Fast read stdin into buffer with no checking

Question

I have buffer, let's say 65536 bytes long. How can I read stdin into that buffer as fast as possible (using IO hardware) without any check for newline or '\0' characters. I have a guarantee that the number of characters in stdin will always match my buffer.

So far I have this:

#include <iostream>
#include <stdio.h>

#define BUFFER_LENGTH 65536

int main()
{
    std::ios::sync_with_stdio(false);
    setvbuf(stdout, NULL, _IONBF, BUFFER_LENGTH);

    char buffer[BUFFER_LENGTH];

    // now read stdin into buffer
    // fast print:
    puts(buffer);    // given buffer is null terminated

    return 0;
}

Is there something similar to puts() that will fast read into buffer instead of console?

Not sure I understand the request. Does [fread](https://en.cppreference.com/w/c/io/fread) look like what you want? — AProgrammer, Dec 30 '19 at 15:04
@AProgrammer Yeah, fread works. I just didn't know you could pass stdin as FILE*. — sanitizedUser, Dec 30 '19 at 15:18

score 1 · Accepted Answer · answered Dec 30 '19 at 17:38

You can use C's standard fread() function:

#include <iostream>
#include <stdio.h>

#define BUFFER_LENGTH 65536

int main()
{
    std::ios::sync_with_stdio(false);
    setvbuf(stdout, NULL, _IONBF, BUFFER_LENGTH);

    // need space to terminate the C-style string
    char buffer[BUFFER_LENGTH + 1];

    // eliminate stdin buffering too
    setvbuf(stdin, NULL, _IONBF, BUFFER_LENGTH);    

    // now read stdin into buffer
    size_t numRead = fread( buffer, 1, BUFFER_LENGTH, stdin );

    // should check for errors/partial reads here

    // fread() will not terminate any string so it
    // has to be done manually before using puts()
    buffer[ numRead ] = '\0';

    // fast print:
    puts(buffer);    // given buffer is null terminated

    return 0;
}

Note that `puts()` inserts newline at the end. You can use `fputs(buffer, stdout);` to manage newlines yourself. — sanitizedUser, Dec 30 '19 at 19:11

score 0 · Answer 2 · answered Dec 30 '19 at 14:58

If puts gives you the behavior you want except for the fact that it outputs to stdout, you could use dup2 to pipe stdout to a different file descriptor (don't forget to reconnect stdout when you're done).

This post shows a good example of redirecting output in C, and this post has an example of getting a file descriptor for a buffer in memory.

Fast read stdin into buffer with no checking

2 Answers2