/Users/brianlane/projs/aisparser/c/src/nmea.c File Reference

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Detailed Description

NMEA 0183 Sentence Parser Module.

Author:

Copyright 2006-2008 by Brian C. Lane <bcl@brianlane.com>, All Rights Reserved

Version:

1.8

This module provides utility functions for handling NMEA 0183 data streams. The stream data is 8 bit ASCII with comma separated data fields. The functions in this module can be used to generate and verify the checksum, find the start of the sentence, find the next field, convert a field to an integer and copy a field to a destination buffer.

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include "portable.h"
#include "nmea.h"

Functions
char __stdcall	ahextobin (char *c)
	Convert a single ASCII Hex digit (0-9 A-F or a-f) to a binary value.
char *__stdcall	nmea_next_field (char *p)
	Return a pointer to the next field in the string.
unsigned int __stdcall	nmea_uint (char *p)
	Convert the current field to an unsigned integer.
char *__stdcall	nmea_copy_field (char *dest, char *src, int len)
	Copy len bytes from a field to a destination buffer.
char *__stdcall	find_nmea_start (char *buffer)
	Find the start of a NMEA sentence '!' or '$' character.
unsigned int __stdcall	nmea_checksum (char *buffer, unsigned char *checksum)
	Calculate the checksum of a NMEA 0183 sentence.
unsigned int __stdcall	check_nmea_checksum (char *buffer, unsigned char *checksum)
	Check and return the checksum of a NMEA 0183 sentence.

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Function Documentation

char __stdcall ahextobin

(

char *

c

)

Convert a single ASCII Hex digit (0-9 A-F or a-f) to a binary value.

Parameters:

c

ASCII character to convert

Returns:

• binary 0x00-0x0F
• -1 if it was not an ASCII hex character

Example:

    ahextobin( 'D' ) == 0x0D

unsigned int __stdcall check_nmea_checksum	(	char *	buffer,
		unsigned char *	checksum
	)

Check and return the checksum of a NMEA 0183 sentence.

Parameters:

	buffer	pointer to a 0 terminated buffer
	checksum	pointer to variable where checksum will be stored

Returns:

• 0 if it does match
• 1 if it does not match
• 2 if there was an error

This function checks the checksum of a string and returns the result. The string is expected to start with a ! or $ and end after the 2 hex checksum characters. Trailing CR and/or LF are optional. The actual checksum is returned in the variable pointed to by checksum.

Example:

    char buf[255];
    unsigned char checksum;
    
    strcpy( (char *) buf, "!AIVDM,1,1,,A,15N;<J0P00Jro1<H>bAP0?vL00Rb,0*1B\n" );
    if( check_nmea_checksum( buf, &checksum ) != 0 )
    {
        fprintf( stderr, "Checksum failed\n" );
    }

    checksum == 0x1B

char* __stdcall find_nmea_start

(

char *

buffer

)

Find the start of a NMEA sentence '!' or '$' character.

Parameters:

buffer

pointer to a 0 terminated string buffer

Returns:

• pointer to start character or NULL if not found

The NMEA sentence may not always start at the beginning of the buffer, use this routine to make sure the start of the sentence has been found.

Example:

    char buf[255];
    char *p;

    strcpy( (char *) buf, "678,4343,123,585*FF\n$GPGGA,32,121,676,29394,29234*AA\n" );
    p = find_nmea_start( buf ) );

    *p == "$GPGGA,..."

unsigned int __stdcall nmea_checksum	(	char *	buffer,
		unsigned char *	checksum
	)

Calculate the checksum of a NMEA 0183 sentence.

Parameters:

	buffer	pointer to a 0 terminated buffer
	checksum	pointer to variable where checksum will be stored

Returns:

• 0 if no error
• 1 if there was an error
• Fills in *checksum with calculated checksum

This function will calculate the checksum of the string by skipping the ! or $ character and stopping at the * character or end of string.

Example:

    char buf[255];
    unsigned char checksum;
    
    strcpy( (char *) buf, "!AIVDM,1,1,,A,15N;<J0P00Jro1<H>bAP0?vL00Rb,0*1B\n" );
    nmea_checksum( buf, &checksum )

    checksum == 0x1B

char* __stdcall nmea_copy_field	(	char *	dest,
		char *	src,
		int	len
	)

Copy len bytes from a field to a destination buffer.

Parameters:

	dest	pointer to destination buffer
	src	pointer to 0 terminated source buffer
	len	maximum number of characters to copy

Returns:

• unsigned int

This routine stops copying when it hits the end of the field, end of the string or len.

Example:

    char buf1[255];
    char buf2[255];
    char *p;
    
    strcpy( buf1, "!AIVDM,1,1,,A,19NWoq000Wrt<RJHEuuqiWlN061d,0*5F" );
    buf2[0] = 0;
    
    p = buf1;
    nmea_copy_field( buf2, p, 254 );

    buf2 == buf1

char* __stdcall nmea_next_field

(

char *

p

)

Return a pointer to the next field in the string.

Parameters:

p

Pointer to the NMEA 0183 field

Returns:

• pointer to next field in string
• NULL if the end of the string has been reached

Example:

    char buf[255];
    char *p;
    
    strcpy( buf, "!AIVDM,1,1,,A,19NWoq000Wrt<RJHEuuqiWlN061d,0*5F" );
    p = nmea_next_field( p );

    *p == '1'

unsigned int __stdcall nmea_uint

(

char *

p

)

Convert the current field to an unsigned integer.

Parameters:

p

Pointer to the NMEA 0183 field

Returns:

• unsigned int

Example:

    char buf[255];
    char *p;
    unsigned int i;
    
    strcpy( buf, "!AIVDM,1,1,,A,19NWoq000Wrt<RJHEuuqiWlN061d,0*5F" );
    p = nmea_next_field( p );   
    i = nmea_uint( p );
    
    i == 1

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