How to use serial port to communicate between two computers

/* serrecv.c */
/* Receives and saves a file over a serial port */
/* Last modified: Septemeber 21, 2005 */
/* http://www.gomorgan89.com */

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

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

/* 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 receive and save file from serial port */
void get_file_from_serial_port(HANDLE h, char *file_name, unsigned long file_length);

int main(int argc, char **argv)
{
	HANDLE serial_port;					/* Handle to the serial port */
	long baud_rate = 9600;				/* Baud rate */
	char port_name[] = "COM1:";			/* Name of serial port */
	unsigned long file_size;			/* Size of file to receive in bytes */
	unsigned long bytes_received;		/* Bytes received from serial port */
	unsigned long file_name_size;		/* Size of file name in bytes */
	char file_name[256];				/* Name of file to receive */

	/* Check command line */
	if (argc == 3)
	{
		/* Read in baud rate */
		if (argv[1][1] != 'b' || sscanf(argv[2], "%ld", &baud_rate) != 1)
		{
			usage();
			exit(0);
		}
	}
	else if (argc != 1)
	{
		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);

	/* Receive file name size from serial port */
	ReadFile(serial_port, (void *)&file_name_size, sizeof(unsigned long), &bytes_received, NULL);
	if (bytes_received != sizeof(unsigned long))
	{
		fprintf(stderr, "Error getting file name size.\n");
		CloseHandle(serial_port);
		exit(0);
	}

	/* Receive file name from serial port */
	ReadFile(serial_port, (void *)file_name, file_name_size, &bytes_received, NULL);
	if (bytes_received != file_name_size)
	{
		fprintf(stderr, "Error retrieving file name.\n");
		CloseHandle(serial_port);
		exit(0);
	}

	/* Append NULL terminator to end of string */
	file_name[bytes_received] = '\0';
	
	/* Receive file size from serial port */
	ReadFile(serial_port, (void *)&file_size, sizeof(unsigned long), &bytes_received, NULL);
	if (bytes_received != sizeof(unsigned long))
	{
		fprintf(stderr, "Error getting file size.\n");
		CloseHandle(serial_port);
		exit(0);
	}

	/* Get the file from the serial port */
	get_file_from_serial_port(serial_port, file_name, file_size);

	/* Print out success information */
	printf("\n%lu bytes successfully received and saved as %s\n", file_size, file_name);

	/* Close handle */
	CloseHandle(serial_port);

	return 0;
}

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

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\n", baud);
		usage();
		exit(0);
		break;
	}
	
	/* Set the other properties */
	properties.Parity = NOPARITY;
	properties.ByteSize = 8;
	properties.StopBits = ONESTOPBIT;

	SetCommState(h, &properties);

	return;
}

void get_file_from_serial_port(HANDLE h, char *file_name, unsigned long file_length)
{
	FILE *data_file;						/* File to create */
	unsigned long bytes_left = file_length;	/* Bytes left to receive */
	unsigned long bytes_received_total = 0;	/* Total bytes received */
	unsigned long bytes_to_receive;			/* Number of bytes to receive */
	unsigned long bytes_received;			/* Number of bytes receive */
	char buffer[200];						/* Buffer to store data */

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

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

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

		/* Receive data over serial cable */
		ReadFile(h, (void *)buffer, bytes_to_receive, &bytes_received, NULL);
		if (bytes_received != bytes_to_receive)
		{
			fprintf(stderr, "Error reading file.\n");
			CloseHandle(h);
			exit(0);
		}

		/* Save buffer to file */
		fwrite((void *)buffer, 1, bytes_received, data_file);
		
		/* Decrement number of bytes left */
		bytes_left -= bytes_received;

		/* Increment number of bytes received */
		bytes_received_total += bytes_received;

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

	fclose(data_file);

	return;
}