bsls_alignment.h

Go to the documentation of this file.

/// @file bsls_alignment.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bsls_alignment.h                                                   -*-C++-*-
#ifndef INCLUDED_BSLS_ALIGNMENT
#define INCLUDED_BSLS_ALIGNMENT
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bsls_alignment bsls_alignment
/// @brief  Provide a namespace for enumerating memory alignment strategies.
/// @addtogroup bsl
/// @{
/// @addtogroup bsls
/// @{
/// @addtogroup bsls_alignment
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bsls_alignment-purpose"> Purpose</a>
/// * <a href="#bsls_alignment-classes"> Classes </a>
/// * <a href="#bsls_alignment-description"> Description </a>
///   * <a href="#bsls_alignment-alignment-strategy"> Alignment Strategy </a>
///   * <a href="#bsls_alignment-usage"> Usage </a>
///
/// # Purpose {#bsls_alignment-purpose}
/// Provide a namespace for enumerating memory alignment strategies.
///
/// # Classes {#bsls_alignment-classes}
///
/// -  bsls::Alignment: namespace for enumerated alignment strategy values
///
/// @see  bsls_alignmentutil
///
/// # Description {#bsls_alignment-description}
/// This component provides a namespace, `bsls::Alignment`, for
/// enumerating alignment strategies, and provides a function, `toAscii`, that
/// converts each of the enumerators to its corresponding string representation.
///
/// ## Alignment Strategy {#bsls_alignment-alignment-strategy}
///
///
/// This component supports three alignment strategies: 1) MAXIMUM ALIGNMENT,
/// 2) NATURAL ALIGNMENT, and 3) 1-BYTE ALIGNMENT.
///
/// 1. MAXIMUM ALIGNMENT: This strategy, as indicated by the enumerator
///    `BSLS_MAXIMUM`, specifies that a memory block be aligned as per the
///    *most* restrictive alignment requirement on the host platform.
/// 2. NATURAL ALIGNMENT: This strategy, as indicated by the enumerator
///    `BSLS_NATURAL`, specifies that a memory block be aligned based on the
///    size (in bytes) of that block.  An object of a fundamental type (`int`,
///    etc.) is *naturally* *aligned* when its size evenly divides its address.
///    An object of an aggregate type has natural alignment if the alignment of
///    the most-restrictively aligned sub-object evenly divides the address of
///    the aggregate.  Natural alignment is always at least as restrictive as
///    the compiler's required alignment.
/// 3. 1-BYTE ALIGNMENT: This strategy, as indicated by the enumerator
///    `BSLS_BYTEALIGNED`, specifies that a memory block may be aligned
///    arbitrarily on any 1-byte boundary.  This is the *least* restrictive
///    alignment requirement.
///
/// ## Usage {#bsls_alignment-usage}
///
///
/// Suppose that we want to create a static function, `allocateFromBuffer`, that
/// takes a buffer, the size of the buffer, a cursor indicating a position
/// within the buffer, an allocation request size, and a memory alignment
/// strategy; `allocateFromBuffer` returns a pointer to a block of memory,
/// wholly contained within the buffer, having the specified size and alignment.
/// As a side-effect, the cursor is updated to refer to the next available free
/// byte in the buffer.  Such a function could be used by a memory manager to
/// satisfy allocation requests from internally-managed buffers.  Clients of
/// this function indicate which alignment strategy to use based on their
/// specific requirements.
///
/// Our `allocateFromBuffer` function depends on an alignment utility,
/// `my_AlignmentUtil`, whose minimal interface is limited to that required by
/// this usage example.  (See the @ref bsls_alignmentutil  component for a more
/// realistic alignment utility.):
/// @code
/// struct my_AlignmentUtil {
///     // This 'struct' provides a namespace for basic types and utilities
///     // related to memory alignment.
///
///     // TYPES
///
///     /// Provide the *minimal* value that satisfies the alignment
///     /// requirements for *all* types on the host platform.  Note that 8
///     /// is used for illustration purposes only; an actual implementation
///     /// would employ template meta-programming to deduce the value at
///     /// compile time.
///     enum {
///         MY_MAX_PLATFORM_ALIGNMENT = 8
///     };
///
///     // CLASS METHODS
///
