Usage of fmemopen

Question

I don't understand an example of fmemopen from APUE

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

#define BSZ 48

int
main()
{
    FILE *fp;
    char buf[BSZ];

    memset(buf, 'a', BSZ-2);
    buf[BSZ-2] = '\0';
    buf[BSZ-1] = 'X';
    if ((fp = fmemopen(buf, BSZ, "w+")) == NULL)  //"w+" truncate the file by setting the first byte to null byte
        printf("fmemopen failed\n");
    printf("initial buffer contents: %s\n", buf); //print nothing cause the first byte is null byte.
    fprintf(fp, "hello, world"); //write format string to the stream, so the buffer should change and be rewritten.
    printf("before flush: %s\n", buf); //should print "hello, world". Why not?
    fflush(fp);
    printf("after fflush: %s\n", buf);
    printf("len of string in buf = %ld\n", (long)strlen(buf));

    memset(buf, 'b', BSZ-2);
    buf[BSZ-2] = '\0';
    buf[BSZ-1] = 'X';
    fprintf(fp, "hello, world");
    fseek(fp, 0, SEEK_SET);
    printf("after  fseek: %s\n", buf);
    printf("len of string in buf = %ld\n", (long)strlen(buf));

    memset(buf, 'c', BSZ-2);
    buf[BSZ-2] = '\0';
    buf[BSZ-1] = 'X';
    fprintf(fp, "hello, world");
    fclose(fp);
    printf("after fclose: %s\n", buf);
    printf("len of string in buf = %ld\n", (long)strlen(buf));

    return(0);
}

I use Clion to debug and when it finishes fprintf(fp, "hello, world"); and comes to printf("before flush: %s\\n", buf); , I see the content of char array buf does not change and print nothing . Why!?

A debug screenshot for better description:

Also, quoted from the book:

Third, a null byte is written at the current position in the stream whenever we increase the amount of data in the stream's buffer and call fclose , fflush , fseek , fseeko , or fsetpos .

What does this mean?

---

Update:

From the book and it seems that buffer is unchanged until stream is flushed is the expected result.

Answer 1

change the allocated buf array in the stack

char buf[BSZ];

to this

char* buf = new(std::nothrow) chr[BSZ];

and freed buf after close the stream

delete[] buf

and test again.