diff --git a/storm/Common.hpp b/storm/Common.hpp
new file mode 100644
index 0000000..bffb7d5
--- /dev/null
+++ b/storm/Common.hpp
@@ -0,0 +1,16 @@
+#ifndef STORM_COMMON_HPP
+#define STORM_COMMON_HPP
+
+
+#define DECLARE_STRICT_HANDLE(name) \
+    typedef struct name##__ {       \
+        int unused;                 \
+    }* name
+
+#define DECLARE_DERIVED_HANDLE(name, base)      \
+    typedef struct name##__ : public base##__ { \
+        int unused;                             \
+    }* name
+
+
+#endif
diff --git a/storm/Log.cpp b/storm/Log.cpp
new file mode 100644
index 0000000..5b3f624
--- /dev/null
+++ b/storm/Log.cpp
@@ -0,0 +1,606 @@
+#include "storm/Log.hpp"
+#include "storm/Thread.hpp"
+#include "storm/Error.hpp"
+#include "storm/Memory.hpp"
+#include <cstdio>
+#include <cstdlib>
+#include <ctime>
+#include <cstring>
+
+#if defined(WHOA_SYSTEM_LINUX)
+#include <sys/stat.h>
+#include <sys/types.h>
+#endif
+
+#if defined(WHOA_SYSTEM_MAC)
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <mach/mach_time.h>
+#include <mach-o/dyld.h>
+#include <sys/syslimits.h>
+#endif
+
+
+#define STORM_LOG_MAX_CHANNELS 4
+#define STORM_LOG_MAX_BUFFER 0x10000
+#define STORM_LOG_FLUSH_POINT 0xC000
+
+
+typedef struct _LOG {
+    HSLOG log;
+    _LOG* next;
+    char filename[STORM_MAX_PATH];
+    FILE* file;
+    uint32_t flags;
+    size_t bufferused;
+    size_t pendpoint;
+    int32_t indent;
+    int32_t timeStamp;
+    char buffer[STORM_LOG_MAX_BUFFER];
+} LOG;
+
+
+static uint64_t lasttime = 0;
+static size_t timestrlen = 0;
+static char timestr[64] = { '\0' };
+
+static SCritSect* s_critsect[STORM_LOG_MAX_CHANNELS] = { nullptr };
+static SCritSect* s_defaultdir_critsect = nullptr;
+
+static LOG* s_loghead[STORM_LOG_MAX_CHANNELS] = { nullptr };
+static HSLOG s_sequence = 0;
+
+static char s_defaultdir[STORM_MAX_PATH] = { '\0' };
+static bool s_logsysteminit = false;
+
+
+static LOG* LockLog(HSLOG log, HLOCKEDLOG* lockedhandle, bool createifnecessary) {
+    if (!log) {
+        *lockedhandle = (HLOCKEDLOG)-1;
+        return nullptr;
+    }
+
+    size_t index = reinterpret_cast<size_t>(log) & 3;
+    s_critsect[index]->Enter();
+    *lockedhandle = (HLOCKEDLOG)index;
+
+    LOG* result = s_loghead[index];
+    LOG** p_next = &s_loghead[index];
+
+    while (result && result->log != log) {
+        p_next = &result->next;
+        result = result->next;
+    }
+
+    if (result) {
+        return result;
+    }
+
+    if (createifnecessary) {
+        result = (LOG*)SMemAlloc(sizeof(LOG), __FILE__, __LINE__, 0);
+    }
+
+    if (!result) {
+        s_critsect[index]->Leave();
+        *lockedhandle = (HLOCKEDLOG)-1;
+        return nullptr;
+    }
+
+    *p_next = result;
+    result->log = log;
+    result->next = nullptr;
+    result->filename[0] = '\0';
+    result->file = nullptr;
+    result->bufferused = 0;
+    result->pendpoint = 0;
+
+    return result;
+}
+
+static void UnlockLog(HLOCKEDLOG lockedhandle) {
+    s_critsect[reinterpret_cast<size_t>(lockedhandle)]->Leave();
+}
+
