How to use serial port to communicate between two computers

/* sertrans.c */
/* Transmits a file to another computer over a serial cable */
/* Last modified: September 20, 2005 */
/* http://www.gomorgan89.com */

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

/* Function to set up the serial port settings with the specified baud rate,
   no parity, and one stop bit */
void set_up_serial_port(HANDLE h, long baud);

/* Function to print out usage */
void usage(void);

/* Function to get size of file */
unsigned long get_file_size(char *file_name);

/* Function to write the file to the serial port */
void write_file_to_serial_port(HANDLE h, char *file_name, unsigned long file_size);

int main(int argc, char **argv)
{
	HANDLE serial_port;				/* Handle to the serial port */
	long baud_rate = 9600;			/* Specified baud rate */
	char port_name[] = "COM1:";		/* Name of the serial port */
	unsigned long file_size;		/* File size to transmit in bytes */
	unsigned long bytes_written;	/* Bytes written to serial port */
	unsigned long file_name_size;	/* Size of file name */

	/* Check command line arguments */
	if (argc == 4)
	{
		/* Read in baud rate */
		if (argv[2][1] != 'b' || sscanf(argv[3], "%ld", &baud_rate) != 1)
		{
			usage();
			exit(0);
		}
	}
	else if (argc != 2)
	{
		usage();
		exit(0);
	}
	
	/* Open up a handle to the serial port */
	serial_port = CreateFile(port_name, GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, 0);

	/* Make sure port was opened */
	if (serial_port == INVALID_HANDLE_VALUE)
	{
		fprintf(stderr, "Error opening port\n");
		CloseHandle(serial_port);
		exit(0);
	}

	/* Set up the serial port */
	set_up_serial_port(serial_port, baud_rate);

	/* Get the file size */
	file_size = get_file_size(argv[1]);

	/* Print out information */
	printf("Preparing to transmit file %s: %lu bytes\n", argv[1], file_size);

	/* Write file name size to serial port */
	file_name_size = (unsigned long)strlen(argv[1]);
	WriteFile(serial_port, (void *)&file_name_size, sizeof(unsigned long), &bytes_written, NULL);
	if (bytes_written != sizeof(unsigned long))
	{
		fprintf(stderr, "Error writing file name size.\n");
		CloseHandle(serial_port);
		exit(0);
	}

	/* Write file name to serial port */
	WriteFile(serial_port, (void *)argv[1], file_name_size, &bytes_written, NULL);
	if (bytes_written != file_name_size)
	{
		fprintf(stderr, "Error writing file name.\n");
		CloseHandle(serial_port);
		exit(0);
	}


	/* Write file size to serial port */
	WriteFile(serial_port, (void *)&file_size, sizeof(unsigned long), &bytes_written, NULL);
	if (bytes_written != sizeof(unsigned long))
	{
		fprintf(stderr, "Error writing file size.\n");
		CloseHandle(serial_port);
		exit(0);
	}

	/* Write file to serial port */
	write_file_to_serial_port(serial_port, argv[1], file_size);

	printf("\n%lu bytes successfully transmitted.\n", file_size);

	/* Close the handle */
	CloseHandle(serial_port);

	return 0;
}

void set_up_serial_port(HANDLE h, long baud)
{
	DCB properties;			/* Properties of serial port */

	/* Get the properties */
	GetCommState(h, &properties);

	/* Set the baud rate */
	switch(baud)
	{
	case 1200:
		properties.BaudRate = CBR_1200;
		break;
	case 2400:
		properties.BaudRate = CBR_2400;
		break;
	case 4800:
		properties.BaudRate = CBR_4800;
		break;
	case 9600:
		properties.BaudRate = CBR_9600;
		break;
	case 14400:
		properties.BaudRate = CBR_14400;
		break;
	case 19200:
		properties.BaudRate = CBR_19200;
		break;
	case 38400:
		properties.BaudRate = CBR_38400;
		break;
	default:
		fprintf(stderr, "Invalid baud rate: %ld", baud);
		usage();
		exit(0);
		break;
	}
	
	/* Set the other properties */
	properties.Parity = NOPARITY;
	properties.ByteSize = 8;
	properties.StopBits = ONESTOPBIT;

	SetCommState(h, &properties);

	return;
}

void usage(void)
{
	fprintf(stderr, "Usage:\n");
	fprintf(stderr, "\tsertrans file [-b baud_rate]\n");
	fprintf(stderr, "\tDefault baud rate is 9600\n");
	fprintf(stderr, "\tSupported baud rates: 1200, 2400, 4800, 9600, 14400, 19200, 38400\n");
	return;
}

unsigned long get_file_size(char *file_name)
{
	FILE *data_file;
	unsigned long size = 0;
	size_t bytes_read;
	char byte_read;

	/* Open the file */
	data_file = fopen(file_name, "rb");

	/* Quit if file couldn't be opened */
	if (data_file == NULL)
	{
		fprintf(stderr, "Could not open file %s\n", file_name);
		exit(0);
	}

	/* Read in data one byte at a time until we reach the end */
	while (1)
	{
		bytes_read = fread((void *)&byte_read, 1, 1, data_file);
		if (bytes_read <= 0)
		{
			break;
		}
		++size;
	}

	/* Close file */
	fclose(data_file);

	return size;
}

void write_file_to_serial_port(HANDLE h, char *file_name, unsigned long file_size)
{
	FILE *data_file;
	unsigned long bytes_left = file_size;
	unsigned long bytes_sent;
	unsigned long bytes_read;
	unsigned long total_bytes_sent = 0;
	size_t bytes_to_send;
	char buffer[200];

	/* Open the file */
	data_file = fopen(file_name, "rb");

	/* Quit if file couldn't be opened */
	if (data_file == NULL)
	{
		fprintf(stderr, "Could not open file %s\n", file_name);
		exit(0);
	}

	while (1)
	{
		/* Determine how many bytes to send */
		if (bytes_left == 0)
		{
			break;
		}
		else if (bytes_left < 200)
		{
			bytes_to_send = bytes_left;
		}
		else
		{
			bytes_to_send = 200;
		}

		/* Read in specified number of bytes */
		bytes_read = (unsigned long)fread((void *)buffer, 1, bytes_to_send, data_file);

		/* Send data over serial cable */
		WriteFile(h, (void *)buffer, bytes_read, &bytes_sent, NULL);
		if (bytes_sent != bytes_read)
		{
			fprintf(stderr, "Error writing file.\n");
			CloseHandle(h);
			exit(0);
		}
		
		/* Decrement number of bytes left */
		bytes_left -= bytes_sent;

		/* Increment number of bytes sent */
		total_bytes_sent += bytes_sent;

		/* Print out progress */
		printf("\r%5lu bytes transmitted.", total_bytes_sent);
	}

	fclose(data_file);

	return;
}