seekgb.c

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#include <stdio.h>
#include <stdlib.h>
#include "grib2.h"
 
void seekgb(FILE *lugb,g2int iseek,g2int mseek,g2int *lskip,g2int *lgrib)
{
    g2int k,k4,ipos,nread,lim,start,vers,lengrib;
    int    end;
    unsigned char *cbuf;
 
// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    *lgrib=0;
    cbuf=(unsigned char *)malloc(mseek);
    nread=mseek;
    ipos=iseek;
 
// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
//  LOOP UNTIL GRIB MESSAGE IS FOUND
 
    while (*lgrib==0 && nread==mseek) {
 
//  READ PARTIAL SECTION
 
        fseek(lugb,ipos,SEEK_SET);
        nread=fread(cbuf,sizeof(unsigned char),mseek,lugb);
        lim=nread-8;
 
//  LOOK FOR 'GRIB...' IN PARTIAL SECTION
 
        for (k=0;k<lim;k++) {
            gbit(cbuf,&start,(k+0)*8,4*8);
            gbit(cbuf,&vers,(k+7)*8,1*8);
            if (start==1196575042 && (vers==1 || vers==2)) {
//  LOOK FOR '7777' AT END OF GRIB MESSAGE
                if (vers == 1) gbit(cbuf,&lengrib,(k+4)*8,3*8);
                if (vers == 2) gbit(cbuf,&lengrib,(k+12)*8,4*8);
                fseek(lugb,ipos+k+lengrib-4,SEEK_SET);
//          Hard code to 4 instead of sizeof(g2int)
                k4=fread(&end,4,1,lugb);
                if (k4 == 1 && end == 926365495) {      //GRIB message found
                    *lskip=ipos+k;
                    *lgrib=lengrib;
                    break;
                }
            }
        }
        ipos=ipos+lim;
    }
 
    free(cbuf);
}