seekgb.c

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#include "grib2_int.h"
#include <stdio.h>
#include <stdlib.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;
 
    LOG((3, "seekgb iseek %ld mseek %ld", iseek, mseek));
 
    *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++)
        {
            /* Look at the first 4 bytes - should be 'GRIB'. */
            gbit(cbuf, &start, k * BYTE, 4 * BYTE);
 
            /* Look at the 8th byte, it has the GRIB version. */
            gbit(cbuf, &vers, (k + 7) * BYTE, 1 * BYTE);
 
            /* If the message starts with 'GRIB', and is version 1 or
             * 2, then this is a GRIB message. */
            if (start == 1196575042 && (vers == 1 || vers == 2))
            {
                /* Find the length of the message. */
                if (vers == 1)
                    gbit(cbuf, &lengrib, (k + 4) * BYTE, 3 * BYTE);
                if (vers == 2)
                    gbit(cbuf, &lengrib, (k + 12) * BYTE, 4 * BYTE);
                LOG((4, "lengrib %ld", lengrib));
 
                /* Read the last 4 bytesof the message. */
                fseek(lugb, ipos + k + lengrib - 4, SEEK_SET);
                k4 = fread(&end, 4, 1, lugb);
 
                /* Look for '7777' at end of grib message. */
                if (k4 == 1 && end == 926365495)
                {
                    /* GRIB message found. */
                    *lskip = ipos + k;
                    *lgrib = lengrib;
                    LOG((4, "found end of message lengrib %ld", lengrib));
                    break;
                }
            }
        }
        ipos = ipos + lim;
    }
 
    free(cbuf);
}