+static void UnlockDeleteLog(LOG* logptr, HLOCKEDLOG lockedhandle) {
+    size_t index = reinterpret_cast<size_t>(lockedhandle);
+    LOG* log = s_loghead[index];
+    LOG** p_next = &s_loghead[index];
+
+    while (log && log != logptr) {
+        p_next = &log->next;
+        log = log->next;
+    }
+
+    if (log) {
+        *p_next = log->next;
+        SMemFree(log);
+    }
+
+    s_critsect[index]->Leave();
+}
+
+static void FlushLog(LOG* logptr) {
+    if (!logptr->bufferused) {
+        return;
+    }
+
+    STORM_ASSERT(logptr->file);
+    fwrite(logptr->buffer, 1, logptr->bufferused, logptr->file);
+    fflush(logptr->file);
+    logptr->bufferused = 0;
+    logptr->pendpoint = 0;
+}
+
+static const char* PrependDefaultDir(char* newfilename, uint32_t newfilenamesize, const char* filename) {
+    if (!filename || !filename[0] || filename[1] == ':' || SStrChr(filename, '\\') || SStrChr(filename, '/')) {
+        return filename;
+    }
+
+    s_defaultdir_critsect->Enter();
+
+    if (s_defaultdir[0]) {
+        SStrCopy(newfilename, s_defaultdir, newfilenamesize);
+        SStrPack(newfilename, filename, newfilenamesize);
+    } else {
+#if defined(WHOA_SYSTEM_WIN)
+        GetModuleFileName(NULL, newfilename, newfilenamesize);
+        char* slash = SStrChrR(newfilename, '\\');
+        if (slash) {
+            slash[0] = '\0';
+        }
+
+        SStrPack(newfilename, "\\", newfilenamesize);
+        SStrPack(newfilename, filename, newfilenamesize);
+#endif
+
+#if defined(WHOA_SYSTEM_LINUX)
+        char buffer[PATH_MAX];
+        if (!realpath("/proc/self/exe", buffer)) {
+            buffer[0] = '\0';
+        }
+
+        char* slash = SStrChrR(buffer, '/');
+        if (slash) {
+            slash[0] = '\0';
+        }
+
+        SStrCopy(newfilename, buffer, newfilenamesize);
+        SStrPack(newfilename, "/", newfilenamesize);
+        SStrPack(newfilename, filename, newfilenamesize);
+#endif
+
+#if defined(WHOA_SYSTEM_MAC)
+        newfilename[0] = '\0';
+
+        char path[PATH_MAX] = {0};
+        char actualPath[PATH_MAX] = {0};
+        uint32_t size = PATH_MAX;
+        if (_NSGetExecutablePath(path, &size) == 0) {
+            if (!realpath(path, actualPath)) {
+                actualPath[0] = '\0';
+            }
+        }
+
+        char* slash = SStrChrR(actualPath, '/');
+        if (slash) {
+            slash[0] = '\0';
+        }
+
+        SStrCopy(newfilename, actualPath, newfilenamesize);
+        SStrPack(newfilename, "/", newfilenamesize);
+        SStrPack(newfilename, filename, newfilenamesize);
+#endif
+    }
+
+    s_defaultdir_critsect->Leave();
+    return newfilename;
+}
+
+static size_t PathGetRootChars(const char* path) {
+    if (path[0] == '/') {
+        if (!SStrCmpI(path, "/Volumes/", 9)) {
+            const char* offset = SStrChr(path + 9, '/');
+            if (offset) {
+                return offset - path + 1;
+            }
+        }
+        return 1;
+    }
+
+    if (SStrLen(path) < 2) {
+        return 0;
+    }
+
+    if (path[1] == ':') {
+        return (path[2] == '\\') ? 3 : 2;
