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There are several things worth noting about GDX files
| • | Only one GDX file can be open at a time. |
| • | When the GDX file to be written has the same name as an existing GDX file the existing file will be overwritten. The resultant file will only contain the new data; there is no merge or append option. |
| • | A compile time GDX write using the $Unload will only write out data defined in the compilation at the point where the command appears. No results of any solves or calculations done within the current GAMS program will be reported with $UnLoad. This is not true with Execute_Unload. |
| • | An execution time GDX write using the Execute_Unload will write out data defined in the execution sequence at the point where the GDX command appears. The results of the most recent solve command and any parameter calculations occurring before the GDX write will be reported. |
| • | Any subsequent Execute_Unload to a file written earlier will totally overwrite that file so care must be taken to write all wanted information in the last appearing Execute_Unload. |
| • | A command line GDX write using the GDX=filename command line parameter will write out data defined at the end of the execution sequence. The results of the most recent solve and any parameter calculations will be reported. |
| • | When loading data note that domain checking will not be enforced so that when items are resident in the GDX file for set elements not present in the current file these items will be ignored. GAMS will not generate any message to tell you items are ignored. |
| • | Additional examples of GDX loads and unloads can be found in the library file qp1x and in the all the Performance World examples in the linlib make use of the Gdxin feature. |
| • | Option Savepoint and Execute_Loadpoint provide a GDX way of saving and loading a basis as opposed to GAMSBAS. |
| • | The contents as they differ between GDX files can be examined with Gdxmerge. or Gdxdiff |
| • | GDX files as of version 22.3 are written in compressed form unless the environment variable GDXCOMPRESS is set to zero. |
| • | Compressed GDX files are not readable by older GAMS versions but the utility gdxcopy allows one to transform to older versions. |
| • | Copy GDX files to a different directory converting the file format used. |
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