Copy a string to the end of a string

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

typedef struct
{

  unsigned argc;
  char *args;

}func_t;

func_t concat( func_t s1 ,char s[])
{
  s1.args=realloc(s1.args, s1.argc+strlen(s));
  strcpy(s1.args+s1.argc,s);
  s1.argc=strlen(s1.args);

  return s1;
}



int main(void)
{
 char s[20]="Gummybear";
 func_t s1={0, NULL};

 s1=concat(s1,s);
 printf("%s \n",s1.args);

 s1=concat(s1,"SSS");
 printf("%s \n",s1.args);


 putchar('\n');

 return 0;
}