+    }
+
+    if (path[0] != '\\' || path[1] != '\\') {
+        return 0;
+    }
+
+    const char* slash1 = SStrChr(path + 2, '\\');
+    if (!slash1) {
+        return 0;
+    }
+
+    const char* slash2 = SStrChr(slash1 + 1, '\\');
+    if (!slash2) {
+        return 0;
+    }
+
+    return slash2 - path + 1;
+}
+
+static void PathStripFilename(char* path) {
+    char* slash = SStrChrR(path, '/');
+    char* backslash = SStrChrR(path, '\\');
+    if (slash <= backslash) {
+        slash = backslash;
+    }
+
+    if (!slash) {
+        return;
+    }
+
+    char* relative = &path[PathGetRootChars(path)];
+    if (slash >= relative) {
+        slash[1] = 0;
+    } else {
+        relative[0] = 0;
+    }
+}
+
+static void PathConvertSlashes(char* path) {
+    while (*path) {
+#if defined(WHOA_SYSTEM_WIN)
+        if (*path == '/') {
+            *path = '\\';
+        }
+#else
+        if (*path == '\\') {
+            *path = '/';
+        }
+#endif
+        path++;
+    }
+}
+
+static bool CreateFileDirectory(const char* path) {
+    STORM_ASSERT(path);
+
+    char buffer[STORM_MAX_PATH];
+    SStrCopy(buffer, path, STORM_MAX_PATH);
+    PathStripFilename(buffer);
+
+    char* relative = &buffer[PathGetRootChars(buffer)];
+    PathConvertSlashes(relative);
+
+    for (size_t i = 0; relative[i]; ++i) {
+        if (relative[i] != '\\' && relative[i] != '/') {
+            continue;
+        }
+
+        char slash = relative[i];
+        relative[i] = '\0';
+#if defined(WHOA_SYSTEM_WIN)
+        CreateDirectory(buffer, NULL);
+#else
+        mkdir(buffer, 0755);
+#endif
+        relative[i] = slash;
+    }
+
+#if defined(WHOA_SYSTEM_WIN)
+    return CreateDirectory(buffer, NULL);
+#else
+    return !mkdir(buffer, 0755);
+#endif
+}
+
+static bool OpenLogFile(const char* filename, FILE** file, uint32_t flags) {
+    *file = nullptr;
+
+    if (filename && filename[0]) {
+        char newfilename[STORM_MAX_PATH];
+        PrependDefaultDir(newfilename, STORM_MAX_PATH, filename);
+        CreateFileDirectory(newfilename);
+        *file = fopen(newfilename, (flags & SLOG_FLAG_APPEND) ? "a" : "w");
+        return (*file != nullptr);
+    }
+    return false;
+}
+
+static void OutputIndent(LOG* logptr) {
+    int32_t indent = logptr->indent;
+    if (indent > 0) {
+        if (indent >= 128) {
+            indent = 128;
+        }
+        memset(&logptr->buffer[logptr->bufferused], ' ', indent);
+        logptr->buffer[indent + logptr->bufferused] = 0;
+        logptr->bufferused += indent;
+    }
+}
+
+static void OutputReturn(LOG* logptr) {
+    // strcpy(&logptr->buffer[logptr->bufferused], "\r\n");
+    // logptr->bufferused += 2;
+
+    // No need to write "\r\n" with fopen() in text mode
+    logptr->buffer[logptr->bufferused] = '\n';
+    logptr->bufferused++;
+}
+
+static void OutputTime(LOG* logptr, bool show) {
+    if (logptr->timeStamp == 0) {
+        return;
+    }
+
+    uint64_t ticks = 0;
+
+#if defined(WHOA_SYSTEM_WIN)
+    ticks = GetTickCount64();
+#endif
+
+#if defined(WHOA_SYSTEM_LINUX)
