How to store an int array values in a char array?

If you want that to be on the heap and dynamically allocate memory you can use the asprintf.

char *ip = NULL;
int ret;
ret = asprintf(&ip, "%d.%d.%d.%d", a[0], a[1], a[2], a[3]);
if (ret < 0)
{  
 // error handling
}
// use ip
free(ip); 
ip = NULL;

Otherwise, if you know the size before hand you can use sprintf.

sprintf(ip, "%d.%d.%d.%d", a[0], a[1], a[2], a[3]);

Personally, I prefer to use asprintf, however, you need to remember to free that memory allocated by asprintf otherwise you will get a memory leak.

Side note, asprintf calls realloc internally so you could use it for string concatenation as long as you free successive calls.

You always, always, always check that you aren’t overrunning your buffer in C, and especially when you’re dealing with data you got over a network.

For example, you set the array to fifteen characters, but the string "100.100.100.100" has sixteen with its terminating null. That works for now, but is precariously brittle. Or you might get buggy or malicious input like {-1000000000L, -1000000000L, -1000000000L, -1000000000}. Or you might later refactor to support IPv6.

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

#define DOTTED_IPV4_LEN     16U

int main(void)
{
  static const unsigned char a[4] = {192,168,8,100};
  char ip[DOTTED_IPV4_LEN];

  snprintf( ip, DOTTED_IPV4_LEN, "%u.%u.%u.%u", a[0], a[1], a[2], a[3] );
  puts(ip);

  return EXIT_SUCCESS;
}

In fact, some shops, including I think Microsoft, wouldn’t even accept the destination buffer being declared on the stack like that and would demand the extra precaution of making ip a pointer to dynamically-allocated memory, perhaps:

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

#define DOTTED_IPV4_LEN     16U

int main(void)
{
  static const unsigned char a[4] = {192,168,8,100};
  char (* const ip)[DOTTED_IPV4_LEN] = malloc(sizeof(*ip));

  assert(ip);
  snprintf( *ip, sizeof(*ip), "%u.%u.%u.%u", a[0], a[1], a[2], a[3] );
  puts(*ip);

  free(ip); // Unnecessary because we're about to quit.
  return EXIT_SUCCESS;
}