BoringSSL usually follows the Google C++ style guide, The rest of this document describes differences and clarifications on top of the base guide.
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.
Although BoringSSL is, now, written in C++, it has a long history from C, so some parts of the Google C++ style guide cannot be applied directly. In particular, our public API overlaps highly with OpenSSL, and our public headers must be C-compatible.
The implementation also used to be in C and has been incrementally adapted to C++, so much of our code reflects our C conventions rather than Google C++. If unsure, match surrounding code. Discrepancies between it and Google C++ style will be fixed over time.
In C code, BoringSSL does not limit itself to old versions of C. (See BUILDING.md for current requirements.) In particular:
Variable declarations in the middle of a function or inside a for loop are allowed and preferred where possible. Note that the common goto err cleanup pattern requires lifting some variable declarations.
Comments should be // C99-style for consistency with C++.
For new constants, prefer enums when the values are sequential. If adding values to an existing set of #defines, continue with #define.
In both C and C++, when declaring pointer or reference types, * or & should be placed next to the variable name, not the type. So
uint8_t *ptr; const auto &blah;
not
uint8_t* ptr; const auto& blah;
When allocating memory in libcrypto and libssl, do not use malloc() and free() from C, or new and delete from C++. Instead, use the wrappers OPENSSL_malloc() and OPENSSL_free() from <openssl/mem.h>, or the internal C++ helpers bssl::New, bssl::Delete, and bssl::MakeUnique from crypto/mem_internal.h.
Memory allocation from these functions is fallible. Calling code should check for allocation failure and handle it gracefully. OPENSSL_free additionally overwrites memory before freeing it.
We may relax these requirements in the future to align with standard C++, after evaluating where we need fallible allocations and other ways to handle overwriting memory.
With the exception of memory allocation above, code in BoringSSL can freely use C standard library functions. In particular, use the standard C assert() as appropriate. (Note that assert() is a debug-only assert. Use BSSL_CHECK() for a check that should be run in release builds.)
Some standard library functions are impacted by a language bug around zero-length inputs. Although this will be fixed in C29, we cannot rely on this. Instead, use the following wrappers, found in crypto/internal.h:
OPENSSL_memchrOPENSSL_memcmpOPENSSL_memcpyOPENSSL_memmoveOPENSSL_memsetWith the exception of memory allocation above, code in BoringSSL can freely use C++ standard library functions. Note this excludes most container types, which may allocate. Instead, crypto/mem_internal.h contains some replacement containers that use OPENSSL_malloc. Like OPENSSL_malloc, memory allocation is fallible in this containers.
We may relax these requirements in the future to align with standard C++, after evaluating where we need fallible allocations and other ways to handle overwriting memory.
Single-statement blocks are not allowed. All conditions and loops must use braces:
if (foo) {
do_something();
}
not
if (foo) do_something();
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).
Follow Google naming conventions for C++.
For C symbols, use the following naming conventions for consistency with existing OpenSSL and C styles:
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. All TYPE_NAME_free functions must do nothing on NULL input.
Name enums like enum unix_hacker_t. For instance:
enum should_free_handshake_buffer_t {
free_handshake_buffer,
dont_free_handshake_buffer,
};
In both languages, if a variable is the number of objects at some pointer, it has the suffix _len when the elements are each 1-byte and the prefix num_ otherwise. An output parameter is named out or has an out_ prefix. For instance:
uint8_t *out, size_t *out_len, const uint8_t *in, size_t in_len, const uint32_t *codepoints, size_t num_codepoints,
Where possible, prefer to use bssl::Span from <openssl/span.h>, instead of separating pointer and length parameters. This is, however, not possible in public functions, which must remain C-compatible.
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_CRYPTO_EC_INTERNAL_H.
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.
Where not constrained by legacy code, parameter order should be:
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, unsigned tag);
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|.
Documentation should be concise but completely describe the exposed behavior of the function. Pay special note to success/failure behaviors and caller obligations on object lifetimes. If this sacrifices conciseness, consider simplifying the function's behavior.
// 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.
BoringSSL is used by many projects with many different build tools. Reimplementing and maintaining build logic in each downstream build is cumbersome, so build logic should be avoided where possible. Platform-specific files should be excluded by wrapping the contents in #ifdefs, rather than computing platform-specific file lists. Generated source files such as perlasm and err_data.c may be used in the standalone CMake build but, for downstream builds, they should be pre-generated in generate_build_files.py.