boringssl/API-CONVENTIONS.md

BoringSSL API Conventions

This document describes conventions for BoringSSL APIs. The style guide also includes guidelines, but this document is targeted at both API consumers and developers.

Documentation

All supported public APIs are documented in the public header files, found in include/openssl. The API documentation is also available online.

Some headers lack documention comments. These are functions and structures from OpenSSL's legacy ASN.1, X.509, and PEM implementation. If possible, avoid using them. These are left largely unmodified from upstream and are retained only for compatibility with existing OpenSSL consumers.

Forward declarations

Do not write typedef struct foo_st FOO or try otherwise to define BoringSSL's types. Including openssl/base.h (or openssl/ossl_typ.h for consumers who wish to be OpenSSL-compatible) will forward-declare each type without importing the rest of the library or invasive macros.

Error-handling

Most functions in BoringSSL may fail, either due to allocation failures or input errors. Functions which return an int typically return one on success and zero on failure. Functions which return a pointer typically return NULL on failure. However, due to legacy constraints, some functions are more complex. Consult the API documentation before using a function.

On error, most functions also push errors on the error queue, an errno-like mechanism. See the documentation for err.h for more details.

As with errno, callers must test the function's return value, not the error queue to determine whether an operation failed. Some codepaths may not interact with the error queue, and the error queue may have state from a previous failed operation.

When ignoring a failed operation, it is recommended to call ERR_clear_error to avoid the state interacting with future operations. Failing to do so should not affect the actual behavior of any functions, but may result in errors from both operations being mixed in error logging. We hope to improve this situation in the future.

Where possible, avoid conditioning on specific reason codes and limit usage to logging. The reason codes are very specific and may change over time.

Memory allocation

BoringSSL allocates memory via OPENSSL_malloc, found in mem.h. Use OPENSSL_free, found in the same header file, to release it. BoringSSL functions will fail gracefully on allocation error, but it is recommended to use a malloc implementation that aborts on failure.

Object initialization and cleanup

BoringSSL defines a number of structs for use in its APIs. It is a C library, so the caller is responsible for ensuring these structs are properly initialized and released. Consult the documentation for a module for the proper use of its types. Some general conventions are listed below.

Heap-allocated types

Some types, such as RSA, are heap-allocated. All instances will be allocated and returned from BoringSSL's APIs. It is an error to instantiate a heap- allocated type on the stack or embedded within another object.

Heap-allocated types may have functioned named like RSA_new which allocates a fresh blank RSA. Other functions may also return newly-allocated instances. For example, RSA_parse_public_key is documented to return a newly-allocated RSA object.

Heap-allocated objects must be released by the corresponding free function, named like RSA_free. Like C's free and C++'s delete, all free functions internally check for NULL. Consumers are not required to check for NULL before calling.

A heap-allocated type may be reference-counted. In this case, a function named like RSA_up_ref will be available to take an additional reference count. The free function must be called to decrement the reference count. It will only release resources when the final reference is released. For OpenSSL compatibility, these functions return int, but callers may assume they always successfully return one because reference counts use saturating arithmetic.

C++ consumers are recommended to use bssl::UniquePtr to manage heap-allocated objects. bssl::UniquePtr<T>, like other types, is forward-declared in openssl/base.h. Code that needs access to the free functions, such as code which destroys a bssl::UniquePtr, must include the corresponding module's header. (This matches std::unique_ptr's relationship with forward declarations.) Note, despite the name, bssl::UniquePtr is also used with reference-counted types. It owns a single reference to the object. To take an additional reference, use the bssl::UpRef function, which will return a separate bssl::UniquePtr.

Stack-allocated types

Other types in BoringSSL are stack-allocated, such as EVP_MD_CTX. These types may be allocated on the stack or embedded within another object. However, they must still be initialized before use.

Every stack-allocated object in BoringSSL has a zero state, analogous to initializing a pointer to NULL. In this state, the object may not be completely initialized, but it is safe to call cleanup functions. Entering the zero state cannot fail. (It is usually memset(0).)