///     /// Calculate a usable alignment for a memory block of the specified
///     /// `size` (in bytes) in the absence of compile-time knowledge of
///     /// the block's alignment requirements.  Return the largest power of
///     /// two that evenly divides `size`, up to a maximum of
///     /// `MY_MAX_PLATFORM_ALIGNMENT`.  It is guaranteed that a block of
///     /// `size` bytes can be safely aligned on the return value.   The
///     /// behavior is undefined unless `0 < size`.
///     static int calculateAlignmentFromSize(int size);
///
///     /// Return the smallest non-negative offset (in bytes) that, when
///     /// added to the specified `address`, yields the specified
///     /// `alignment`.  The behavior is undefined unless `0 != alignment`
///     /// and `alignment` is a non-negative, integral power of 2.
///     static int calculateAlignmentOffset(const void *address,
///                                         int         alignment);
/// };
/// @endcode
/// The definition of our `allocateFromBuffer` function is as follows:
/// @code
/// /// Allocate a memory block of the specified `size` (in bytes) from the
/// /// specified `buffer` having the specified `bufferSize` at the
/// /// specified `cursor` position, using the specified alignment
/// /// `strategy`.  Return the address of the allocated memory block if
/// /// `buffer` contains sufficient available memory, and 0 otherwise.  The
/// /// `cursor` is set to the first byte position immediately after the
/// /// allocated memory (which might be 1 byte past the end of `buffer`) if
/// /// there is sufficient memory, and is not modified otherwise.  The
/// /// behavior is undefined unless `0 <= bufferSize`, `0 < size`, and
/// /// `cursor` refers to a valid position in `buffer`.
/// static void *allocateFromBuffer(int                       *cursor,
///                                 char                      *buffer,
///                                 int                        bufferSize,
///                                 int                        size,
///                                 bsls::Alignment::Strategy  strategy)
/// {
/// @endcode
/// First we assert the function pre-conditions:
/// @code
///     assert(cursor);
///     assert(buffer);
///     assert(0 <= bufferSize);
///     assert(0 < size);
/// @endcode
/// Then, based on the alignment `strategy`, we calculate the alignment value
/// that can satisfy the allocation request.  In the case of
/// `bsls::Alignment::BSLS_NATURAL`, we calculate the alignment from `size`; for
/// `bsls::Alignment::BSLS_MAXIMUM`, we use the platform-dependent
/// `my_AlignmentUtil::MY_MAX_PLATFORM_ALIGNMENT` value; and for
/// `bsls::Alignment::BSLS_BYTEALIGNED`, we simply use 1:
/// @code
///     const int alignment =
///                        strategy == bsls::Alignment::BSLS_NATURAL
///                        ? my_AlignmentUtil::calculateAlignmentFromSize(size)
///                        : strategy == bsls::Alignment::BSLS_MAXIMUM
///                          ? my_AlignmentUtil::MY_MAX_PLATFORM_ALIGNMENT
///                          : 1;
/// @endcode
/// Now we calculate the offset from the current `cursor` value that can satisfy
/// the `alignment` requirements:
/// @code
///     const int offset = my_AlignmentUtil::calculateAlignmentOffset(
///                                                           buffer + *cursor,
///                                                           alignment);
///
/// @endcode
/// Next we check if the available free memory in `buffer` can satisfy the
/// allocation request; 0 is returned if the request cannot be satisfied:
/// @code
///     if (*cursor + offset + size > bufferSize) {
///         return 0;                                                 // RETURN
///     }
///
///     void *result = &buffer[*cursor + offset];
///     *cursor += offset + size;
///
/// @endcode
/// Finally, return the address of the correctly aligned memory block:
/// @code
///     return result;
/// }
/// @endcode
/// The `allocateFromBuffer` function may be used by a memory manager that needs
/// to appropriately align memory blocks that are allocated from
/// internally-managed buffers.  For an example, see the @ref bslma_bufferimputil 
/// component.
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bsls
 * @{
 */
/** @addtogroup bsls_alignment
 * @{
 */
 
