The _searchenv routine searches for the target file in the specified domain. The varname variable can be any environment or user-defined variable that specifies a list of directory paths, such as PATH, LIB, and INCLUDE.

The routine searches first for the file in the current working directory. If it does not find the file, it looks next through the directories specified by the environment variable. If the target file is in one of those directories, the newly created path is copied into pathname. If the filename file is not found, pathname contains an empty null-terminated string.