The function to enter the zero state is named like EVP_MD_CTX_init or CBB_zero and will always return void. To release resources associated with the type, call the cleanup function, named like EVP_MD_CTX_cleanup. The cleanup function must be called on all codepaths, regardless of success or failure. For example:

uint8_t md[EVP_MAX_MD_SIZE];
unsigned md_len;
EVP_MD_CTX ctx;
EVP_MD_CTX_init(&ctx);  /* Enter the zero state. */
int ok = EVP_DigestInit_ex(&ctx, EVP_sha256(), NULL) &&
         EVP_DigestUpdate(&ctx, "hello ", 6) &&
         EVP_DigestUpdate(&ctx, "world", 5) &&
         EVP_DigestFinal_ex(&ctx, md, &md_len);
EVP_MD_CTX_cleanup(&ctx);  /* Release |ctx|. */

Note that EVP_MD_CTX_cleanup is called whether or not the EVP_Digest* operations succeeded. More complex C functions may use the goto err pattern:

  int ret = 0;
  EVP_MD_CTX ctx;
  EVP_MD_CTX_init(&ctx);

  if (!some_other_operation()) {
    goto err;
  }

  uint8_t md[EVP_MAX_MD_SIZE];
  unsigned md_len;
  if (!EVP_DigestInit_ex(&ctx, EVP_sha256(), NULL) ||
      !EVP_DigestUpdate(&ctx, "hello ", 6) ||
      !EVP_DigestUpdate(&ctx, "world", 5) ||
      !EVP_DigestFinal_ex(&ctx, md, &md_len) {
    goto err;
  }

  ret = 1;

err:
  EVP_MD_CTX_cleanup(&ctx);
  return ret;

Note that, because ctx is set to the zero state before any failures, EVP_MD_CTX_cleanup is safe to call even if the first operation fails before EVP_DigestInit_ex. However, it would be illegal to move the EVP_MD_CTX_init below the some_other_operation call.

As a rule of thumb, enter the zero state of stack-allocated structs in the same place they are declared.

C++ consumers are recommended to use the wrappers named like bssl::ScopedEVP_MD_CTX, defined in the corresponding module's header. These wrappers are automatically initialized to the zero state and are automatically cleaned up.

Data-only types

A few types, such as SHA_CTX, are data-only types and do not require cleanup. These are usually for low-level cryptographic operations. These types may be used freely without special cleanup conventions.

Ownership and lifetime

When working with allocated objects, it is important to think about ownership of each object, or what code is responsible for releasing it. This matches the corresponding notion in higher-level languages like C++ and Rust.

Ownership applies to both uniquely-owned types and reference-counted types. For the latter, ownership means the code is responsible for releasing one reference. Note a reference in BoringSSL refers to an increment (and eventual decrement) of an object's reference count, not T& in C++. Thus, to "take a reference" means to increment the reference count and take ownership of decrementing it.

As BoringSSL's APIs are primarily in C, ownership and lifetime obligations are not rigorously annotated in the type signatures or checked at compile-time. Instead, they are described in API documentation. This section describes some conventions.

Unless otherwise documented, functions do not take ownership of pointer arguments. The pointer typically must remain valid for the duration of the function call. The function may internally copy information from the argument or take a reference, but the caller is free to release its copy or reference at any point after the call completes.

A function may instead be documented to take or transfer ownership of a pointer. The caller must own the object before the function call and, after transfer, no longer owns it. As a corollary, the caller may no longer reference the object without a separate guarantee on the lifetime. The function may even release the object before returning. Callers that wish to independently retain a transfered object must therefore take a reference or make a copy before transferring. Callers should also take note of whether the function is documented to transfer pointers unconditionally or only on success. Unlike C++ and Rust, functions in BoringSSL typically only transfer on success.

Likewise, output pointers may be owning or non-owning. Unless otherwise documented, functions output non-owning pointers. The caller is not responsible for releasing the output pointer, but it must not use the pointer beyond its lifetime. The pointer may be released when the parent object is released or even sooner on state change in the parent object.

If documented to output a newly-allocated object or a reference or copy of one, the caller is responsible for releasing the object when it is done.

By convention, functions named get0 return non-owning pointers. Functions named new or get1 return owning pointers. Functions named set0 take ownership of arguments. Functions named set1 do not. They typically take a reference or make a copy internally. These names originally referred to the effect on a reference count, but the convention applies equally to non-reference-counted types.

API documentation may also describe more complex obligations. For instance, an object may borrow a pointer for longer than the duration of a single function call, in which case the caller must ensure the lifetime extends accordingly.

Memory errors are one of the most common and dangerous bugs in C and C++, so callers are encouraged to make use of tools such as AddressSanitizer and higher-level languages.

Thread safety

BoringSSL is internally aware of the platform threading library and calls into it as needed. Consult the API documentation for the threading guarantees of particular objects. In general, stateless reference-counted objects like RSA or EVP_PKEY which represent keys may typically be used from multiple threads simultaneously, provided no thread mutates the key.