Compatibility of preexisting ESQL/C applications with current library versions

You specify the esql command-line options (in The esql command-line options for Informix general libraries) to tell the esql command which version of the HCL Informix® libraries to link with the Informix® ESQL/C application. After the esql command successfully compiles and links your application, the version of the Informix® general libraries is fixed. When you install a new version of Informix® ESQL/C, you receive new copies of the Informix® general libraries. Whether you need to recompile and relink your existing Informix® ESQL/C applications to run with these new copies depends on the factors that the following table describes.
Change to the Informix® general library Version of the Informix® general library Need to recompile or relink?
New release of the Informix® general libraries Static

Thread-safe static

 Only if the application needs to take advantage of a new feature in the new release
Informix® general libraries in new release have a new major-version number Shared

Thread-safe shared

 Only if the application needs to take advantage of a new feature in the new release
Informix® general libraries in new release have a new API-version number Shared

Thread-safe shared

 Must recompile and relink

On UNIX™, you can use the ifx_getversion utility to determine the version of the Informix® library that is installed on your system.

In Windows™ environments, use the following find command to find the occurrence of the string that contains the version number in the isqlt09A.dll. The command needs to be issued from the %INFORMIXDIR%\bin directory. 
C: cd %INFORMIXDIR%\bin
C: find INFORMIX-SQL isqlt09a.dll
The output of the find command is shown:
- - - - - - - - - ISQLT09A.DLL
INFORMIX-SQL Version 9.20T1N79