[PATCH] stdio-common: Add the fgetln function

[Paul Eggert]

If the stream is not already oriented, FreeBSD getln sets the stream to 
byte-orientation. Should glibc getln do the same?

On 5/3/22 00:36, Florian Weimer via Libc-alpha wrote:
> +  /* Discard the old buffer.  This optimizes for a buffered stream,
> +     with multiple lines in each buffer.  */
> +  if (fp->_fgetln_buf != NULL)
> +    {
> +      free (fp->_fgetln_buf);
> +      fp->_fgetln_buf = NULL;
> +    }

Hope you don't mind a bit of bikeshedding here....

Why free the fgetln buffer eagerly? Instead, free it only when closing. 
That would lessen pressure on the memory allocator and would save a few 
insns in fgetln's usual case.

Come to think of it, how about if we restrict fgetln to streams for 
which either (1) the user has not called setvbuf with a nonnull buffer, 
or (2) the input line fits in the user-supplied setvbuf buffer.  Then we 
wouldn't need to worry about adding a _fgetln_buf slot, as fgetln could 
always return a pointer into the already-existing stdio buffer, possibly 
by enlarging the buffer in case (1). (In case (2) fgetln could fail with 
ENOMEM if the input line is longer than the user-supplied buffer.) This 
would suffice for 99.9% of applications and would be more efficient than 
what FreeBSD does, and the whole point of fgetln is low-level efficiency 
right?