NMEA checksum calculator in C

nmea_checksum_creator.c

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

int nmea0183_checksum(char *nmea_data)
{
    int crc = 0;
    int i;

    // the first $ sign and the last two bytes of original CRC + the * sign
    for (i = 1; i < strlen(nmea_data) - 3; i ++) {
        crc ^= nmea_data[i];
    }

    return crc;
}

You may want to hoist strlen(nmea_data) out of the loop.