Merge pull request #4561 from ctiller/StyleGuide

Initial style guide notes for C

doc/c-style-guide.md

+GRPC C STYLE GUIDE
+=====================
+
+Background
+----------
+
+Here we document style rules for C usage in the gRPC Core library.
+
+General
+-------
+
+- Layout rules are defined by clang-format, and all code should be passed through
+  clang-format. A (docker-based) script to do so is included in 
+  tools/distrib/clang_format_code.sh.
+
+Header Files
+------------
+
+- Public header files (those in the include/grpc tree) should compile as pedantic C89
+- Public header files should be includable from C++ programs. That is, they should 
+  include the following:
+  ```c
+  #ifdef __cplusplus
+  extern "C" {
+  # endif
+
+  /* ... body of file ... */
+
+  #ifdef __cplusplus
+  }
+  # endif
+  ```
+- Header files should be self-contained and end in .h.
+- All header files should have a #define guard to prevent multiple inclusion.
+  To guarantee uniqueness they should be based on the file's path.
+
+  For public headers: include/grpc/grpc.h --> GRPC_GRPC_H
+
+  For private headers: 
+  src/core/channel/channel_stack.h --> GRPC_INTERNAL_CORE_CHANNEL_CHANNEL_STACK_H
+
+C99 Features
+------------
+
+- Variable sized arrays are not allowed
+- Do not use the 'inline' keyword
+- Flexible array members are allowed (https://en.wikipedia.org/wiki/Flexible_array_member)
+
+Comments
+--------
+
+Within public header files, only `/* */` comments are allowed.
+
+Within implementation files and private headers, either single line `//` 
+or multi line `/* */` comments are allowed. Only one comment style per file is
+allowed however (i.e. if single line comments are used anywhere within a file,
+ALL comments within that file must be single line comments).
+
+Symbol Names
+------------
+
+- Non-static functions must be prefixed by grpc_
+- static functions must not be prefixed by grpc_
+- enumeration values and #define names are uppercased, all others are lowercased
+- Multiple word identifiers use underscore as a delimiter (NEVER camel casing)