Very simple hwmon fan speed meter

FanMeter.c

@@ -0,0 +1,44 @@
+/*
+htop - FanMeter.c
+(C) 2026 Murad Karammaev
+Released under the GNU GPLv2+, see the COPYING file
+in the source distribution for its full text.
+*/
+
+#include "config.h" // IWYU pragma: keep
+
+#include "FanMeter.h"
+
+#include "CRT.h"
+#include "Platform.h"
+#include "XUtils.h"
+
+
+static const int FanMeter_attributes[] = {
+   METER_VALUE
+};
+
+static void FanMeter_updateValues(Meter* this) {
+   int fanspeed = Platform_getFanSpeed();
+   if (fanspeed < 0) {
+      xSnprintf(this->txtBuffer, sizeof(this->txtBuffer), "N/A");
+   } else {
+      xSnprintf(this->txtBuffer, sizeof(this->txtBuffer), "%d rpm", fanspeed);
+   }
+}
+
+const MeterClass FanMeter_class = {
+   .super = {
+      .extends = Class(Meter),
+      .delete = Meter_delete
+   },
+   .updateValues = FanMeter_updateValues,
+   .defaultMode = TEXT_METERMODE,
+   .supportedModes = (1 << TEXT_METERMODE) | (1 << LED_METERMODE),
+   .maxItems = 0,
+   .total = 0.0,
+   .attributes = FanMeter_attributes,
+   .name = "Fan",
+   .uiName = "Fan speed",
+   .caption = "Fan speed: ",
+};

FanMeter.h

@@ -0,0 +1,15 @@
+#ifndef HEADER_FanMeter
+#define HEADER_FanMeter
+/*
+htop - FanMeter.h
+(C) 2026 Murad Karammaev
+Released under the GNU GPLv2+, see the COPYING file
+in the source distribution for its full text.
+*/
+
+#include "Meter.h"
+
+
+extern const MeterClass FanMeter_class;
+
+#endif

Makefile.am

@@ -114,6 +114,7 @@ myhtopheaders = \
 	DynamicMeter.h \
 	DynamicScreen.h \
 	EnvScreen.h \
+	FanMeter.h \
 	FileDescriptorMeter.h \
 	FunctionBar.h \
 	GPUMeter.h \
@@ -213,6 +214,7 @@ linux_platform_headers = \
 	zfs/ZfsCompressedArcMeter.h
 
 linux_platform_sources = \
+	FanMeter.c \
 	GPUMeter.c \
 	generic/gettime.c \
 	generic/hostname.c \

linux/Platform.c

@@ -29,6 +29,7 @@ in the source distribution for its full text.
 #include "DiskIOMeter.h"
 #include "FileDescriptorMeter.h"
 #include "GPUMeter.h"
+#include "FanMeter.h"
 #include "HostnameMeter.h"
 #include "HugePageMeter.h"
 #include "LoadAverageMeter.h"
@@ -280,6 +281,7 @@ const MeterClass* const Platform_meterTypes[] = {
    &OpenRCUserMeter_class,
    &FileDescriptorMeter_class,
    &GPUMeter_class,
+   &FanMeter_class,
    NULL
 };
 
@@ -441,6 +443,88 @@ void Platform_setGPUValues(Meter* this, double* totalUsage, unsigned long long*
    this->values[residueIndex] = residuePercentage;
 }
 
+static int Platform_getFanSpeedFromHwmonDevice(openat_arg_t parent_dfd, const char *hwmon_device) {
+   DIR* dirp;
+
+#ifdef HAVE_OPENAT
+   openat_arg_t dfd = Compat_openat(parent_dfd, hwmon_device, O_RDONLY | O_CLOEXEC | O_DIRECTORY);
+   if (dfd == -1)
+      return -1;
+
+   dirp = fdopendir(dfd);
+   if (!dirp) {
+      close(dfd);
+      return -1;
+   }
+#else
+   char dfd[4096];
+   if (xSnprintf(dfd, sizeof(dfd), "%s/%s", parent_dfd, hwmon_device) < 0)
+      return -1;
+
+   dirp = opendir(dfd);
+   if (!dirp)
+      return -1;
+#endif
+
+   struct dirent* dir;
+   while ((dir = readdir(dirp)) != NULL) {
+      int fan_num = 0, scan_len = 0;
+      if (sscanf(dir->d_name, "fan%d%n", &fan_num, &scan_len) != 1 ||
+          !String_eq(&dir->d_name[scan_len], "_input"))
+         continue;
+
+      char buffer[32];
+      ssize_t ret = Compat_readfileat(dfd, dir->d_name, buffer, sizeof(buffer));
+      if (ret <= 0 || (size_t)ret > sizeof(buffer))
+         continue;
+
+      int fan_speed;
+      if (sscanf(buffer, "%d", &fan_speed) != 1)
+         continue;
+
+      closedir(dirp);
+      return fan_speed;
+   }
+
+   closedir(dirp);
+   return -1;
+}
+
+/* Traverse hwmon sensors, find first fan, read and return its speed. */
+int Platform_getFanSpeed(void) {
+   DIR* dirp;
+   openat_arg_t dfd;
+
+   dirp = opendir(HWMONDIR);
+   if (!dirp)
+      return -1;
+
+#ifdef HAVE_OPENAT
+   dfd = dirfd(dirp);
+   if (dfd == -1) {
+      closedir(dirp);
+      return -1;
+   }
+#else
+   dfd = HWMONDIR;
+#endif
+
+   struct dirent* dir;
+   while ((dir = readdir(dirp)) != NULL) {
+      if (String_eq(dir->d_name, ".") || String_eq(dir->d_name, ".."))
+         continue;
+
+      int fan_speed = Platform_getFanSpeedFromHwmonDevice(dfd, dir->d_name);
+      if (fan_speed >= 0) {
+         closedir(dirp);
+         return fan_speed;
+      }
+   }
+
+   closedir(dirp);
+   return -1;
+}
+
 void Platform_setMemoryValues(Meter* this) {
    const Machine* host = this->host;
    const LinuxMachine* lhost = (const LinuxMachine*) host;

linux/Platform.h

@@ -38,6 +38,8 @@ in the source distribution for its full text.
    #define PATH_MAX 4096
 #endif
 
+#define HWMONDIR "/sys/class/hwmon"
+
 
 extern const ScreenDefaults Platform_defaultScreens[];
 
@@ -70,6 +72,8 @@ double Platform_setCPUValues(Meter* this, unsigned int cpu);
 
 void Platform_setGPUValues(Meter* this, double* totalUsage, unsigned long long* totalGPUTimeDiff);
 
+int Platform_getFanSpeed(void);
+
 void Platform_setMemoryValues(Meter* this);
 
 void Platform_setSwapValues(Meter* this);