src/libcamera/file_descriptor.cpp


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/* SPDX-License-Identifier: LGPL-2.1-or-later */
/*
 * Copyright (C) 2019, Google Inc.
 *
 * file_descriptor.cpp - File descriptor wrapper
 */

#include <libcamera/file_descriptor.h>

#include <string.h>
#include <unistd.h>
#include <utility>

#include "log.h"

/**
 * \file file_descriptor.h
 * \brief File descriptor wrapper
 */

namespace libcamera {

LOG_DEFINE_CATEGORY(FileDescriptor)

/**
 * \class FileDescriptor
 * \brief RAII-style wrapper for file descriptors
 *
 * The FileDescriptor class provides RAII-style lifetime management of file
 * descriptors with an efficient mechanism for ownership sharing. At its core,
 * an internal Descriptor object wraps a file descriptor (expressed as a signed
 * integer) with an RAII-style interface. The Descriptor is then implicitly
 * shared with all FileDescriptor instances constructed as copies.
 *
 * When constructed from a numerical file descriptor, the FileDescriptor
 * instance duplicates the file descriptor and wraps the duplicate as a
 * Descriptor. The copy constructor and assignment operator create copies that
 * share the Descriptor, while the move versions of those methods additionally
 * make the other FileDescriptor invalid. When the last FileDescriptor that
 * references a Descriptor is destroyed, the file descriptor is closed.
 *
 * The numerical file descriptor is available through the fd() method. As
 * constructing a FileDescriptor from a numerical file descriptor duplicates
 * the file descriptor, the value returned by fd() will be different than the
 * value passed to the constructor. All FileDescriptor instances created as
 * copies of a FileDescriptor will report the same fd() value. Callers can
 * perform operations on the fd(), but shall never close it manually.
 */

/**
 * \brief Create a FileDescriptor wrapping a copy of a given \a fd
 * \param[in] fd File descriptor
 *
 * Constructing a FileDescriptor from a numerical file descriptor duplicates the
 * \a fd and takes ownership of the copy. The original \a fd is left untouched,
 * and the caller is responsible for closing it when appropriate. The duplicated
 * file descriptor will be closed automatically when all FileDescriptor
 * instances that reference it are destroyed.
 *
 * If the \a fd is negative, the FileDescriptor is constructed as invalid and
 * the fd() method will return -1.
 */
FileDescriptor::FileDescriptor(int fd)
{
	if (fd < 0)
		return;

	fd_ = std::make_shared<Descriptor>(fd);
	if (fd_->fd() < 0)
		fd_.reset();
}

/**
 * \brief Copy constructor, create a FileDescriptor from a copy of \a other
 * \param[in] other The other FileDescriptor
 *
 * Copying a FileDescriptor implicitly shares ownership of the wrapped file
 * descriptor. The original FileDescriptor is left untouched, and the caller is
 * responsible for destroying it when appropriate. The wrapped file descriptor
 * will be closed automatically when all FileDescriptor instances that
 * reference it are destroyed.
 */
FileDescriptor::FileDescriptor(const FileDescriptor &other)
	: fd_(other.fd_)
{
}

/**
 * \brief Move constructor, create a FileDescriptor by taking over \a other
 * \param[in] other The other FileDescriptor
 *
 * Moving a FileDescriptor moves the reference to the wrapped descriptor owned
 * by \a other to the new FileDescriptor. The \a other FileDescriptor is
 * invalidated and its fd() method will return -1. The wrapped file descriptor
 * will be closed automatically when all FileDescriptor instances that
 * reference it are destroyed.
 */
FileDescriptor::FileDescriptor(FileDescriptor &&other)
	: fd_(std::move(other.fd_))
{
}

/**
 * \brief Destroy the FileDescriptor instance
 *