/in/foo.c: In function 'main':
/in/foo.c:24:22: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
   p = (char *)malloc(strlen(A) + strlen(B));
                      ^~~~~~
/in/foo.c:24:22: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:24:22: note: include '<string.h>' or provide a declaration of 'strlen'
/in/foo.c:25:3: warning: implicit declaration of function 'strcpy' [-Wimplicit-function-declaration]
   strcpy(p, B);
   ^~~~~~
/in/foo.c:25:3: warning: incompatible implicit declaration of built-in function 'strcpy'
/in/foo.c:25:3: note: include '<string.h>' or provide a declaration of 'strcpy'
/in/foo.c:26:3: warning: implicit declaration of function 'strcat' [-Wimplicit-function-declaration]
   strcat(p, A);
   ^~~~~~
/in/foo.c:26:3: warning: incompatible implicit declaration of built-in function 'strcat'
/in/foo.c:26:3: note: include '<string.h>' or provide a declaration of 'strcat'
/in/foo.c:33:6: warning: incompatible implicit declaration of built-in function 'strlen'
  if (strlen(A) >= 100000)
      ^~~~~~
/in/foo.c:33:6: note: include '<string.h>' or provide a declaration of 'strlen'
/in/foo.c:37:13: warning: format '%c' expects argument of type 'int', but argument 2 has type 'char *' [-Wformat=]
    printf("%c", A + i);
             ^