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Rotating and daily sinks

Two different answers to "the log file cannot grow forever": rotate by size whenever the current file gets too big, or start a fresh file on a schedule regardless of size.

rotating_file_sink​

auto sink = std::make_shared<spdlog::sinks::rotating_file_sink_mt>(
    "logs/app.log",
    1024 * 1024 * 10,   // max_size: 10 MB per file
    5                    // max_files: keep 5 rotated files
);

When app.log reaches max_size, it's renamed app.1.log, the previous app.1.log becomes app.2.log, and so on up to max_files; the oldest file beyond that count is deleted, and a fresh app.log is opened.

daily_file_sink​

auto sink = std::make_shared<spdlog::sinks::daily_file_sink_mt>(
    "logs/app", 2, 30);   // roll at 02:30 local time

The filename gets a date stamp (app_2026-08-07.log), a new file opens at the given hour:minute each day, and an optional max_files retention parameter (a later overload) deletes files older than that many days.

Comparison table​

rotatingdaily
Disk-usage boundYes — max_size × max_filesOnly if max_files retention is set
Filename predictabilityNumbered suffix, changes as it rotatesDate-stamped, one file per day
Process running minutesMay never rotate at allStill produces one dated file
Process running monthsRotates repeatedly, bounded total sizeOne new file every day, unbounded unless retention is set
External logrotateRedundant/conflicting — spdlog already rotatesSame — don't combine

Rotation on open​

rotating_file_sink_mt accepts a rotate_on_open flag — when true, the sink rotates immediately on construction rather than waiting for max_size to be hit. Use it when you want a guaranteed fresh file per process run instead of appending to whatever's already there.

Interaction with logrotate​

Do not point external logrotate at a file spdlog holds open — the writes go to the unlinked inode

If an external tool renames or removes the file spdlog has open, spdlog keeps writing to the original (now unlinked) file descriptor — the data goes nowhere anyone can read it until the process restarts and reopens the path. Let spdlog own rotation for files it writes; don't layer logrotate on top.

Choosing​

Bound by disk quota → rotating. Bound by "one file per day for ops" → daily.

Reach for rotating_file_sink when the constraint is "don't let logs eat the disk." Reach for daily_file_sink when the constraint is operational — humans or tooling expect one file per calendar day regardless of volume.

See also​