Resource use fopen-vs-freopen

Resource use fopen-vs-freopen

[David Seifert via Libc-alpha]

Hi glibc,
this came up recently as a point of interest. Say we have snippet 1:

  #include <stdio.h>

  void reset_std_streams(void) {
    stdin = fopen("/dev/null", "r");
  }

  int main(void) {
    reset_std_streams();
    // A
    /*
    -- actual program code --
    */
  }

and snippet 2:

  #include <stdio.h>

  void reset_std_streams(void) {
    freopen("/dev/null", "r", stdin);
  }

  int main(void) {
    reset_std_streams();
    // B
    /*
    -- actual program code --
    */
  }

When comparing the total tally of all resources used at point A in
snippet 1 and point B in snippet 2, is there an actual difference? Are
there any resources in terms of buffers, memory mapped pages or other
static or dynamic resources that freopen() variant would use that the
fopen() variant would not?

Re: Resource use fopen-vs-freopen

[Zack Weinberg via Libc-alpha]

On 2022-12-04 4:34 PM, David Seifert via Libc-alpha wrote:
...
>    void reset_std_streams(void) {
>      stdin = fopen("/dev/null", "r");
>    }
...
>    void reset_std_streams(void) {
>      freopen("/dev/null", "r", stdin);
>    }
...

> When comparing the total tally of all resources used at point A in
> snippet 1 and point B in snippet 2, is there an actual difference?

Yes.  Snippet 1 leaks the old FILE object and, critically, *does not 
change file descriptor 0*, which remains open on the old stdin.

zw