etl/docs/containers/queues/queue-mpmc-mutex.md

---
title: "queue_mpmc_mutex"
---

{{< callout >}}
  Header: `queue_mpmc_mutex.h`  
{{< /callout >}}

A fixed capacity multi-producer, multi-consumer queue for multi-threaded systems that uses etl::mutex to control access.  

```cpp
etl::queue_mpmc_mutex<typename T, 
                      const size_t SIZE, 
                      const size_t MEMORY_MODEL = etl::memory_model::MEMORY_MODEL_LARGE>
```

Inherits from `etl::iqueue_mpmc_mutex<T>`.  
`etl::iqueue_mpmc_mutex` may be used as a size independent pointer or reference type for any `etl::queue_mpmc_mutex` instance of the same implementation.  

The memory model determines the type used internally for indexes and size, to allow for the most efficient implementation for the application.

## Maximum queue sizes

```cpp
MEMORY_MODEL_SMALL   255
MEMORY_MODEL_MEDIUM  65535
MEMORY_MODEL_LARGE   2147483647
MEMORY_MODEL_HUGE    9223372036854775807
```

See `memory_model.h`

## Member types

```cpp
value_type      T
size_type       <based on memory model>
pointer         value_type*
const_pointer   const value_type*
reference       value_type&
const_reference const value_type&
```

## Constructor

```cpp
queue_mpmc_mutex()
```

## Capacity

```cpp
bool empty() const
```
**Description**  
Returns `true` if the size of the queue is zero, otherwise `false`.

---

```cpp
bool full() const
```
**Description**  
Returns `true` if the size of the queue is SIZE, otherwise `false`.

---

```cpp
size_t size() const
```
**Description**  
Returns the size of the queue.

---

```cpp
size_t available() const
size_t available_from_isr() const
```
**Description**  
Returns the remaining available capacity in the queue.

---

```cpp
ETL_CONSTEXPR size_t max_size() const
```
**Description**  
Returns the maximum possible size of the queue.

---

```cpp
ETL_CONSTEXPR size_t capacity() const
```
**Description**  
Returns the maximum possible size of the queue.

## Modifiers

```cpp
bool push(const T& value)
bool push(T&& value)
```
**Description**  
Pushes a value to the back of the queue.  
Returns `true` if successful, otherwise `false`.

---

```cpp
bool pop()
```
**Description**  
Pop a value from the front of the list.  
Returns `true` if successful, otherwise `false`.

---

```cpp
bool pop(T& value)
bool pop(T&& value)
```
**Description**  
Pop a value from the front of the list and place it in value.  
Returns `true` if successful, otherwise `false`.

---

```cpp
void clear()
```
**Description**  
Clears the queue to a size of zero

---

### C++03
```cpp
bool emplace(const T1& value1)
bool emplace(const T1& value1, const T2& value2)
bool emplace(const T1& value1, const T2& value2, const T3& value3)
bool emplace(const T1& value1, const T2& value2, const T3& value3, const T4& value4)
```
**Description**  
Constructs an item in the the queue 'in place'.

### C++11
```cpp
bool emplace(Args&&… args)
```
**Description**  
Constructs an item in the the queue 'in place'.

## Notes

Remember that thread context switches may occur between calls to the access protected functions. For example, a call to `empty()` may return `true`, but a subsequent call to `pop()` may succeed if a context switch occurred between the two and pushed a new value.

## Example

```cpp
etl::queue_mpmc_mutex<char, 10> queue;

int main()
{
  while (true)
  {
    char c;

    if (queue.pop(c))
    {
      Print(c);
    }
  }
}

void FunctionInThread1(char c)
{
  while (!queue.push(c))
  {
  }
}

void FunctionInThread2(char c)
{
  while (!queue.push(c))
  {
  }
}
```