external/io_grib1/WGRIB/seekgrib.c

   1 /*
   2  * find next grib header
   3  *
   4  * file = what do you think?
   5  * pos = initial position to start looking at  ( = 0 for 1st call)
   6  *       returns with position of next grib header (units=bytes)
   7  * len_grib = length of the grib record (bytes)
   8  * buffer[buf_len] = buffer for reading/writing
   9  *
  10  * returns (char *) to start of GRIB header+PDS
  11  *         NULL if not found
  12  *
  13  * adapted from SKGB (Mark Iredell)
  14  *
  15  * v1.1 9/94 Wesley Ebisuzaki
  16  * v1.2 3/96 Wesley Ebisuzaki handles short records at end of file
  17  * v1.3 8/96 Wesley Ebisuzaki increase NTRY from 3 to 100 for the folks
  18  *      at Automation decided a 21 byte WMO bulletin header wasn't long
  19  *      enough and decided to go to an 8K header.
  20  * v1.4 11/10/2001 D. Haalman, looks at entire file, does not try
  21  *      to read past EOF
  22  */
  23 #include <stdio.h>
  24 #include <stdlib.h>
  25 #include <stddef.h>
  26 #include "grib.h"
  27
  28 #ifndef min
  29    #define min(a,b)  ((a) < (b) ? (a) : (b))
  30 #endif
  31
  32 #define NTRY 100
  33 /* #define LEN_HEADER_PDS (28+42+100) */
  34 #define LEN_HEADER_PDS (28+8)
  35
  36 unsigned char *seek_grib(FILE *file, long *pos, long *len_grib,
  37         unsigned char *buffer, unsigned int buf_len) {
  38
  39     int i, j, len;
  40
  41     j = 1;
  42     clearerr(file);
  43     while ( !feof(file) ) {
  44
  45         if (fseek(file, *pos, SEEK_SET) == -1) break;
  46         i = fread(buffer, sizeof (unsigned char), buf_len, file);
  47         if (ferror(file)) break;
  48         len = i - LEN_HEADER_PDS;
  49
  50         for (i = 0; i < len; i++) {
  51             if (buffer[i] == 'G' && buffer[i+1] == 'R' && buffer[i+2] == 'I'
  52                 && buffer[i+3] == 'B' && buffer[i+7] == 1) {
  53                     *pos = i + *pos;
  54                     *len_grib = (buffer[i+4] << 16) + (buffer[i+5] << 8) +
  55                             buffer[i+6];
  56                     return (buffer+i);
  57             }
  58         }
  59
  60         if (j++ == NTRY) {
  61             fprintf(stderr,"found unidentified data \n");
  62            /* break; // stop seeking after NTRY records */
  63         }
  64
  65         *pos = *pos + (buf_len - LEN_HEADER_PDS);
  66     }
  67
  68     *len_grib = 0;
  69     return (unsigned char *) NULL;
  70 }
  71
  72