Writing bytes to a serial port in C

This is a (relatively) simple example of a C program to send five bytes to a serial port in Windows. In this case, I’m sending the five characters “hello” via COM22 at 38400 baud, but of course the program can easily be modified to send a different string, or to use a different serial port or baudrate.

This program can be compiled using cl.exe (the C++ compiler used in Visual C++ / Visual Studio) or using gcc (the C compiler in MinGW). In the former case, the file should be saved as “serial.cpp” and compiled as C++, whereas in the latter case, it can simply be saved as “serial.c” and compiled as plain C. I show an example of each approach in the screenshots at the end of this post. Either way, the resulting program “serial.exe” works the exact same.

By the way, for a simpler way to send arbitrary characters to a serial port without compiling anything at all, see my previous post.

//
// serial.c / serial.cpp
// A simple serial port writing example
// Written by Ted Burke - last updated 13-2-2013
//
// To compile with MinGW:
//
//		gcc -o serial.exe serial.c
//
// To compile with cl, the Microsoft compiler:
//
//		cl serial.cpp
//
// To run:
//
//		serial.exe
//

#include <windows.h>
#include <stdio.h>

int main()
{
	// Define the five bytes to send ("hello")
	char bytes_to_send[5];
	bytes_to_send[0] = 104;
	bytes_to_send[1] = 101;
	bytes_to_send[2] = 108;
	bytes_to_send[3] = 108;
	bytes_to_send[4] = 111;

	// Declare variables and structures
	HANDLE hSerial;
	DCB dcbSerialParams = {0};
	COMMTIMEOUTS timeouts = {0};
		
	// Open the highest available serial port number
	fprintf(stderr, "Opening serial port...");
	hSerial = CreateFile(
				"\\\\.\\COM22", GENERIC_READ|GENERIC_WRITE, 0, NULL,
				OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL );
	if (hSerial == INVALID_HANDLE_VALUE)
	{
			fprintf(stderr, "Error\n");
			return 1;
	}
	else fprintf(stderr, "OK\n");
	
	// Set device parameters (38400 baud, 1 start bit,
	// 1 stop bit, no parity)
	dcbSerialParams.DCBlength = sizeof(dcbSerialParams);
	if (GetCommState(hSerial, &dcbSerialParams) == 0)
	{
		fprintf(stderr, "Error getting device state\n");
		CloseHandle(hSerial);
		return 1;
	}
	
	dcbSerialParams.BaudRate = CBR_38400;
	dcbSerialParams.ByteSize = 8;
	dcbSerialParams.StopBits = ONESTOPBIT;
	dcbSerialParams.Parity = NOPARITY;
	if(SetCommState(hSerial, &dcbSerialParams) == 0)
	{
		fprintf(stderr, "Error setting device parameters\n");
		CloseHandle(hSerial);
		return 1;
	}

	// Set COM port timeout settings
	timeouts.ReadIntervalTimeout = 50;
	timeouts.ReadTotalTimeoutConstant = 50;
	timeouts.ReadTotalTimeoutMultiplier = 10;
	timeouts.WriteTotalTimeoutConstant = 50;
	timeouts.WriteTotalTimeoutMultiplier = 10;
	if(SetCommTimeouts(hSerial, &timeouts) == 0)
	{
		fprintf(stderr, "Error setting timeouts\n");
		CloseHandle(hSerial);
		return 1;
	}

	// Send specified text (remaining command line arguments)
	DWORD bytes_written, total_bytes_written = 0;
	fprintf(stderr, "Sending bytes...");
	if(!WriteFile(hSerial, bytes_to_send, 5, &bytes_written, NULL))
	{
		fprintf(stderr, "Error\n");
		CloseHandle(hSerial);
		return 1;
	}	
	fprintf(stderr, "%d bytes written\n", bytes_written);
	
	// Close serial port
	fprintf(stderr, "Closing serial port...");
	if (CloseHandle(hSerial) == 0)
	{
		fprintf(stderr, "Error\n");
		return 1;
	}
	fprintf(stderr, "OK\n");

	// exit normally
	return 0;
}

Here’s how it looked when I compiled and ran it using the cl compiler (that’s the C++ compiler used by Visual C++ / Visual Studio).

Here’s how it looked when I compiled and ran it with gcc (from MinGW).