Strange stamp on gettimeofday()

Question

I have written this code for a sampling assignment at my university.

#include <sys/time.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int main(int argc, char **argv){
    struct timeval tv;

    float t = atoi(argv[1]);  //sampling time period in sec's
    float dt = atoi(argv[2]);   //sampling rate in msec's
    double time;
    int nsamples = t/dt * 1000; //number of samples floored

    //samples storage array
    double *samples;
    samples = malloc(nsamples);

    printf("%d\n\n",nsamples);

    int c = 0;  //array index
    double divergance;

    gettimeofday(&tv, NULL);
    time =(double) tv.tv_sec + tv.tv_usec / 1000000.0f;
    samples[c] = time;
    printf("time: %f\n", samples[c]);

    usleep(dt * 1000);

    while(c<nsamples){
      c++;

      gettimeofday(&tv, NULL);
      time = (double) tv.tv_sec + tv.tv_usec / 1000000.0f;
      samples[c] = time;

      //divergance calculated in msec's
      divergance = (samples[c] - samples[c-1]);
      if (c==9){
        printf("%f \n \n%f", samples[c-1], samples[c]);
      }
      printf("time: %f\ndivergance: %f ms\n\n", samples[c], divergance*1000);

      usleep(dt *1000);
    }

}

and this is my output

time: 1557335682.435666 divergance: 200.127125 ms

time: 1557335682.635813 divergance: 200.146914 ms

time: 1557335682.835952 divergance: 200.139046 ms

time: 1557335683.036075 divergance: 200.123072 ms

time: 1557335683.236192 divergance: -50328976507548121002151598324465532616014103321089770750300716493231241208217866953937760599346823570331739493744117764925654540012842402655523878795775819489233146901362588461216017208320541658368563434403808909221817741213696.000000 ms

time: 1557335683.436400 divergance: 1557335683436.399902 ms

time: 1557335683.636521 divergance: 1557335683636.520752 ms

time: 1557335683.836647 divergance: 1557335683836.646973 ms

Does anyone understand what's that weird output there on the fifth calculation. I can not imagine any logical explanation as I have not encountered any similar "bug" before. Does it have to do with some certain feature of gettimeofday() function?

Νote: the input is 10 and 200

Answer 1

You haven't allocated enough space for samples:

samples = malloc(nsamples);

The malloc function allocates space for the specified number of bytes, not the number of array elements. So your array is much shorter that you think it is. This means you'll eventually write past the end of the array, invoking undefined behavior.

You need to multiply the number of elements by the element size to allocate the correct amount of space:

samples = malloc(nsamples * sizeof(*samples));

You also have an off-by-one error when accessing the array:

int c = 0;
...
while(c<nsamples){
  c++;
  ...
  samples[c] = time;
  ...
}

This will also write past the end of the array, specifically one array element too many.

Change the loop to start with the value 1 and increment at the end.

int c = 0;
...
c = 1;
while(c<nsamples){
  ...
  samples[c] = time;
  ...
  c++;
}