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grib-api - this is available from the OpenIFS ftp download site. Although this is also available from the ECMWF software download page, it's recommended the one provided on the ftp site is used as it will have been tested and known to work with OpenIFS.
grib_api
The grib_api software library must be build and installed before you try to build OpenIFS on your system. Although you may have a version of grib_api on your system already that would most likely work, it is recommended that you build and install the version of the software provided on the OpenIFS ftp site. This version will have been tested with OpenIFS and known to work.