Mutex: ..., Not allowed in ISR context when type casting float

Mbed OS
1

Hi,

I receive
Error Status: 0x80010133 Code: 307 Module: 1
Error Message: Mutex: 0x200019CC, Not allowed in ISR context

when I change
std::string s1 = to_string(5);
to
std::string s1 = to_string(5.1);
inside an ISR context for my serial output.

How can I circumvent this?

Thank you!

Code:

#include “ThisThread.h”
#include “mbed.h”
#include “cstddef”
#include “iostream”
#include “string”
#include “iomanip”

// Maximum number of element the application buffer can contain
#define MAXIMUM_BUFFER_SIZE 32

InterruptIn button(SW2);
DigitalOut led(LED1);

// Create a BufferedSerial object with a default baud rate.
static UnbufferedSerial SerialPort(USBTX, USBRX);

// Application buffer to receive the data
char buf[MAXIMUM_BUFFER_SIZE] = {0};
uint8_t bufPos;

void sendMotorStatusMessage(double speed)
{
bufPos = 0;
std::string s1 = to_string(5.1);
strncpy(buf + bufPos, s1.c_str(), s1.length());
bufPos += s1.length();

buf[bufPos] = '\0';

SerialPort.write(buf, sizeof(buf));
memset(buf, 0, sizeof(buf));

}

void flip()
{
led = !led;
sendMotorStatusMessage(0);
}

int main(void)
{
// Set desired properties (9600-8-N-1).
SerialPort.baud(115200);
SerialPort.format(
/* bits / 8,
/ parity / BufferedSerial::None,
/ stop bit */ 1
);
button.rise(&flip);

while (1)
{
    ThisThread::sleep_for(250ms);
}

}

2

Hi Marcel,

it would help people who might be able to answer if you provided which version of Mbed OS you are using and perhaps a large code snippet .

Thanks
Anna

3

It’s most likely because you’re calling ‘sendMotorStatusMessage’ (which contains a printf call) from within the interrupt.

The way I got around this is to use EventQueues (in mBedOS 5.13 anyway). Post an message to the queue and let the queue handler print the status message.

HTH

4

Done.

5

I’m aware of that but I use UnbufferedSerial class and it works with writing 5 to the serial interface. Only floats like 5.1 do not work.