Add a style guide.

I hear all the cool projects have those.

Change-Id: I0fb82ddb3f7c1768523311637099baf26c574c75
Reviewed-on: https://boringssl-review.googlesource.com/3062
Reviewed-by: Adam Langley <agl@google.com>

STYLE

@@ -0,0 +1,189 @@
+BoringSSL Style Guide.
+
+BoringSSL usually follows the Google C++ style guide, found below. The
+rest of this document describes differences and clarifications on top
+of the base guide.
+
+https://google-styleguide.googlecode.com/svn/trunk/cppguide.html
+
+
+Legacy code.
+
+As a derivative of OpenSSL, BoringSSL contains a lot of legacy code
+that does not follow this style guide. Particularly where public API
+is concerned, balance consistency within a module with the benefits of
+a given rule. Module-wide deviations on naming should be respected
+while integer and return value conventions take precedence over
+consistency.
+
+Some modules have seen few changes, so they still retain the original
+indentation style for now. When editing these, try to retain the
+original style. For Emacs, doc/c-indentation.el from OpenSSL may be
+helpful in this.
+
+
+Language.
+
+The majority of the project is in C, so C++-specific rules in the
+Google style guide do not apply. Support for C99 features depends on
+our target platforms. Typically, Chromium's target MSVC is the most
+restrictive.
+
+Variable declarations in the middle of a function are allowed.
+
+Comments should be /* C-style */ for consistency.
+
+When declaration pointer types, * should be placed next to the variable
+name, not the type. So
+
+  uint8_t *ptr;
+
+not
+
+  uint8_t* ptr;
+
+Rather than malloc() and free(), use the wrappers OPENSSL_malloc() and
+OPENSSL_free(). Use the standard C assert() function freely.
+
+For new constants, prefer enums when the values are sequential and typed
+constants for flags. If adding values to an existing set of #defines, continue
+with #define.
+
+
+Formatting.
+
+Single-statement blocks are not allowed. All conditions and loops must
+use braces:
+
+  if (foo) {
+    do_something();
+  }
+
+not
+
+  if (foo)
+    do_something();
+
+
+Integers.
+
+Prefer using explicitly-sized integers where appropriate rather than
+generic C ones. For instance, to represent a byte, use uint8_t, not
+unsigned char. Likewise, represent a two-byte field as uint16_t, not
+unsigned short.
+
+Sizes are represented as size_t.
+
+Within a struct that is retained across the lifetime of an SSL
+connection, if bounds of a size are known and it's easy, use a smaller
+integer type like uint8_t. This is a "free" connection footprint
+optimization for servers. Don't make code significantly more complex
+for it, and do still check the bounds when passing in and out of the
+struct. This narrowing should not propagate to local variables and
+function parameters.
+
+When doing arithmetic, account for overflow conditions.
+
+Except with platform APIs, do not use ssize_t. MSVC lacks it, and
+prefer out-of-band error signaling for size_t (see Return values).
+
+
+Naming.
+
+Define structs with typedef named TYPE_NAME. The corresponding struct
+should be named struct type_name_st.
+
+Name public functions as MODULE_function_name, unless the module
+already uses a different naming scheme for legacy reasons. The module
+name should be a type name if the function is a method of a particular
+type.
+
+Some types are allocated within the library while others are
+initialized into a struct allocated by the caller, often on the
+stack. Name these functions TYPE_NAME_new/TYPE_NAME_free and
+TYPE_NAME_init/TYPE_NAME_cleanup, respectively.
+
+If a variable is the length of a pointer value, it has the suffix
+_len. An output parameter is named out or has an out_ prefix. For
+instance, For instance:
+
+  uint8_t *out,
+  size_t *out_len,
+  const uint8_t *in,
+  size_t in_len,
+
+Name public headers like include/openssl/evp.h with header guards like
+OPENSSL_HEADER_EVP_H. Name internal headers like crypto/ec/internal.h
+with header guards like OPENSSL_HEADER_EC_INTERNAL_H.
+
+Name enums like unix_hacker_t. For instance:
+
+enum should_free_handshake_buffer_t {
+  free_handshake_buffer,
+  dont_free_handshake_buffer,
+};
+
+
+Return values.
+
+As even malloc may fail in BoringSSL, the vast majority of functions
+will have a failure case. Functions should return int with one on
+success and zero on error. Do not overload the return value to both
+signal success/failure and output an integer. For example:
+
+  OPENSSL_EXPORT int CBS_get_u16(CBS *cbs, uint16_t *out);
+
+If a function needs more than a true/false result code, define an enum
+rather than arbitrarily assigning meaning to int values.
+
+If a function outputs a pointer to an object on success and there are no
+other outputs, return the pointer directly and NULL on error.
+
+
+Parameters.
+
+Where not constrained by legacy code, parameter order should be:
+
+1. context parameters
+2. output parameters
+3. input parameters
+
+For example,
+
+/* CBB_add_asn sets |*out_contents| to a |CBB| into which the contents of an
+ * ASN.1 object can be written. The |tag| argument will be used as the tag for
+ * the object. It returns one on success or zero on error. */
+OPENSSL_EXPORT int CBB_add_asn1(CBB *cbb, CBB *out_contents, uint8_t tag);
+
+
+Documentation.
+
+All public symbols must have a documentation comment in their header
+file. The style is based on that of Go. The first sentence begins with
+the symbol name, optionally prefixed with "A" or "An". Apart from the
+initial mention of symbol, references to other symbols or parameter
+names should be surrounded by |pipes|. Don't be too verbose, but do
+document success and failure behaviors.
+
+/* EVP_DigestVerifyUpdate appends |len| bytes from |data| to the data which
+ * will be verified by |EVP_DigestVerifyFinal|. It returns one on success and
+ * zero otherwise. */
+OPENSSL_EXPORT int EVP_DigestVerifyUpdate(EVP_MD_CTX *ctx, const void *data,
+                                          size_t len);
+
+Explicitly mention any surprising edge cases or deviations from common
+return value patterns in legacy functions.
+
+/* RSA_private_encrypt encrypts |flen| bytes from |from| with the private key in
+ * |rsa| and writes the encrypted data to |to|. The |to| buffer must have at
+ * least |RSA_size| bytes of space. It returns the number of bytes written, or
+ * -1 on error. The |padding| argument must be one of the |RSA_*_PADDING|
+ * values. If in doubt, |RSA_PKCS1_PADDING| is the most common.
+ *
+ * WARNING: this function is dangerous because it breaks the usual return value
+ * convention. Use |RSA_sign_raw| instead. */
+OPENSSL_EXPORT int RSA_private_encrypt(int flen, const uint8_t *from,
+                                       uint8_t *to, RSA *rsa, int padding);
+
+Document private functions in their internal.h header or, if static,
+where defined.