process.h is the C header file which contains function declarations and macros used in working with threads and processes.
Microsoft's version of the file dates back to at least 1985, according to its copyright statement.
The Lattice C compiler version 3.30 (Aug-24-1988) did not have such a header file, but offered similar functions.
Given the fact there is no standard on which to base the implementation, the functions declared by process.h differ, depending on the compiler in use.
Another aspect that might vary is the combined length of exec* and spawn* parameters.