read lock on a POSIX reader-writer lock.

Multiple readers may hold the lock simultaneously, provided no writer currently owns the lock.

This allows concurrent read access to shared data.

#include <pthread.h>

Prototype¶

int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);

Parameters¶

rwlock¶

Pointer to an initialized reader-writer lock.

Example:

pthread_rwlock_t lock;

Return Value¶

Returns:

on success.

Otherwise returns a POSIX error code.

Common errors:

Error	Description
EDEADLK	Deadlock detected
EINVAL	Invalid rwlock object

Usage Pattern¶

pthread_rwlock_rdlock(&lock);

/* read shared data */

pthread_rwlock_unlock(&lock);

Reader Behavior¶

Multiple readers may enter simultaneously:

Reader-1
Reader-2
Reader-3

All can acquire:

pthread_rwlock_rdlock()

at the same time.

Blocking Behavior¶

Readers block when:

Writer Holds Lock

Example:

Writer Active
        ↓
Reader Waits

until the writer releases the lock.

Example¶

pthread_rwlock_rdlock(&lock);

printf("value=%d\n", shared_value);

pthread_rwlock_unlock(&lock);

Typical Use Cases¶

Configuration Tables¶

Many readers
Rare updates

Lookup Tables¶

Routing tables
Device tables

Shared Read-Mostly Data¶

Read frequency
>>
Write frequency

Advantages¶

Concurrent Readers¶

Readers do not block each other.

Simple Programming Model¶

Very similar to mutex usage.

Limitations¶

Reader Locking Overhead¶

Readers still perform synchronization operations.

Every reader acquires and releases a lock.

Writer Starvation¶

Some implementations may allow continuous readers to delay writers.

Fairness policy is implementation-dependent.

Comparison¶

Mutex¶

1 Reader

or

1 Writer

only.

RWLock¶

Multiple Readers

or

1 Writer

Seqlock¶

Readers avoid locking but may need retries.

RCU¶

Readers avoid locking and focus on object lifetime management.

Locking Primitives