Optimize read + realloc using fstat in C

Question

I want to mmap stdin. But I can't mmap stdin.

So I have to call read and realloc in a loop, and I want to optimize it by choosing a good buffer size.

fstat of the file descriptor 0 gives a struct stat with a member named st_size, which seems to represent the amount of bytes that are already in the pipe buffer corresponding to stdin.

Depending on how I call my program, st_size varies between 0 and the full pipe buffer size of stdin, which is about 65520. For example in this program:

#include <stdio.h>
#include <sys/stat.h>
#include <unistd.h>

int main()
{
    struct stat buf;
    int i;
    for (i=0; i<20; i++)
    {
        fstat(0, &buf);
        printf("%lld\n", buf.st_size);
        usleep(10);
    }
}

We can observe the buffer is still being filled:

$ cc fstat.c
$ for (( i=0; i<10000; i++ )) do echo "0123456789" ; done | ./a.out
9196
9306
9350
9394
9427
9471
9515
9559
...

And the output of this program changes everytime I re-run it.

I'd like to use st_size for the initial buffer size so I have to do less calls to realloc.

I have three questions, most important one first:

• Can I 'flush' stdin, i.e. wait until the buffer is not being filled anymore ?

• Could I do better ? Maybe there is another way to optimize this, to make the realloc in a loop make feel less dirty.

• Can you confirm I can't use mmap with stdin ?

Few details:

• I wanted to use mmap because I already wrote a program, for educational purpose, that does what nm does, using mmap. I'm wondering how it does handle stdin.

• I know that st_size can be 0, in which case I will have no choice but to define a buffer size.

• I know stdin can give more bytes than the pipe buffer size.

• I want to optimize for educational purpose. There is no imperative need to it.

Thank you