Thread safety

LwPRINTF uses re-entrant functions, especially the one that format string to user application buffer. It is fully allowed to access to the same LwPRINTF instance from multiple operating-system threads.

However, when it comes to direct print functions, such as lwprintf_printf_ex() (or any other similar), calling those functions from multiple threads may introduce mixed output stream of data.

This is due to the fact that direct printing functions use same output function to print single character. When called from multiple threads, one thread may preempt another, causing strange output string.

Multiple threads printing at the same time without thread-safety enabled
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#include "lwprintf/lwprintf.h"

/* Assuming LwPRINTF has been initialized before */

void
task_1(void* arg) {
	lwprintf_printf("Hello world\r\n");
}

void
task_2(void* arg) {
	lwprintf_printf("This is Task 2\r\n");
}

/*
 * If thread safety is not enabled,
 * running above example may print:
 *
 * "Hello This is Task 2\r\nworld\r\n"
 */

LwPRINTF therefore comes with a solution that introduces mutexes to lock print functions when in use from within single thread context.

Note

If application does not have any issues concerning mixed output, it is safe to disable OS support in OS environment. This will not have any negative effect on performance or memory corruption.

Tip

To enable thread-safety support, parameter LWPRINTF_CFG_OS must be set to 1. Please check Configuration for more information about other options.

After thread-safety features has been enabled, it is necessary to implement 4 low-level system functions.

Tip

System function template example is available in lwprintf/src/system/ folder.

Example code for CMSIS-OS V2

Note

Check System functions section for function description

System function implementation for CMSIS-OS based operating systems
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/**
 * \file            lwprintf_sys_cmsis_os.c
 * \brief           System functions for CMSIS-OS based operating system
 */

/*
 * Copyright (c) 2020 Tilen MAJERLE
 *
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without restriction,
 * including without limitation the rights to use, copy, modify, merge,
 * publish, distribute, sublicense, and/or sell copies of the Software,
 * and to permit persons to whom the Software is furnished to do so,
 * subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE
 * AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 *
 * This file is part of LwPRINTF - Lightweight stdio manager library.
 *
 * Author:          Tilen MAJERLE <tilen@majerle.eu>
 * Version:         $_version_$
 */
#include "system/lwprintf_sys.h"

#if LWPRINTF_CFG_OS && !__DOXYGEN__

#include "cmsis_os.h"

uint8_t
lwprintf_sys_mutex_create(LWPRINTF_CFG_OS_MUTEX_HANDLE* m) {
    *m = osMutexNew(NULL);
    return lwprintf_sys_mutex_isvalid(m);
}

uint8_t
lwprintf_sys_mutex_isvalid(LWPRINTF_CFG_OS_MUTEX_HANDLE* m) {
    return *m != NULL;
}

uint8_t
lwprintf_sys_mutex_wait(LWPRINTF_CFG_OS_MUTEX_HANDLE* m) {
    return osMutexAcquire(*m, osWaitForever) == osOK;
}

uint8_t
lwprintf_sys_mutex_release(LWPRINTF_CFG_OS_MUTEX_HANDLE* m) {
    return osMutexRelease(*m) == osOK;
}

#endif /* LWPRINTF_CFG_OS && !__DOXYGEN__ */