NMEA checksum checker

#include <stdio.h>

int hextoint(char c)
{
  if((c>='0')&&(c<='9'))
    return c-'0';
  if((c>='A')&&(c<='F'))
    return c-'A'+10;
  if((c>='a')&&(c<='f'))
    return c-'a'+10;
  return -1;
}

int main(int argc,char **argv)
{
  if(argc!=2)
  {
    fprintf(stderr,"%s needs exactly 1 argument\n",argv[0]);
    return -1;
  }

  char *nmea=argv[1];
  if(*nmea++!='$')
  {
    fprintf(stderr,"NMEA doesn't begin with '$' %s\n",argv[1]);
    return 2;
  }

  int csum=0;
  while((*nmea)&&(*nmea!='*'))
    csum^=*nmea++;

  if(!*nmea)
  {
    fprintf(stderr,"NMEA doesn't contain '*' %s\n",argv[1]);
    return 3;
  }

  ++nmea;
  if((!nmea[0])||(!nmea[1]))
  {
    fprintf(stderr,"Checksum missing from the end: %s\n",argv[1]);
    return 4;
  }
  int tmp=hextoint(nmea[0]);
  int checksum=hextoint(nmea[1]);
  if((tmp==-1)||(checksum==-1))
  {
    fprintf(stderr,"Invalid hex digit: %s\n",argv[1]);
    return 5;
  }

  checksum+=tmp<<4;
  if(csum!=checksum)
  {
    fprintf(stderr,"Checksums are different: %02x!=%02x %s\n",csum,checksum,argv[1]);
    return 1;
  }

  return 0; // everything was OK
}