ClickHouse/src/IO/ReadBufferFromFileDescriptor.h
2020-07-31 17:53:41 +03:00

47 lines
1.2 KiB
C++

#pragma once
#include <IO/ReadBufferFromFileBase.h>
#include <unistd.h>
namespace DB
{
/** Use ready file descriptor. Does not open or close a file.
*/
class ReadBufferFromFileDescriptor : public ReadBufferFromFileBase
{
protected:
int fd;
size_t file_offset_of_buffer_end; /// 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_), file_offset_of_buffer_end(0) {}
int getFD() const
{
return fd;
}
off_t getPosition() override
{
return file_offset_of_buffer_end - (working_buffer.end() - pos);
}
/// If 'offset' is small enough to stay in buffer after seek, then true seek in file does not happen.
off_t seek(off_t off, int whence) override;
private:
/// Assuming file descriptor supports 'select', check that we have data to read or wait until timeout.
bool poll(size_t timeout_microseconds);
};
}