bdls_filedescriptorguard.h

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/// @file bdls_filedescriptorguard.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bdls_filedescriptorguard.h                                         -*-C++-*-
#ifndef INCLUDED_BDLS_FILEDESCRIPTORGUARD
#define INCLUDED_BDLS_FILEDESCRIPTORGUARD
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bdls_filedescriptorguard bdls_filedescriptorguard
/// @brief  Provide a RAII guard class used to close files.
/// @addtogroup bdl
/// @{
/// @addtogroup bdls
/// @{
/// @addtogroup bdls_filedescriptorguard
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bdls_filedescriptorguard-purpose"> Purpose</a>
/// * <a href="#bdls_filedescriptorguard-classes"> Classes </a>
/// * <a href="#bdls_filedescriptorguard-description"> Description </a>
///   * <a href="#bdls_filedescriptorguard-usage"> Usage </a>
///     * <a href="#bdls_filedescriptorguard-example-1-close-a-file-descriptor"> Example 1: Close a File Descriptor </a>
///
/// # Purpose {#bdls_filedescriptorguard-purpose}
/// Provide a RAII guard class used to close files.
///
/// # Classes {#bdls_filedescriptorguard-classes}
///
/// -  bdls::FileDescriptorGuard: RAII guard class used to close files
///
/// @see  bdls_filesystemutil
///
/// # Description {#bdls_filedescriptorguard-description}
/// This component defines a class, `bdls::FileDescriptorGuard`, an
/// object of which manages an open file descriptor, and closes it when the
/// guard goes out of scope and is destroyed.  A `release` method is provided,
/// which will release the descriptor from management by the guard.  When a
/// released guard is destroyed, nothing happens.  A `closeAndRelease` method
/// is also provided, which closes the managed file handle and puts the guard
/// into a released state.
///
/// ## Usage {#bdls_filedescriptorguard-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Close a File Descriptor {#bdls_filedescriptorguard-example-1-close-a-file-descriptor}
///
///
/// Suppose we want to open a file and perform some I/O operations.  We use an
/// object of type `bdls::FileDescriptorGuard` to ensure this handle is closed
/// after the operations are complete.
///
/// First, we create a name for our temporary file name and a few local
/// variables.
/// @code
/// const bsl::string fileName = "essay.txt";
/// int rc;
/// @endcode
/// Then, we open the file:
/// @code
/// Util::FileDescriptor fd = Util::open(fileName,
///                                      Util::e_CREATE,
///                                      Util::e_READ_WRITE);
/// assert(Util::k_INVALID_FD != fd);
/// @endcode
/// Next, we enter a lexical scope and create a guard object to manage `fd`:
/// @code
/// {
///     bdls::FileDescriptorGuard guard(fd);
/// @endcode
/// Then, we declare an essay we would like to write to the file:
/// @code
///     const char essay[] = {
///                        "If you can't annoy somebody, there is little\n"
///                        "point in writing.\n"
///                        "                Kingsley Amis\n"
///                        "\n"
///                        "It takes a lifetime to build a reputation, and\n"
///                        "five minutes to lose it.\n"
///                        "                Warren Buffet\n"
///                        "\n"
///                        "Originality is stubborn but not indestructible.\n"
///                        "You can't tell it what to do, and if you try too\n"
///                        "hard to steer it, you either chase it away or\n"
///                        "murder it.\n"
///                        "                Salman Khan\n" };
/// @endcode
/// Next, we write our essay to the file:
/// @code
///     rc = Util::write(fd, essay, sizeof(essay));
///     assert(sizeof(essay) == rc);
/// @endcode
/// Now, `guard` goes out of scope, and its destructor closes the file
/// descriptor.
/// @code
/// }
/// @endcode
/// Finally, we observe that further attempts to access `fd` fail because the
/// descriptor has been closed:
/// @code
/// Util::Offset off = Util::seek(fd,
///                               0,
///                               Util::e_SEEK_FROM_BEGINNING);
/// assert(-1 == off);
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bdl
 * @{
 */
/** @addtogroup bdls
 * @{
 */
/** @addtogroup bdls_filedescriptorguard
 * @{
 */
 
#include <bdlscm_version.h>
 
#include <bdls_filesystemutil.h>
 
#include <bsls_assert.h>
 
 
 
namespace bdls {
                         // ==========================
                         // struct FileDescriptorGuard
                         // ==========================
 
/// This class implements a guard that conditionally closes an open file
/// descriptor upon its destruction.
///
/// See @ref bdls_filedescriptorguard 
struct FileDescriptorGuard {
 
    // DATA
    FilesystemUtil::FileDescriptor d_descriptor;    // handle for the
                                                    // merged file
 
  private:
    // NOT IMPLEMENTED
    FileDescriptorGuard(const FileDescriptorGuard&);
    FileDescriptorGuard& operator=(const FileDescriptorGuard&);
 
  public:
    // CREATORS
 
    /// Create a guard object that will manage the specified `descriptor`,
    /// closing it upon destruction (unless either `realease` or
    /// `closeAndRelease` has been called).  It is permissible for
    /// `descriptor` to be `FilesystemUtil::k_INVALID_FD`, in which case the
    /// guard created will not manage anything.
    explicit
    FileDescriptorGuard(FilesystemUtil::FileDescriptor descriptor);
 
    /// If this guard object manages a file, close that file.
    ~FileDescriptorGuard();
 
    // MANIPULATORS
 
    /// Close the file managed by this guard and release that file from management by this object.  
    /// 
    /// \pre The behavior is undefined   unless this
    /// object is managing a file.
    void closeAndRelease();
 
    /// Release the file from management by this object (without closing it)
    /// and return the formerly managed descriptor.  
    /// 
    /// \pre The behavior is undefined   unless this object is managing a file.
    FilesystemUtil::FileDescriptor release();
 
    // ACCESSORS
 
    /// If this guard is managing a file, return the file descriptor
    /// referring to that file, and return `FilesystemUtil::k_INVALID_FD`
    /// otherwise.
    FilesystemUtil::FileDescriptor descriptor() const;
};
 
// ============================================================================
//                            INLINE DEFINITIONS
// ============================================================================
 
// CREATORS
inline
FileDescriptorGuard::FileDescriptorGuard(
                               FilesystemUtil::FileDescriptor descriptor)
: d_descriptor(descriptor)
{
}
 
inline
FileDescriptorGuard::~FileDescriptorGuard()
{
    if (FilesystemUtil::k_INVALID_FD != d_descriptor) {
        closeAndRelease();
    }
}
 
// MANIPULATORS
inline
FilesystemUtil::FileDescriptor FileDescriptorGuard::release()
{
    BSLS_ASSERT(FilesystemUtil::k_INVALID_FD != d_descriptor);
 
    FilesystemUtil::FileDescriptor ret = d_descriptor;
    d_descriptor = FilesystemUtil::k_INVALID_FD;
 
    return ret;
}
 
// ACCESSORS
inline
FilesystemUtil::FileDescriptor FileDescriptorGuard::descriptor()
                                                                          const
{
    return d_descriptor;
}
 
}  // close package namespace
 
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2015 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */