gcc -g -Wall -o raspi_temp raspi_temp.c && sudo cp raspi_temp /etc/munin/plugins/

raspi_temp.c

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define TEMP_PATH "/sys/class/thermal/thermal_zone0/temp"
#define BUFFER_SIZE (1024)

int main(int ac, char **av)
{
    if (ac == 2)
    {
        if (strcmp(av[1], "config") == 0)
        {
            printf("graph_title Temperature of BCM283x SoC\n");
            printf("graph_vlabel temperature in Celcius\n");
            printf("graph_args -l %d\n", 0);
            printf("graph_scale no\n");
            printf("graph_category sensors\n");
            printf("vcgencmd_temp.label vcgencmd_temp\n");
            printf("vcgencmd_temp.critical %4.2f\n", 85.0);
            printf("vcgencmd_temp.warning %4.2f\n", 80.0);
            return 0;
        }
        if (strcmp(av[1], "autoconf") == 0)
        {
            printf("no\n");
            return 0;
        }
    }
    FILE *fp = fopen(TEMP_PATH, "rb");
    if (!fp)
    {
        fprintf(stderr, "Cannot open %s: %s\n", TEMP_PATH, strerror(errno));
        return 1;
    }

    char line[BUFFER_SIZE];
    fgets(line, sizeof line, fp);
    double f = strtod(line, NULL);

    printf("vcgencmd_temp.value %5.4f\n", f / 1000.0);
    fclose(fp);

    return 0;
}