/usr/include/dballe/core/structbuf.h is in libdballe-dev 7.7-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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* core/structbuf - memory or file-backed storage of structures
*
* Copyright (C) 2014 ARPA-SIM <urpsim@smr.arpa.emr.it>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*
* Author: Enrico Zini <enrico@enricozini.com>
*/
#ifndef DBALLE_CORE_STRUCTBUF_H
#define DBALLE_CORE_STRUCTBUF_H
#include <wreport/error.h>
#include <cstdlib>
#include <unistd.h>
#include <sys/mman.h>
namespace dballe {
namespace structbuf {
int make_anonymous_tmpfile();
void write_buffer(int fd, void* buf, size_t size);
}
/**
* Buffer of simple structures that becomes file backed if it grows beyond a
* certain size.
*
* bufsize is the number of T items that we keep in memory before becoming
* file-backed.
*/
template<typename T, int bufsize=1024>
class Structbuf
{
protected:
/**
* In-memory buffer using during appending. When it becomes full, it is
* flushed out to a temporary file.
*/
T* membuf = nullptr;
/// Number of items in membuf
unsigned membuf_last = 0;
/**
* Memory area used for reading. It points to membuf if we are
* memory-backed, or it is the mmap view of the file if we are file-backed
*/
const T* readbuf = (const T*)MAP_FAILED;
/// Number of items appended so far
size_t m_count = 0;
/// Unix file descriptor to the temporary file, or -1 if we are memory
/// backed
int tmpfile_fd = -1;
public:
Structbuf()
: membuf(new T[bufsize])
{
}
~Structbuf()
{
delete[] membuf;
if (tmpfile_fd != -1)
{
if (readbuf != MAP_FAILED)
munmap((void*)readbuf, m_count * sizeof(T));
::close(tmpfile_fd);
}
}
/// Get the number of structures that have been added to the buffer so far
size_t size() const { return m_count; }
/// Return true if the buffer has become file-backed
bool is_file_backed() const { return tmpfile_fd != -1; }
/// Append an item to the buffer
void append(const T& val)
{
if (readbuf != MAP_FAILED)
throw wreport::error_consistency("writing to a Structbuf that is already being read");
if (membuf_last == bufsize)
write_to_file();
membuf[membuf_last++] = val;
++m_count;
}
/// Stop appending and get ready to read back the data
void ready_to_read()
{
if (tmpfile_fd == -1)
readbuf = membuf;
else
{
// Flush the remaining memory data to file
if (membuf_last) write_to_file();
// mmap the file for reading
readbuf = (const T*)mmap(nullptr, m_count * sizeof(T), PROT_READ, MAP_SHARED, tmpfile_fd, 0);
if (readbuf == MAP_FAILED)
throw wreport::error_system("cannot map temporary file contents to memory");
}
}
/// Read back an item
const T& operator[](size_t idx) const
{
return readbuf[idx];
}
protected:
void write_to_file()
{
if (tmpfile_fd == -1)
tmpfile_fd = structbuf::make_anonymous_tmpfile();
structbuf::write_buffer(tmpfile_fd, membuf, sizeof(T) * membuf_last);
membuf_last = 0;
}
};
}
#endif
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