scan string with unknown number of elements to an array of int in C

// Adapted from strtok(3) example.

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

int
main(int argc, char *argv[])
{
  char *str1, *str2, *token, *subtoken;
  char *saveptr1,*saveptr2;
  int j;
  char datatemp[ 120 ];
  char delim = ' ';

  fgets(datatemp, 119, stdin);

  /*
   * strtok has to be called with a pointer to the input
   * the first time then with a pointer to the string
   * subsequently
   */

  for(j=1, str1 = datatemp;; j++, str1 = NULL)
  {
    token = strtok_r(str1, &delim, &saveptr1);

    if(token==NULL)
      break;

    for (str2 = token; ; str2 = NULL)
    {
      subtoken = strtok_r(str2, &delim, &saveptr2);
      if (subtoken == NULL)
        break;
      printf(" --> %s\n", subtoken);
    }
  }

  exit(0);

}

To use fgets()/sscanf() as requested by OP, see below.

Using fgets()/strtol() offers better error handling than sscanf(). Recommend strtol()

To get the best answer, OP needs to provide info like how data is to be store (VLA, malloc, 1 at-a time), error handle concerns, etc.

Hard to tell why OP code did not work as the defalcation of array[] is missing, but it may have been too small for the test data and certain was not resized as needed for various data input.

void foo() {
  char buf[100];
  if (fgets(buf, sizeof buf, stdin) == NULL) Handle_EOForIOError();

  int i = 0;
  int n = 0;
  int number;
  // Count the numbers
  while (sscanf(&buf[i], "%d %n", &number, &i) == 1) {
    n++;
  }

  int num[n];
  i = 0;
  int j;
  // scan again, this time save the numbers
  while (sscanf(&buf[i], "%d %n", &num[i], &j) == 1) {
    i = j;
  }

  // use the numbers
  bar(num, n);
}

Above code could use likely skip the j and use while (sscanf(&buf[i], "%d %n", &num[i], &i) == 1);, but I need to double check some code-point issues.

strtol() is better than sscanf(). To use, suggest a helper function:

// return non-zero on failure 
static int user_strtoi(char **s, int *result) {
  char *endptr;
  errno = 0;
  long num = strtol(*s, &endptr, 10);
  if (endptr == *s) return 1; //  fail as nothing was parsed
  if (errno != 0) return 2; //  fail due to overflow
  if (num < INT_MIN || num > INT_MAX)  return 2; //  fail due to overflow
  *result = (int) num;
  *s = endptr;
  return 0;
}

You can use standard function strtol declared in header <stdlib.h>. For example

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


int main( void ) 
{
    char s[] = "5 13 12 7 3 0";
    size_t l = strlen( s );

    size_t n = 0;

    char *end_ptr = s;
    char **next_ptr = &end_ptr;

    errno = 0;

    while ( end_ptr != s + l )
    {
        strtol( *next_ptr, next_ptr, 10 );
        if ( errno != 0 ) break;
        ++n;
    }

    int a[n];
    memset( a, 0, n * sizeof( int ) );

    end_ptr = s;
    size_t i = 0;
    while ( end_ptr != s + l )
    {
        int value = ( int )strtol( *next_ptr, next_ptr, 10 );
        if ( errno != 0 ) break;
        a[i++] = value;
    }

    for ( i = 0; i < n; i++ ) printf( "%d ", a[i] );
    puts( "" );

    return 0;
}

The output is

5 13 12 7 3 0

I suggest to use strtol in a loop, is faster (direct convert without an intermediate buffer) and you can check for errors

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

int main(void)
{
    char s[128], *p, *q;
    int i, n, *v;

    /* get user input */
    fgets(s, sizeof s, stdin);
    /* count integers */
    for (p = s, n = 0; ; p = q, n++) {
        strtol(p, &q, 10);
        if (p == q) break;
    }
    v = malloc(sizeof(int) * n);
    /* populate array */
    for (p = s, i = 0; i < n; i++) {
        v[i] = strtol(p, &p, 10);
    }
    /* print array */
    for (i = 0; i < n; i++) {
        printf("%d\n", v[i]);
    }
    free(v);
    return 0;
}