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GRIB-API support is being discontinued at the end of 2018. Please consider upgrading to ecCodes

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 !  Description: how to set a bitmap in a GRIB message
 program set_bitmap
   use grib_api
   implicit none
   integer                         :: infile,outfile
   integer                         :: igrib, iret
   integer                         :: numberOfValues
   real, dimension(:), allocatable :: values
   real                            :: missingValue
   logical                         :: grib1Example
   if (grib1Example) then
     ! GRIB 1 example
     call grib_open_file(infile,'../../data/regular_latlon_surface.grib1','r')
     ! GRIB 2 example
     call grib_open_file(infile,'../../data/regular_latlon_surface.grib2','r')
   end if
   call grib_open_file(outfile,'out.grib','w')
   !     a new grib message is loaded from file
   !     igrib is the grib id to be used in subsequent calls
   call grib_new_from_file(infile,igrib)
   ! The missingValue is not coded in the message. 
   ! It is a value we define as a placeholder for a missing value
   ! in a point of the grid.
   ! It should be choosen in a way that it cannot be confused 
   ! with a valid field value
   call grib_set(igrib, 'missingValue',missingValue)
   write(*,*) 'missingValue=',missingValue
   ! get the size of the values array
   call grib_get_size(igrib,'values',numberOfValues)
   write(*,*) 'numberOfValues=',numberOfValues
   allocate(values(numberOfValues), stat=iret)
   ! get data values
   call grib_get(igrib,'values',values)
   ! enable bitmap 
   call grib_set(igrib,"bitmapPresent",1)
   ! some values are missing
   values(1:10) = missingValue
   ! set the values (the bitmap will be automatically built)
   call grib_set(igrib,'values', values)
   !  write modified message to a file
   call grib_write(igrib,outfile)
   call grib_release(igrib)
   call grib_close_file(infile)
   call grib_close_file(outfile)
 end program set_bitmap
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