The z/OS® Language Environment® message file is managed by z/OS Language Environment and may not be directly opened or closed with fopen(), freopen() or fclose() within a C or C++ application. In z/OS Language Environment, output from stderr is directed to the z/OS Language Environment message file by default. You can use freopen() and fclose() to manage stderr, or you can redirect it to another destination. There are application writer interfaces (AWIs) that enable you to access the z/OS Language Environment message file directly. These are documented in z/OS Language Environment Programming Guide.
See Language Environment Message file operations for more information on z/OS Language Environment message files.