ClickHouse/dbms/include/DB/IO/ReadBufferFromFileDescriptor.h
2016-10-25 09:49:24 +03:00

48 lines
1.1 KiB
C++

#pragma once
#include <unistd.h>
#include <DB/IO/ReadBufferFromFileBase.h>
#include <DB/IO/ReadBuffer.h>
namespace DB
{
/** Use ready file descriptor. Does not open or close a file.
*/
class ReadBufferFromFileDescriptor : public ReadBufferFromFileBase
{
protected:
int fd;
off_t pos_in_file; /// What offset in file corresponds to working_buffer.end().
bool nextImpl() override;
/// Name or some description of file.
std::string getFileName() const override;
public:
ReadBufferFromFileDescriptor(int fd_, size_t buf_size = DBMS_DEFAULT_BUFFER_SIZE, char * existing_memory = nullptr, size_t alignment = 0)
: ReadBufferFromFileBase(buf_size, existing_memory, alignment), fd(fd_), pos_in_file(0) {}
int getFD() const override
{
return fd;
}
off_t getPositionInFile() override
{
return pos_in_file - (working_buffer.end() - pos);
}
private:
/// If 'offset' is small enough to stay in buffer after seek, then true seek in file does not happen.
off_t doSeek(off_t offset, int whence) override;
/// Assuming file descriptor supports 'select', check that we have data to read or wait until timeout.
bool poll(size_t timeout_microseconds);
};
}