first re-commit.

pigpio-master/EXAMPLES/C/ROTARY_ENCODER/README

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+Function to decode a mechanical rotary encoder.
+
+Follow the instructions in the test file to build and run.
+

pigpio-master/EXAMPLES/C/ROTARY_ENCODER/rotary_encoder.c

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+#include <stdio.h>
+#include <stdlib.h>
+
+#include <pigpio.h>
+
+#include "rotary_encoder.h"
+
+struct _Pi_Renc_s
+{
+   int gpioA;
+   int gpioB;
+   Pi_Renc_CB_t callback;
+   int levA;
+   int levB;
+   int lastGpio;
+};
+
+/*
+
+             +---------+         +---------+      0
+             |         |         |         |
+   A         |         |         |         |
+             |         |         |         |
+   +---------+         +---------+         +----- 1
+
+       +---------+         +---------+            0
+       |         |         |         |
+   B   |         |         |         |
+       |         |         |         |
+   ----+         +---------+         +---------+  1
+
+*/
+
+static void _cb(int gpio, int level, uint32_t tick, void *user)
+{
+   Pi_Renc_t *renc;
+
+   renc = user;
+
+   if (gpio == renc->gpioA) renc->levA = level; else renc->levB = level;
+
+   if (gpio != renc->lastGpio) /* debounce */
+   {
+      renc->lastGpio = gpio;
+
+      if ((gpio == renc->gpioA) && (level == 1))
+      {
+         if (renc->levB) (renc->callback)(1);
+      }
+      else if ((gpio == renc->gpioB) && (level == 1))
+      {
+         if (renc->levA) (renc->callback)(-1);
+      }
+   }
+}
+
+Pi_Renc_t * Pi_Renc(int gpioA, int gpioB, Pi_Renc_CB_t callback)
+{
+   Pi_Renc_t *renc;
+
+   renc = malloc(sizeof(Pi_Renc_t));
+
+   renc->gpioA = gpioA;
+   renc->gpioB = gpioB;
+   renc->callback = callback;
+   renc->levA=0;
+   renc->levB=0;
+   renc->lastGpio = -1;
+
+   gpioSetMode(gpioA, PI_INPUT);
+   gpioSetMode(gpioB, PI_INPUT);
+
+   /* pull up is needed as encoder common is grounded */
+
+   gpioSetPullUpDown(gpioA, PI_PUD_UP);
+   gpioSetPullUpDown(gpioB, PI_PUD_UP);
+
+   /* monitor encoder level changes */
+
+   gpioSetAlertFuncEx(gpioA, _cb, renc);
+   gpioSetAlertFuncEx(gpioB, _cb, renc);
+}
+
+void Pi_Renc_cancel(Pi_Renc_t *renc)
+{
+   if (renc)
+   {
+      gpioSetAlertFunc(renc->gpioA, 0);
+      gpioSetAlertFunc(renc->gpioB, 0);
+
+      free(renc);
+   }
+}
+

pigpio-master/EXAMPLES/C/ROTARY_ENCODER/rotary_encoder.h

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+#ifndef ROTARY_ENCODER_H
+#define ROTARY_ENCODER_H
+
+typedef void (*Pi_Renc_CB_t)(int);
+
+struct _Pi_Renc_s;
+
+typedef struct _Pi_Renc_s Pi_Renc_t;
+
+Pi_Renc_t * Pi_Renc(int gpioA, int gpioB, Pi_Renc_CB_t callback);
+/*
+   This function establishes a rotary encoder on gpioA and gpioB.
+
+   When the encoder is turned the callback function is called.
+
+   A pointer to a private data type is returned.  This should be passed
+   to Pi_Renc_cancel if the rotary encoder is to be cancelled.
+*/
+
+void Pi_Renc_cancel(Pi_Renc_t *renc);
+/*
+   This function releases the resources used by the decoder.
+*/
+
+#endif

pigpio-master/EXAMPLES/C/ROTARY_ENCODER/test_rotary_encoder.c

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+#include <stdio.h>
+
+#include <pigpio.h>
+
+#include "rotary_encoder.h"
+
+/*
+
+REQUIRES
+
+A rotary encoder contacts A and B connected to separate gpios and
+the common contact connected to Pi ground.
+
+TO BUILD
+
+gcc -o rot_enc_c test_rotary_encoder.c rotary_encoder.c -lpigpio -lrt
+
+TO RUN
+
+sudo ./rot_enc_c
+
+*/
+
+void callback(int way)
+{
+   static int pos = 0;
+
+   pos += way;
+
+   printf("pos=%d\n", pos);
+}
+
+int main(int argc, char *argv[])
+{
+   Pi_Renc_t * renc;
+
+   if (gpioInitialise() < 0) return 1;
+
+   renc = Pi_Renc(7, 8, callback);
+
+   sleep(300);
+
+   Pi_Renc_cancel(renc);
+
+   gpioTerminate();
+}
+