Re: new module 'freadptr'

[Jim Meyering]

Bruno Haible <address@hidden> wrote:
> Hi,
>
> Here's one more module in the "stdio internals" series. freadptr (stream)
> returns a pointer to the read buffer of stream.
>
> For example, if you want to read the contents of the stream up to but not
> including the next newline into an obstack, here's the naïve code:
>
>   struct obstack *obs = ...;
>   FILE *fp = ...;
>   for (;;)
>     {
>       int ch = getc_unlocked (fp);
>       if (ch == EOF || ch == '\n')
>         break;
>       obstack_1grow (obs, ch);
>     }
>
> Optimization using fread_unlocked is not possible, because we cannot push
> back the read bytes after the newline back to the stream.
>
> And here's the code with freadptr. It does a maximum of operations "en bloc":
> memchr and obstack_grow (which expands into a memcpy call).

Looks useful.

>   struct obstack *obs = ...;
>   FILE *fp = ...;
>   for (;;)
>     {
>       size_t available = freadahead (fp);
>       if (available > 0)
>         {
>           const char *buf = freadptr (fp);
>           const char *nl = (const char *) memchr (buf, '\n', n);

Surely you meant this:

            const char *nl = (const char *) memchr (buf, '\n', available);


>           if (nl != NULL)
>             {
>               obstack_grow (obs, buf, nl - buf);
>               fseek (fp, nl - buf + 1, SEEK_CUR);
>             }
>           else
>             {
>               obstack_grow (obs, buf, available);
>               fseek (fp, available, SEEK_CUR);
>             }
>         }
>       else
>         {
>           int ch = getc (fp);
>           if (ch == EOF || ch == '\n')
>             break;
>           obstack_1grow (obs, ch);
>         }
>     }