main.c:99: error: ‘LangParser_decl_return’ undeclared (first use in this function)

Page in ANTLR3C API : http://antlr.org/api/C/index.html

Something must be changed to fit your project:

1. All variables containing /.*Lang.*/, change "Lang" to "JavaScript" or your language which your grammar described. (I wrote a JavaScript.g file to describe JavaScript Language Grammar.)

2. Change "LangParser_decl_return" to "JavaScriptParser_program_return", where "JavaScript" is what I said ablow, and where "program" is the first token you define below the "option" token, in Grammar file (JavaScript.g).
ps: You may want to change all "decl" to "program", because "decl" only in file "ANTLR_Examples.zip/C/treeparser/Lang.g"