+    struct timespec ts;
+    if (clock_gettime(CLOCK_MONOTONIC_COARSE, &ts) == 0) {
+        ticks = ts.tv_sec * 1000 + ts.tv_nsec / 1000000;
+    }
+#endif
+
+#if defined(WHOA_SYSTEM_MAC)
+    ticks = mach_absolute_time();
+#endif
+
+    if (ticks != lasttime) {
+        lasttime = ticks;
+
+        time_t t = time(nullptr);
+        struct tm* ts = localtime(&t);
+        // No milliseconds for now
+        timestrlen = SStrPrintf(timestr,
+            sizeof(timestr),
+            "%d/%d %02d:%02d:%02d.%03d  ",
+            ts->tm_mon + 1,
+            ts->tm_mday,
+            ts->tm_hour,
+            ts->tm_min,
+            ts->tm_sec,
+            0);
+    }
+
+    if (show) {
+        memcpy(&logptr->buffer[logptr->bufferused], timestr, timestrlen);
+    } else {
+        memset(&logptr->buffer[logptr->bufferused], ' ', timestrlen);
+    }
+    logptr->bufferused += timestrlen;
+    logptr->buffer[logptr->bufferused] = '\0';
+}
+
+static bool PrepareLog(LOG* logptr) {
+    if (logptr->file) {
+        return true;
+    }
+
+    if (OpenLogFile(logptr->filename, &logptr->file, logptr->flags)) {
+        return true;
+    }
+
+    logptr->filename[0] = '\0';
+    return false;
+}
+
+void SLogInitialize() {
+    if (s_logsysteminit) {
+        return;
+    }
+
+    for (size_t i = 0; i < STORM_LOG_MAX_CHANNELS; ++i) {
+        s_critsect[i] = new SCritSect();
+    }
+
+    s_defaultdir_critsect = new SCritSect();
+
+    s_logsysteminit = true;
+}
+
+int SLogIsInitialized() {
+    return s_logsysteminit ? 1 : 0;
+}
+
+void SLogDestroy() {
+    for (size_t i = 0; i < STORM_LOG_MAX_CHANNELS; ++i) {
+        s_critsect[i]->Enter();
+        LOG* log = s_loghead[i];
+        while (log) {
+            if (log->file) {
+                FlushLog(log);
+                fclose(log->file);
+            }
+            LOG* next = log->next;
+            SMemFree(log);
+            log = next;
+        }
+        s_loghead[i] = nullptr;
+        s_critsect[i]->Leave();
+        delete s_critsect[i];
+    }
+
+    delete s_defaultdir_critsect;
+    s_logsysteminit = false;
+}
+
+int SLogCreate(const char* filename, uint32_t flags, HSLOG* log) {
+    STORM_ASSERT(filename);
+    STORM_ASSERT(*filename);
+    STORM_ASSERT(log);
+
+    FILE* file = nullptr;
+    HLOCKEDLOG lockedhandle;
+
+    *log = 0;
+
+    if (flags & SLOG_FLAG_NO_FILE) {
+        filename = "";
+        flags &= ~SLOG_FLAG_OPEN_FILE;
+    }
+
+    if ((flags & SLOG_FLAG_OPEN_FILE) == 0 || OpenLogFile(filename, &file, flags)) {
+        s_sequence = reinterpret_cast<HSLOG>(reinterpret_cast<size_t>(s_sequence) + 1);
+        *log = s_sequence;
+        LOG* result = LockLog(s_sequence, &lockedhandle, true);
+        if (result) {
+            result->file = file;
+            SStrCopy(result->filename, filename, STORM_MAX_PATH);
+            result->flags = flags;
+            result->timeStamp = 1;
+            result->indent = 0;
+            UnlockLog(lockedhandle);
+            return 1;
+        } else {
+            *log = 0;
+        }
