valgrind：为什么我的小编程分配了这么多空间？

Question

I'm reading through chapter 4 of "Learn C the Hard Way", where we start to work with valgrind.

One thing I noticed is that my very small programs are allocating 1,024 bytes:

==19896== HEAP SUMMARY:
==19896==     in use at exit: 0 bytes in 0 blocks
==19896==   total heap usage: 1 allocs, 1 frees, 1,024 bytes allocated

In the book, and in other people's code it shows 0 bytes allocated.

Here is the code:

#include <stdio.h>

int main(int argc, char *argv[])
{
  int distance = 100;

  // this is also a comment
  printf("You are %d miles away.\n", distance);

  return 0;
}

I don't understand why there needs to be 1kb of space allocated for this thing.

This bothers me and I would like to know what is going on.

Any help is appreciated. Thanks for your time!

Edit: 1KB, not 1MB

Answer 1

That's 1KB, not 1MB.. which isn't much memory these days (35 years ago, it was a lot).

As to why it's using that much: printf uses buffered I/O, which allocates buffers. The exact answer really depends upon the platform since c libraries and operating systems will vary. But if you were to write the same program using just system calls eg. write instead of printf , you'd probably see the memory usage go down.

Answer 2

printf buffers I/O before it actually writes that data to standard out (as @little_birdie mentioned). Buffered I/O refers to the practice of temporarily storing I/O operation in your application (user-space) prior to transmitting it to the kernel, which can be slow. In order to minimize these so called system calls , your application will ask for such memory ahead of time.

It is not uncommon for certain features of the system to disable this feature entirely, or even perhaps for a historical system to not have buffered I/O at all (although I'm not familiar with any).

If you want to disabled buffering on your stdout here (and thus allocation "0" bytes of heap memory), you can ask for it with setbuf like so:

#include <stdio.h>

int main()
{
    int distance = 100;
    setbuf(stdout, NULL);
    printf("You are %d miles away.\n", distance);
    return 0;
}

If you want to learn more about this, check out the excellent Linux Programming Interface