Skip to main content

Console sinks

Console output is the default destination because it's the one that always exists — no file permissions to worry about, no disk to fill. The interesting decisions here are which stream you write to and whether you color the output.

stdout vs stderr​

stdout_color_sink_mt/_st and stderr_color_sink_mt/_st write colored output to stdout or stderr respectively; stdout_sink_mt/_st and stderr_sink_mt/_st are the plain, uncolored equivalents.

Log errors to stderr so pipelines can separate them

If your process's stdout is piped somewhere meaningful (another program, a file), routing warn+ to a stderr sink keeps operational noise out of that pipe while still surfacing problems on the terminal.

Color​

Color is applied per level using a default mapping (red for errors, yellow for warnings, and so on), which you can override:

sink->set_color(spdlog::level::info, sink->green);

Only the part of the pattern between %^ and %$ is colored — everything outside that range prints in the terminal's default color:

sink->set_pattern("[%H:%M:%S] %^[%l]%$ %v");   // only "[info]" (etc.) is colored

See Pattern flags for the rest of the pattern syntax.

Color and redirection​

ANSI codes end up in your log file when stdout is redirected

./myapp > out.log still sends spdlog raw ANSI escape codes if you used a color sink — spdlog does not detect the redirection for you. Use a non-color sink (stdout_sink_mt) whenever the destination might not be an interactive terminal, or detect TTY-ness yourself and pick the sink accordingly.

Windows​

Windows console color historically didn't support ANSI escapes the way Unix terminals do, so spdlog provides wincolor_stdout_sink_mt using the Windows console API directly instead of writing escape codes. Watch for the UTF-8 code page caveat — non-ASCII output can come out mangled unless the console is switched to UTF-8 (chcp 65001) or you use the Windows-native sink's built-in handling.

Throughput​

Console I/O is slow and serialized by the OS terminal driver — orders of magnitude slower than writing to a file, let alone an in-memory buffer.

If console logging is in your hot path, move it to a file or async

A tight loop logging to console at info level will bottleneck on terminal I/O long before it bottlenecks on formatting. See Async vs sync trade-offs for when moving console output off the caller's thread is worth the complexity.

See also​