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feat(sinks): custom setvbuf buffer size for file sinks (#1799) #3645

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e674be7
feat(sinks): custom setvbuf buffer size for file sinks (#1799)
rachelwelchio Aug 14, 2026
e2ff0cd
feat(sinks): custom setvbuf buffer size for file sinks (#1799)
rachelwelchio Aug 14, 2026
60eccf2
feat(sinks): custom setvbuf buffer size for file sinks (#1799)
rachelwelchio Aug 14, 2026
dc65b9f
feat(sinks): custom setvbuf buffer size for file sinks (#1799)
rachelwelchio Aug 14, 2026
662270a
feat(sinks): custom setvbuf buffer size for file sinks (#1799)
rachelwelchio Aug 14, 2026
2fdd2bb
feat(sinks): custom setvbuf buffer size for file sinks (#1799)
rachelwelchio Aug 14, 2026
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33 changes: 33 additions & 0 deletions include/spdlog/details/file_helper-inl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@
#include <cstdio>
#include <string>
#include <tuple>
#include <vector>

SPDLOG_NAMESPACE_BEGIN
namespace details {
Expand Down Expand Up @@ -48,6 +49,7 @@ SPDLOG_INLINE void file_helper::open(const filename_t &fname, bool truncate) {
std::fclose(tmp);
}
if (!os::fopen_s(&fd_, fname, mode)) {
apply_buffer_();
if (event_handlers_.after_open) {
event_handlers_.after_open(filename_, fd_);
}
Expand All @@ -61,6 +63,37 @@ SPDLOG_INLINE void file_helper::open(const filename_t &fname, bool truncate) {
errno);
}

SPDLOG_INLINE void file_helper::set_buffer(std::size_t size) {
if (size == 0) {
if (fd_ != nullptr) {
if (std::setvbuf(fd_, nullptr, _IOFBF, BUFSIZ) != 0) {
throw_spdlog_ex("Failed setvbuf on file " + os::filename_to_str(filename_), errno);
}
}
buffer_.clear();
return;
}

// Detach any previous user buffer before resizing (avoids dangling FILE* buffer ptr).
if (fd_ != nullptr && !buffer_.empty()) {
if (std::setvbuf(fd_, nullptr, _IOFBF, BUFSIZ) != 0) {
throw_spdlog_ex("Failed setvbuf on file " + os::filename_to_str(filename_), errno);
}
}

buffer_.resize(size);
apply_buffer_();
}

SPDLOG_INLINE void file_helper::apply_buffer_() {
if (fd_ == nullptr || buffer_.empty()) {
return;
}
if (std::setvbuf(fd_, buffer_.data(), _IOFBF, buffer_.size()) != 0) {
throw_spdlog_ex("Failed setvbuf on file " + os::filename_to_str(filename_), errno);
}
}

SPDLOG_INLINE void file_helper::reopen(bool truncate) {
if (filename_.empty()) {
throw_spdlog_ex("Failed re opening file - was not opened before");
Expand Down
9 changes: 9 additions & 0 deletions include/spdlog/details/file_helper.h
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@

#include <spdlog/common.h>
#include <tuple>
#include <vector>

SPDLOG_NAMESPACE_BEGIN
namespace details {
Expand All @@ -31,6 +32,11 @@ class SPDLOG_API file_helper {
size_t size() const;
const filename_t &filename() const;

// Set custom FILE stream buffer size via setvbuf(_IOFBF).
// size == 0 clears the custom buffer and restores default buffering if the file is open.
// Re-applied automatically on open/reopen when size > 0.
void set_buffer(std::size_t size);

//
// return file path and its extension:
//
Expand All @@ -47,11 +53,14 @@ class SPDLOG_API file_helper {
static std::tuple<filename_t, filename_t> split_by_extension(const filename_t &fname);

private:
void apply_buffer_();

const int open_tries_ = 5;
const unsigned int open_interval_ = 10;
std::FILE *fd_{nullptr};
filename_t filename_;
file_event_handlers event_handlers_;
std::vector<char> buffer_;
};
} // namespace details
SPDLOG_NAMESPACE_END
Expand Down
12 changes: 11 additions & 1 deletion include/spdlog/sinks/basic_file_sink-inl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -16,8 +16,12 @@ namespace sinks {
template <typename Mutex>
SPDLOG_INLINE basic_file_sink<Mutex>::basic_file_sink(const filename_t &filename,
bool truncate,
const file_event_handlers &event_handlers)
const file_event_handlers &event_handlers,
std::size_t buffer_size)
: file_helper_{event_handlers} {
if (buffer_size > 0) {
file_helper_.set_buffer(buffer_size);
}
file_helper_.open(filename, truncate);
}

