ring/STYLE.md

*ring* inherited C, C++, and assembly language code from BoringSSL, and the
style guidelines for that code are in the second section of this document.


# *ring* Style Guide (for code not in [crypto/](crypto))

*ring* usually follows the [Rust Guidelines](https://aturon.github.io/), but
there are some differences and *ring* adds additional guidelines.

## Arrays

When creating a slice from the start of a indexable value, use `x[..n]`, not
`x[0..n]`. Similarly, use `x[n..]`, not `x[n..x.len()]` for creating a slice
from a specific point to the end of the value.

## Unsafe

In general, avoid using `unsafe` whenever it is practical to do so. The *ring*
developers chose to use Rust because of the goodness of the safe subset; stuff
that requires `unsafe` is generally better off being written in C or assembly
language code. Generally, this means that `unsafe` is only used to call
functions written in C or assembly language. Even if your goal is to replace C
and/or assembly language code with Rust code, don't be afraid to leave, or even
add, C code to avoid adding a load of `unsafe` Rust code.

In particular, prefer references and indexing (which is checked at runtime) to
pointers and pointer arithmetic. Example:
```rust
fn good_example(x: &[u8], n: usize) {
    unsafe {
        unsafe_fn(x[n..].as_ptr()) // The compiler inserts bounds checks for us.
    }
}

fn bad_example(x: &[u8], n: usize) {
    unsafe {
        // If we do things this way, the compiler won't do bounds checking for
        // us. Also, since `offset` takes an `isize`, we have to do a cast from
        // `usize` to `isize` which is potentially unsafe because an `isize`
        // cannot hold every positive value of `usize`.
        unsafe_fn(x.as_ptr().offset(n as isize))
    }
}
```

When you must use `unsafe`, minimize the scope of `unsafe`. Example:
```rust
fn good_example() {
   unsafe { unsafe_fn(); }
   safe_fn();
   unsafe { unsafe_fn(); }
}

fn bad_example {
    unsafe {
        unsafe_fn();
        safe_fn(); // No safe statements allowed in an unsafe block.
        unsafe();
    }
}
```

But, don't go overboard:
```rust
fn ok_example(x: &[u8]) {
    unsafe {
        unsafe_fn1(x[n]); // `x[n]` is a safe expression
    }
}

fn bad_example(x: &[u8]) {
    let x_n = x[n]; // This is going overboard.
    unsafe {
        unsafe_fn1(x_n);
    }
}
```



# BoringSSL Style Guide (for code in [crypto/](crypto))

BoringSSL usually follows the
[Google C++ style guide](https://google.github.io/styleguide/cppguide.html),
The rest of this section describes differences and clarifications on
top of the base guide.


## 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 `#define`s,
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

Follow Google naming conventions in C++ files. In C files, 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.

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 `enum 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|.

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.