I think you already provided solution in your problem itself.
Problem is with permission issues.
Apache user/group is not having permission to read/execute your 'C' server binaries.
Please check whether
1. Apache user/group has permission to directory where server files resides.
2. Apache has permission to execute binaries ( if there are ).
3. Check whether you are using absolute path/relative path and its accessible by Apache.
Please update us if you still have problems.