C99 specifies that the EOF condition on a file is "sticky": once EOF has been encountered, all subsequent reads should continue to return EOF until the file is closed or something clears the "end-of-file indicator" (e.g. fseek, clearerr). This is arguably a change from C89, where the wording was ambiguous; the BSDs always had sticky EOF, but the System V lineage would attempt to read from the underlying fd again. GNU libc has followed System V for as long as we've been using libio, but nowadays C99 conformance and BSD compatibility are more important than System V compatibility.
You might wonder if changing the _underflow impls is sufficient to apply the C99 semantics to all of the many stdio functions that perform input. It should be enough to cover all paths to _IO_SYSREAD, and the only other functions that call _IO_SYSREAD are the _seekoff impls, which is OK because seeking clears EOF, and the _xsgetn impls, which, as far as I can tell, are unused within glibc.
The test programs in this patch use a pseudoterminal to set up the necessary conditions. To facilitate this I added a new test-support function that sets up a pair of pty file descriptors for you; it's almost the same as BSD openpty, the only differences are that it allocates the optionally-returned tty pathname with malloc, and that it crashes if anything goes wrong.
[BZ #1190] [BZ #19476]
- libio/fileops.c (_IO_new_file_underflow): Return EOF immediately if the _IO_EOF_SEEN bit is already set; update commentary.
- libio/oldfileops.c (_IO_old_file_underflow): Likewise.
- libio/wfileops.c (_IO_wfile_underflow): Likewise.
- support/support_openpty.c, support/tty.h: New files.
- support/Makefile (libsupport-routines): Add support_openpty.
- libio/tst-fgetc-after-eof.c, wcsmbs/test-fgetwc-after-eof.c: New test cases.
- libio/Makefile (tests): Add tst-fgetc-after-eof.
- wcsmbs/Makefile (tests): Add tst-fgetwc-after-eof.
2cc7bad0ae [BZ 1190] Make EOF sticky in stdio.
ChangeLog | 17 +++++++
NEWS | 8 +++
libio/Makefile | 3 +-
libio/fileops.c | 7 ++-
libio/oldfileops.c | 7 ++-
libio/tst-fgetc-after-eof.c | 109 ++++++++++++++++++++++++++++++++++++++++
libio/wfileops.c | 4 ++
support/Makefile | 1 +
support/support_openpty.c | 109 ++++++++++++++++++++++++++++++++++++++++
support/tty.h | 45 +++++++++++++++++
wcsmbs/Makefile | 2 +-
wcsmbs/tst-fgetwc-after-eof.c | 114 ++++++++++++++++++++++++++++++++++++++++++
12 files changed, 416 insertions(+), 10 deletions(-)