The -set_sec_size option is DANGEROUS.
Grib messages are made of sections where the section number varies from
0 to 8. Suppose you want to modify a grib message by changing
section 4 (Product Definition Section). You would
use -set_sec_size to change the size of section 4
if necessary and then use -set_byte to change the contents
of your new section 4. This is not pretty but when testing a new template,
you have to do ugly things.
After you have altered the grib message, you can save the
message by either -grib or
the -grib_out. You need to use
the latter option if the grid values were altered because the
data section needs to be updated.
-set_sec_size SECTION SIZE
SECTION=0 .. 8
SIZE=integer, size of new section
can be zero for a missing section
generally greater than 5
The -set_sec_size option expands
or contracts a section. For expansions, the new octets
are filled with 255. This option puts the size of the
new section in octets 1..4 and the section number in octet 5
assuming the size is greater than equal to 5. Some
sections can be missing.