+    }
+    return 0;
+}
+
+void SLogClose(HSLOG log) {
+    if (!s_logsysteminit) {
+        return;
+    }
+
+    HLOCKEDLOG lockedhandle;
+    LOG* logptr = LockLog(log, &lockedhandle, false);
+    if (!logptr) {
+        return;
+    }
+
+    if (logptr->file) {
+        FlushLog(logptr);
+        fclose(logptr->file);
+    }
+
+    UnlockDeleteLog(logptr, lockedhandle);
+}
+
+void SLogFlush(HSLOG log) {
+    HLOCKEDLOG lockedhandle;
+    LOG* logptr = LockLog(log, &lockedhandle, false);
+    if (logptr) {
+        if (logptr->file) {
+            FlushLog(logptr);
+        }
+        UnlockLog(lockedhandle);
+    }
+}
+
+void SLogFlushAll() {
+    for (size_t i = 0; i < STORM_LOG_MAX_CHANNELS; ++i) {
+        s_critsect[i]->Enter();
+        for (LOG* log = s_loghead[i]; log; log = log->next) {
+            if (log->file) {
+                FlushLog(log);
+            }
+        }
+        s_critsect[i]->Leave();
+    }
+}
+
+void SLogGetDefaultDirectory(char* dirname, size_t dirnamesize) {
+    s_defaultdir_critsect->Enter();
+    SStrCopy(dirname, s_defaultdir, dirnamesize);
+    s_defaultdir_critsect->Leave();
+}
+
+void SLogSetDefaultDirectory(const char* dirname) {
+    s_defaultdir_critsect->Enter();
+    size_t size = SStrCopy(s_defaultdir, dirname, STORM_MAX_PATH);
+    PathConvertSlashes(s_defaultdir);
+#if defined(WHOA_SYSTEM_WIN)
+    char slash = '\\';
+#else
+    char slash = '/';
+#endif
+    if (size < STORM_MAX_PATH - 1 && s_defaultdir[size - 1] != slash) {
+        s_defaultdir[size] = slash;
+        s_defaultdir[size + 1] = 0;
+    }
+    s_defaultdir_critsect->Leave();
+}
+
+int32_t SLogSetAbsIndent(HSLOG log, int32_t indent) {
+    HLOCKEDLOG lockedhandle;
+    LOG* logptr = LockLog(log, &lockedhandle, false);
+    if (logptr) {
+        int32_t result = logptr->indent;
+        logptr->indent = indent;
+        UnlockLog(lockedhandle);
+        return result;
+    }
+    return 0;
+}
+
+int32_t SLogSetIndent(HSLOG log, int32_t deltaIndent) {
+    HLOCKEDLOG lockedhandle;
+    LOG* logptr = LockLog(log, &lockedhandle, false);
+    if (logptr) {
+        int32_t result = logptr->indent;
+        logptr->indent = result + deltaIndent;
+        UnlockLog(lockedhandle);
+        return result;
+    }
+    return 0;
+}
+
+void SLogSetTimestamp(HSLOG log, int32_t timeStamp) {
+    HLOCKEDLOG lockedhandle;
+    LOG* logptr = LockLog(log, &lockedhandle, false);
+    if (logptr) {
+        logptr->timeStamp = timeStamp;
+        UnlockLog(lockedhandle);
+    }
+}
+
+void SLogVWrite(HSLOG log, const char* format, va_list arglist) {
+    HLOCKEDLOG lockedhandle;
+    LOG* logptr = LockLog(log, &lockedhandle, false);
+    if (!logptr) {
+        return;
+    }
+
+    if (PrepareLog(logptr)) {
+        OutputTime(logptr, true);
+        OutputIndent(logptr);
+
+        int count = vsnprintf(
+            &logptr->buffer[logptr->bufferused],
+            STORM_LOG_MAX_BUFFER - logptr->bufferused,
+            format,
+            arglist);
+        if (count > 0) {
+            logptr->bufferused += count;
