FIX parsing and checksum

Parsing

// Sample
char *lsFixStrBuf = "8=FIX.4.3\0019=60\00135=0\00149=ServerX\00156=ClientX\00134=1991\00152=20191126-15:14:34.016\00110=249\001";
char *lsTagBuf = "35";
char lsValBuf[1024] = { 0x00, };

if (getFixVal(lsFixStrBuf, lsTagBuf, lsValBuf) != 0x00)
{
  printf("%s = %s\n", lsTagBuf, lsValBuf);
}

// get value from fix string
char* getFixVal(char *data, char *tag, char *osVal)
{
  char *p, *q;
  int i, j;
  int maxlen = 0;
  char sTmpTag[1024];

  sprintf(sTmpTag, "\001%s=", tag);

  if ((p = strstr(data, sTmpTag)) == 0x00)
  {
    return 0x00;
  }

  // Add this in order to prevent infinite loop
  maxlen = strlen(data) - (p+strlen(tag) - data);

  for(i=0, j=0 ; i < maxlen ; i++) {
    if (*(p+strlen(tag)+i) == '\001')
    {
      osVal[j++] = 0x00;
      break;
    }
    else if (*(p+strlen(tag)+i) {
      continue;
    }
    osVal[j++] = *(p+strlen(tag)+i);
  }
  return p;
}

CheckSum

This is an example of fix checksum.

// Generate checksum value from fix string
char *GenerateCheckSum( char *buf, long bufLen )
{
    static char tmpBuf[8];
    long idx;
    unsigned int cks;

    for( idx = 0L, cks = 0; idx < bufLen; cks += (unsigned int)buf[ idx++ ] );
    sprintf( tmpBuf, "%03d", (unsigned int)( cks % 256 ) );
    return( tmpBuf );
}

Fix checksum is generally calculated by fix engine. You can check the function with the following code.

#include <stdio.h>

char *GenerateCheckSum( char *buf, long bufLen );

int main()
{
    char *rslt;
    char tmpstr[] = "8=FIX.4.2^A9=61^A35=04^A49=TARGETUAT^A56=SENDERUAT^A34=8970^A52=20111014-08:16:37";
    rslt = GenerateCheckSum(tmpstr, sizeof(tmpstr));
    printf("[%s]\n", rslt);
}

char *GenerateCheckSum( char *buf, long bufLen )
{
        static char tmpBuf[8];
        long idx;
        unsigned int cks;

        for( idx = 0L, cks = 0; idx < bufLen; cks += (unsigned int)buf[ idx++ ] );
        sprintf( tmpBuf, "%03d", (unsigned int)( cks % 256 ) );
        return( tmpBuf );
}