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On webpage http://abyz.me.uk/rpi/pigpio/examples.html you can find Tiny GPIO Access that defines minimal set of C functions needed to control the GPIO without needing root privileges (it uses /dev/gpiomem to access the GPIO).

I am using this code for bitbanging on I2C dedicated channels (GPIO2 and GPIO3). However, after bitbanging, I2C kernel communication (linux/i2c-dev.h) is no longer working. Is there a way to de-initialize GPIO access and let kernel do its work as previously?

The initialization part of the code is

#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>

int gpioInitialise(void)
{
   int fd;

   piRev = gpioHardwareRevision(); /* sets piModel and piRev */

   fd = open("/dev/gpiomem", O_RDWR | O_SYNC) ;

   if (fd < 0)
   {
      fprintf(stderr, "failed to open /dev/gpiomem\n");
      return -1;
   }

   gpioReg = (uint32_t *)mmap(NULL, 0xB4, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

   close(fd);

   if (gpioReg == MAP_FAILED)
   {
      fprintf(stderr, "Bad, mmap failed\n");
      return -1;
   }
   return 0;
}
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Yes, set the GPIO back into mode ALT0 (needed by GPIO 2/3 to be in I2C mode).

Tiny GPIO has the following define to use to set that mode.

#define PI_ALT0 4

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