Sink overview

The split of responsibility is deliberate: a logger decides whether to log at all, a sink decides where and how an accepted message ends up. Everything about spdlog's sink family — the built-in ones and the ones you write — follows from that one interface.

The sink interface

class sink {
public:
    virtual ~sink() = default;
    virtual void log(const spdlog::details::log_msg& msg) = 0;
    virtual void flush() = 0;
    virtual void set_pattern(const std::string& pattern) = 0;
    virtual void set_formatter(std::unique_ptr<spdlog::formatter> sink_formatter) = 0;
};

log receives an already level-checked message; flush forces buffered output out; the two set_pattern/set_formatter calls control how that message is rendered before it's written.

Decision diagram

The _mt / _st suffix

Every built-in sink ships in a mutex-guarded (_mt) and unguarded (_st) flavor — same class, different locking.

	_st	_mt
Locking cost	None	One mutex lock/unlock per log() call
Safe from multiple threads	No	Yes
When to use	A logger you've confirmed is single-threaded	The default choice

Sinks are shareable

A sink_ptr (a std::shared_ptr<sink>) can back more than one logger. That's how two independent components write to the same file without interleaving corrupted lines — the mutex lives on the sink, so every writer through it serializes correctly regardless of which logger they came through. See Multi-sink loggers for the construction pattern.

Built-in sink families

Family	Header	Page
Console	spdlog/sinks/stdout_color_sinks.h	Console sinks
File	spdlog/sinks/basic_file_sink.h	File sinks
Rotating / daily	spdlog/sinks/rotating_file_sink.h, daily_file_sink.h	Rotating and daily sinks
Syslog / platform	spdlog/sinks/syslog_sink.h and others	Syslog and platform sinks

See also

• Console sinks — the destination that's always there.
• Custom sinks — the extension point when nothing built-in fits.
• Multi-sink loggers — attaching several sinks to one logger.
• spdlog overview.