#ifndef BDE_OMIT_INTERNAL_DEPRECATED
 
#ifndef BDE_DONT_ALLOW_TRANSITIVE_INCLUDES
#include <bsls_alignmentfromtype.h>
#include <bsls_alignmentimp.h>
#include <bsls_alignmenttotype.h>
#include <bsls_platform.h>
 
#include <cstddef>      // for std::size_t
#endif // BDE_DONT_ALLOW_TRANSITIVE_INCLUDES
 
// Temporarily define the legacy 'bsls_AlignmentOf' to refer to its
// replacement, 'bsls::AlignmentFromType'.
 
#ifndef bsls_AlignmentOf
#define bsls_AlignmentOf bsls::AlignmentFromType
#endif
 
// required for some code below wrapped in ifndef BDE_OMIT_INTERNAL_DEPRECATED
 
#include <bsls_alignmentutil.h>
#endif // BDE_OMIT_INTERNAL_DEPRECATED
 
 
 
namespace bsls {
 
                        // ================
                        // struct Alignment
                        // ================
 
/// This `struct` provides a namespace for enumerating the set of strategies
/// for aligning arbitrary blocks of memory.
struct Alignment {
 
    // TYPES
 
    /// Types of alignment strategy.
    enum Strategy {
        /// Align memory block based on the most restrictive alignment
        /// requirements of the host platform.
        BSLS_MAXIMUM = 0,
 
        /// Align memory block on an address that is the largest power of
        /// two that evenly divides the size (in bytes) of the block.
        BSLS_NATURAL = 1,
 
        /// Align memory block based on the least restrictive alignment
        /// requirements of the host platform (1-byte aligned).
        BSLS_BYTEALIGNED = 2
    };
 
#ifndef BDE_OMIT_INTERNAL_DEPRECATED
    /// Define the minimum alignment that satisfies all types.
    ///
    /// @deprecated  This `enum` is deprecated.  Use
    /// `AlignmentUtil::BSLS_MAX_ALIGNMENT` instead.
    enum {
        MAX_ALIGNMENT = AlignmentUtil::BSLS_MAX_ALIGNMENT
    };
 
    /// Primitive type with most stringent alignment requirement.
    ///
    /// @deprecated  Use @ref AlignmentUtil::MaxAlignedType instead.
    typedef AlignmentUtil::MaxAlignedType MaxAlignedType;
 
    /// `struct` with most stringent alignment requirement.
    ///
    /// @deprecated  Use @ref Alignment::MaxAlignedType instead.
    struct Align {
        MaxAlignedType d_align;
    };
#endif // BDE_OMIT_INTERNAL_DEPRECATED
 
    // CLASS METHODS
 
    /// Return the string representation of the specified enumerator
    /// `value`.  The string representation of `value` matches its
    /// corresponding enumerator name with the "BSLS_" prefix elided.  For
    /// example:
    /// @code
    /// bsl::cout << Alignment::toAscii(Alignment::BSLS_NATURAL);
    /// @endcode
    /// will print the following on standard output:
    /// @code
    /// NATURAL
    /// @endcode
    static const char *toAscii(Alignment::Strategy value);
 