+        }
+
+        OutputReturn(logptr);
+
+#if defined(WHOA_SYSTEM_WIN)
+        // if (g_opt.echotooutputdebugstring)
+        OutputDebugString(&logptr->buffer[logptr->pendpoint]);
+#endif
+
+        logptr->pendpoint = logptr->bufferused;
+        if (logptr->bufferused >= STORM_LOG_FLUSH_POINT) {
+            FlushLog(logptr);
+        }
+    }
+    UnlockLog(lockedhandle);
+}
+
+void SLogWrite(HSLOG log, const char* format, ...) {
+    va_list arglist;
+    va_start(arglist, format);
+    SLogVWrite(log, format, arglist);
+}
diff --git a/storm/Log.hpp b/storm/Log.hpp
new file mode 100644
index 0000000..0935d0c
--- /dev/null
+++ b/storm/Log.hpp
@@ -0,0 +1,38 @@
+#ifndef STORM_LOG_HPP
+#define STORM_LOG_HPP
+
+
+#include <cstdarg>
+#include <cstdint>
+
+#include "storm/Common.hpp"
+#include "storm/String.hpp"
+
+
+enum : uint32_t {
+    SLOG_FLAG_DEFAULT   = 0, // Create or open log file with first SLogWrite() call
+    SLOG_FLAG_OPEN_FILE = 1, // Create or open log file with SLogCreate()
+    SLOG_FLAG_NO_FILE   = 2, // Don't use log file (use OutputDebugString or console only)
+    SLOG_FLAG_APPEND    = 4  // Don't truncate existing log file
+};
+
+DECLARE_STRICT_HANDLE(HSLOG);
+DECLARE_STRICT_HANDLE(HLOCKEDLOG);
+
+
+void SLogInitialize();
+int  SLogIsInitialized();
+void SLogDestroy();
+int  SLogCreate(const char* filename, uint32_t flags, HSLOG* log);
+void SLogClose(HSLOG log);
+void SLogFlush(HSLOG log);
+void SLogFlushAll();
+void SLogGetDefaultDirectory(char* dirname, size_t dirnamesize);
+void SLogSetDefaultDirectory(const char* dirname);
+int32_t SLogSetAbsIndent(HSLOG log, int32_t indent);
+int32_t SLogSetIndent(HSLOG log, int32_t deltaIndent);
+void SLogVWrite(HSLOG log, const char* format, va_list arglist);
+void SLogWrite(HSLOG log, const char* format, ...);
+
+
+#endif
diff --git a/test/Log.cpp b/test/Log.cpp
new file mode 100644
index 0000000..b5ba959
--- /dev/null
+++ b/test/Log.cpp
@@ -0,0 +1,51 @@
+#include "storm/Log.hpp"
+#include "test/Test.hpp"
+
+
+TEST_CASE("SLogInitialize", "[log]") {
+    SECTION("constructs correctly") {
+        SLogInitialize();
+        REQUIRE(SLogIsInitialized() == 1);
+        SLogDestroy();
+    }
+
+    SECTION("destructs correctly") {
+        SLogInitialize();
+        SLogDestroy();
+        REQUIRE(SLogIsInitialized() == 0);
+    }
+}
+
+TEST_CASE("SLogCreate", "[log]") {
+    SECTION("creates new log handle") {
+        SLogInitialize();
+
+        HSLOG log;
+        REQUIRE(SLogCreate("test.log", SLOG_FLAG_DEFAULT, &log) == 1);
+        REQUIRE(log != 0);
+
+        SLogDestroy();
+    }
+
+    SECTION("creates new log file") {
+        SLogInitialize();
+
+        HSLOG log;
+        REQUIRE(SLogCreate("test.log", SLOG_FLAG_OPEN_FILE, &log) == 1);
+        REQUIRE(log != 0);
+
+        SLogDestroy();
+    }
+}
+
+TEST_CASE("SLogWrite", "[log]") {
+    SLogInitialize();
+
+    HSLOG log;
+    REQUIRE(SLogCreate("test.log", SLOG_FLAG_DEFAULT, &log) == 1);
+    REQUIRE(log != 0);
+
+    SLogWrite(log, "SLogWrite Test");
+
+    SLogDestroy();
+}