Expand All @@ -32,6 +36,12 @@ SPDLOG_INLINE void basic_file_sink<Mutex>::truncate() {
file_helper_.reopen(true);
}

template <typename Mutex>
SPDLOG_INLINE void basic_file_sink<Mutex>::set_buffer(std::size_t size) {
std::lock_guard<Mutex> lock(base_sink<Mutex>::mutex_);
file_helper_.set_buffer(size);
}

template <typename Mutex>
SPDLOG_INLINE void basic_file_sink<Mutex>::sink_it_(const details::log_msg &msg) {
memory_buf_t formatted;
Expand Down
14 changes: 9 additions & 5 deletions include/spdlog/sinks/basic_file_sink.h
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Original file line number Diff line number Diff line change
Expand Up @@ -21,9 +21,11 @@ class basic_file_sink final : public base_sink<Mutex> {
public:
explicit basic_file_sink(const filename_t &filename,
bool truncate = false,
const file_event_handlers &event_handlers = {});
const file_event_handlers &event_handlers = {},
std::size_t buffer_size = 0);
const filename_t &filename() const;
void truncate();
void set_buffer(std::size_t size);

protected:
void sink_it_(const details::log_msg &msg) override;
Expand All @@ -45,18 +47,20 @@ template <typename Factory = synchronous_factory>
inline std::shared_ptr<logger> basic_logger_mt(const std::string &logger_name,
const filename_t &filename,
bool truncate = false,
const file_event_handlers &event_handlers = {}) {
const file_event_handlers &event_handlers = {},
std::size_t buffer_size = 0) {
return Factory::template create<sinks::basic_file_sink_mt>(logger_name, filename, truncate,
event_handlers);
event_handlers, buffer_size);
}

template <typename Factory = synchronous_factory>
inline std::shared_ptr<logger> basic_logger_st(const std::string &logger_name,
const filename_t &filename,
bool truncate = false,
const file_event_handlers &event_handlers = {}) {
const file_event_handlers &event_handlers = {},
std::size_t buffer_size = 0) {
return Factory::template create<sinks::basic_file_sink_st>(logger_name, filename, truncate,
event_handlers);
event_handlers, buffer_size);
}

SPDLOG_NAMESPACE_END
Expand Down
12 changes: 11 additions & 1 deletion include/spdlog/sinks/rotating_file_sink-inl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -28,7 +28,8 @@ SPDLOG_INLINE rotating_file_sink<Mutex>::rotating_file_sink(
std::size_t max_size,
std::size_t max_files,
bool rotate_on_open,
const file_event_handlers &event_handlers)
const file_event_handlers &event_handlers,
std::size_t buffer_size)
: base_filename_(std::move(base_filename)),
max_size_(max_size),
max_files_(max_files),
Expand All @@ -40,6 +41,9 @@ SPDLOG_INLINE rotating_file_sink<Mutex>::rotating_file_sink(
if (max_files > MaxFiles) {
throw_spdlog_ex("rotating sink constructor: max_files arg cannot exceed MaxFiles");
}
if (buffer_size > 0) {
file_helper_.set_buffer(buffer_size);
}
file_helper_.open(calc_filename(base_filename_, 0));
current_size_ = file_helper_.size(); // expensive. called only once
if (rotate_on_open && current_size_ > 0) {
Expand Down Expand Up @@ -109,6 +113,12 @@ std::size_t rotating_file_sink<Mutex>::get_current_size() {
return current_size_;
}

template <typename Mutex>
SPDLOG_INLINE void rotating_file_sink<Mutex>::set_buffer(std::size_t size) {
std::lock_guard<Mutex> lock(base_sink<Mutex>::mutex_);
file_helper_.set_buffer(size);
}

template <typename Mutex>
SPDLOG_INLINE void rotating_file_sink<Mutex>::sink_it_(const details::log_msg &msg) {
memory_buf_t formatted;
Expand Down
16 changes: 11 additions & 5 deletions include/spdlog/sinks/rotating_file_sink.h
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Original file line number Diff line number Diff line change
Expand Up @@ -25,7 +25,8 @@ class rotating_file_sink final : public base_sink<Mutex> {
std::size_t max_size,
std::size_t max_files,
bool rotate_on_open = false,
const file_event_handlers &event_handlers = {});
const file_event_handlers &event_handlers = {},
std::size_t buffer_size = 0);
static filename_t calc_filename(const filename_t &filename, std::size_t index);
filename_t filename();
void rotate_now();
Expand All @@ -34,6 +35,7 @@ class rotating_file_sink final : public base_sink<Mutex> {
void set_max_files(std::size_t max_files);
std::size_t get_max_files();
std::size_t get_current_size();
void set_buffer(std::size_t size);

protected:
void sink_it_(const details::log_msg &msg) override;
Expand Down Expand Up @@ -72,9 +74,11 @@ std::shared_ptr<logger> rotating_logger_mt(const std::string &logger_name,
size_t max_file_size,
size_t max_files,
bool rotate_on_open = false,
const file_event_handlers &event_handlers = {}) {
const file_event_handlers &event_handlers = {},
std::size_t buffer_size = 0) {
return Factory::template create<sinks::rotating_file_sink_mt>(
logger_name, filename, max_file_size, max_files, rotate_on_open, event_handlers);
logger_name, filename, max_file_size, max_files, rotate_on_open, event_handlers,
buffer_size);
}

template <typename Factory = synchronous_factory>
Expand All @@ -83,9 +87,11 @@ std::shared_ptr<logger> rotating_logger_st(const std::string &logger_name,
size_t max_file_size,
size_t max_files,
bool rotate_on_open = false,
const file_event_handlers &event_handlers = {}) {
const file_event_handlers &event_handlers = {},
std::size_t buffer_size = 0) {
return Factory::template create<sinks::rotating_file_sink_st>(
logger_name, filename, max_file_size, max_files, rotate_on_open, event_handlers);
logger_name, filename, max_file_size, max_files, rotate_on_open, event_handlers,
buffer_size);
}
SPDLOG_NAMESPACE_END

Expand Down