#ifndef BDE_OMIT_INTERNAL_DEPRECATED
    /// Calculate a usable alignment for an object of specified `size` bytes
    /// in the absence of compile-time knowledge of the object's alignment
    /// requirements.  Return the largest power of two that evenly divides
    /// size, up to a maximum of `MAX_ALIGNMENT`.  It is guaranteed that an
    /// object of `size` bytes can be safely aligned on this return value.
    /// Depending on the machine architecture and compiler options, the
    /// returned alignment may be more than required for two reasons:
    /// @code
    ///   1. The object may be composed entirely of elements, such as
    ///      'char', which have minimal alignment restrictions.
    ///
    ///   2. The architecture may have lenient alignment requirements.
    ///      Even if this is the case, aligning on a stricter boundary may
    ///      improve performance.
    /// @endcode
    /// The behavior is undefined unless `0 < size`.
    ///
    /// @deprecated  Use @ref AlignmentUtil::calculateAlignmentFromSize instead.
    static int calculateAlignmentFromSize(int size);
 
    /// Return the smallest non-negative offset in bytes that, when added to
    /// the specified `address`, yields the specified `alignment`.  The
    /// behavior is undefined unless `0 < alignment` and `alignment` is a
    /// power of 2.
    ///
    /// @deprecated  Use @ref AlignmentUtil::calculateAlignmentOffset instead.
    static int calculateAlignmentOffset(const void *address, int alignment);
 
    /// Return `true` if the specified `address` is aligned on a 2-byte
    /// boundary (i.e., the integer value of `address` is divisible by 2),
    /// and `false` otherwise.
    ///
    /// @deprecated  Use @ref AlignmentUtil::is2ByteAligned instead.
    static bool is2ByteAligned(const void *address);
 
    /// Return `true` if the specified `address` is aligned on a 4-byte
    /// boundary (i.e., the integer value of `address` is divisible by 4),
    /// and `false` otherwise.
    ///
    /// @deprecated  Use @ref AlignmentUtil::is4ByteAligned instead.
    static bool is4ByteAligned(const void *address);
 
    /// Return `true` if the specified `address` is aligned on an 8-byte
    /// boundary (i.e., the integer value of `address` is divisible by 8),
    /// and `false` otherwise.
    ///
    /// @deprecated  Use @ref AlignmentUtil::is8ByteAligned instead.
    static bool is8ByteAligned(const void *address);
#endif // BDE_OMIT_INTERNAL_DEPRECATED
};
 
}  // close package namespace
 
#ifndef BDE_OMIT_INTERNAL_DEPRECATED
 
namespace bsls {
 
// ============================================================================
//                      INLINE FUNCTION DEFINITIONS
// ============================================================================
 
                            // ----------------
                            // struct Alignment
                            // ----------------
 
// CLASS METHODS
inline
int Alignment::calculateAlignmentFromSize(int size)
{
    return AlignmentUtil::calculateAlignmentFromSize(size);
}
 
inline
int Alignment::calculateAlignmentOffset(const void *address,
                                        int         alignment)
{
    return AlignmentUtil::calculateAlignmentOffset(address, alignment);
}
 
inline
bool Alignment::is2ByteAligned(const void *address)
{
    return AlignmentUtil::is2ByteAligned(address);
}
 
inline
bool Alignment::is4ByteAligned(const void *address)
{
    return AlignmentUtil::is4ByteAligned(address);
}
 
inline
bool Alignment::is8ByteAligned(const void *address)
{
    return AlignmentUtil::is8ByteAligned(address);
}
 
}  // close package namespace
 
#endif // BDE_OMIT_INTERNAL_DEPRECATED
 
#ifndef BDE_OPENSOURCE_PUBLICATION  // BACKWARD_COMPATIBILITY
// ============================================================================
//                           BACKWARD COMPATIBILITY
// ============================================================================
 
/// This alias is defined for backward compatibility.
typedef bsls::Alignment bsls_Alignment;
#endif  // BDE_OPENSOURCE_PUBLICATION -- BACKWARD_COMPATIBILITY
 
 
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2013 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------
 
/** @} */
/** @} */
/** @} */