Rob Landley wrote, on 11 Apr 2022:
>
> A bunch of protocols (git, http, mbox, etc) start with lines of data followed
> by
> a block of data, so it's natural to want to call getline() and then handle the
> data block. But getline() takes a FILE * and things like zlib and sendfile()
> take an integer file descriptor.
>
> Posix lets me get the file descriptor out of a FILE * with fileno(), but the
> point of FILE * is to readahead and buffer. How do I get the buffered data out
> without reading more from the file descriptor?
>
> I can't find a portable way to do this?
I tried this sequence of calls on a few systems, and it worked in the
way you would expect:
fgets(buf, sizeof buf, fp);
int fd = dup(fileno(fp));
close(fileno(fp));
while ((ret = fread(buf, 1, sizeof buf, fp)) > 0) { ... }
read(fd, buf, sizeof buf);
It relies on fread() not detecting EBADF until it tries to read more
data from the underlying fd.
It has some caveats:
1. It needs a file descriptor to be available.
2. The close() will remove any fcntl() locks that the calling process
holds for the file.
3. In a multi-threaded process it has the usual problem around fd
inheritance, but that's addressed in Issue 8 with the addition
of dup3().
Also, for the standard to require it to work, I think we would need to
tweak the EBADF error for fgetc() (which fread() references) to say:
The file descriptor underlying stream is not a valid file
descriptor open for reading and there is no buffered data
available to be returned.
(adding the "and ..." part).
--
Geoff Clare <[email protected]>
The Open Group, Apex Plaza, Forbury Road, Reading, RG1 1AX, England
