what's the specific of memory allocation in valgrind

Context: How to make Valgrind log all allocations?
https://valgrind.org/docs/manual/ms-manual.html

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

enum { NBOOK = 10};
typedef struct{
    char *title;
    int pages;
} book;

typedef struct{
    int num_books,max_books;
    book *array;
} stack;

stack* create_stack();
book *add_book(stack **s,char *title,int pages);
void prn_stack(stack *s);
void free_stack(stack *s);

int main(void){
    stack *s1 = NULL; /* initialize pointers */

    add_book(&s1,"Huck Finn", 631);
    add_book(&s1,"Tom sawyer", 582);
    add_book(&s1,"The qucik brown fox", 1);

    prn_stack(s1);
    free_stack(s1);

    return 0;
}

stack* create_stack()
{
    stack *s = calloc(1,sizeof *s);

    if(!s){
        fprintf(stderr,"error: virtual memory exhausted.--stack\n");
        exit(EXIT_FAILURE);
    }
    
    s->array = calloc(NBOOK,sizeof *(s->array));
    if(!s->array){
        fprintf(stderr,"error: virtual memory exhausted.--array \n");
        exit(EXIT_FAILURE);
    }

    s->num_books = 0;
    s->max_books = NBOOK;
    return s;
}

/** since add_book may create the stack, you must pass the address
 *  of the stack to add_book so that any changes to s are available
 *  back in the calling funciton.
 */
book *add_book(stack **s,char *title,int pages)
{
    if(!title) return NULL;

    if(!*s) *s = create_stack();

    /* check num_books against max_books and realloc as required */
    if((*s)->num_books == (*s)->max_books){
        void *tmp = realloc((*s)->array,((*s)->max_books + NBOOK) * sizeof *((*s)->array));
        if(!tmp){
            fprintf(stderr, "error: memory exhausted - realloc array.\n");
            return NULL;
        }

        (*s)->array = tmp;
        (*s)->max_books += NBOOK;
    }

    /*  allocate/copy title, assign pages, increment num_books  */
    (*s)->array[(*s)->num_books].title = strdup(title);
    (*s)->array[(*s)->num_books].pages = pages;
    ((*s) ->num_books) ++;

    return &((*s)->array[(*s)->num_books - 1]);
}


void prn_stack(stack *s)
{
    if(!s) return;
    
    printf("There are %d books in the stack. maxbook is %d:\n",s->num_books,s->max_books);

    for(int i = 0; i < s->num_books ; i++)
        printf("\t%2d.\t%-20s\t(%3d pages)\n", i,s->array[i].title,s->array[i].pages);

    putchar('\n');
}

void free_stack(stack *s)
{
    if(!s) return;

    for(int i = 0; i < s->num_books; i++)
        free(s->array[i].title);
    free(s->array);
    free(s);
}

gcc -Wformat=2 -Werror=return-type array_struct35.c &&./a.out
valgrind ./a.out

return

-- total heap usage: 6 allocs, 6 frees, 1,241 bytes allocated

valgrind --tool=massif ./a.out
ms_print massif.out.69359

return

94.59% (1,241B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc.
->78.05% (1,024B) 0x48E0C23: _IO_file_doallocate (filedoalloc.c:101)
| ->78.05% (1,024B) 0x48EFD5F: _IO_doallocbuf (genops.c:347)
|   ->78.05% (1,024B) 0x48EEFDF: _IO_file_overflow@@GLIBC_2.2.5 (fileops.c:744)
|     ->78.05% (1,024B) 0x48ED754: _IO_new_file_xsputn (fileops.c:1243)
|       ->78.05% (1,024B) 0x48ED754: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1196)
|         ->78.05% (1,024B) 0x48D71CC: outstring_func (vfprintf-internal.c:239)
|           ->78.05% (1,024B) 0x48D71CC: __vfprintf_internal (vfprintf-internal.c:1263)
|             ->78.05% (1,024B) 0x48C281E: printf (printf.c:33)
|               ->78.05% (1,024B) 0x10955F: prn_stack (in /home/jian/helloc/a.out)
|                 ->78.05% (1,024B) 0x1092C8: main (in /home/jian/helloc/a.out)
|
->12.20% (160B) 0x109351: create_stack (in /home/jian/helloc/a.out)
| ->12.20% (160B) 0x1093F0: add_book (in /home/jian/helloc/a.out)
|   ->12.20% (160B) 0x109286: main (in /home/jian/helloc/a.out)
|
->03.12% (41B) 0x490A60E: strdup (strdup.c:42)
| ->03.12% (41B) 0x1094C6: add_book (in /home/jian/helloc/a.out)
|   ->01.60% (21B) in 2 places, all below massif's threshold (1.00%)
|   |
|   ->01.52% (20B) 0x1092BC: main (in /home/jian/helloc/a.out)
|
->01.22% (16B) 0x10930A: create_stack (in /home/jian/helloc/a.out)
  ->01.22% (16B) 0x1093F0: add_book (in /home/jian/helloc/a.out)
    ->01.22% (16B) 0x109286: main (in /home/jian/helloc/a.out)

6 allocated roughly means 6 memory related function call. So far I can only count 5. 5 == 3 strdup() function calls plus 2 calloc) function calls.

update
From https://pubs.opengroup.org/onlinepubs/9699919799/functions/strdup.html I know that strdup function related to memory allocation.

But printf reference https://pubs.opengroup.org/onlinepubs/9699919799/functions/fprintf.html I cannot found memory allocation related info.

I found out this greate question. Does fprintf use malloc() under the hood?