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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.
110 lines
3.7 KiB
C
110 lines
3.7 KiB
C
/* Bug 1190: EOF conditions are supposed to be sticky.
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Copyright (C) 2018 Free Software Foundation.
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Copying and distribution of this file, with or without modification,
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are permitted in any medium without royalty provided the copyright
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notice and this notice are preserved. This file is offered as-is,
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without any warranty. */
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/* ISO C1999 specification of fgetc:
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#include <stdio.h>
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int fgetc (FILE *stream);
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Description
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If the end-of-file indicator for the input stream pointed to by
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stream is not set and a next character is present, the fgetc
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function obtains that character as an unsigned char converted to
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an int and advances the associated file position indicator for
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the stream (if defined).
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Returns
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If the end-of-file indicator for the stream is set, or if the
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stream is at end-of-file, the end-of-file indicator for the
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stream is set and the fgetc function returns EOF. Otherwise, the
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fgetc function returns the next character from the input stream
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pointed to by stream. If a read error occurs, the error indicator
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for the stream is set and the fgetc function returns EOF.
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The requirement to return EOF "if the end-of-file indicator for the
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stream is set" was new in C99; the language in the 1989 edition of
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the standard was ambiguous. Historically, BSD-derived Unix always
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had the C99 behavior, whereas in System V fgetc would attempt to
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call read() again before returning EOF again. Prior to version 2.28,
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glibc followed the System V behavior even though this does not
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comply with C99.
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See
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<https://sourceware.org/bugzilla/show_bug.cgi?id=1190>,
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<https://sourceware.org/bugzilla/show_bug.cgi?id=19476>,
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and the thread at
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<https://sourceware.org/ml/libc-alpha/2012-09/msg00343.html>
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for more detail. */
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#include <support/tty.h>
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#include <support/check.h>
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#include <fcntl.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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#define XWRITE(fd, s, msg) do { \
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if (write (fd, s, sizeof s - 1) != sizeof s - 1) \
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{ \
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perror ("write " msg); \
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return 1; \
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} \
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} while (0)
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int
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do_test (void)
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{
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/* The easiest way to set up the conditions under which you can
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notice whether the end-of-file indicator is sticky, is with a
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pseudo-tty. This is also the case which applications are most
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likely to care about. And it avoids any question of whether and
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how it is legitimate to access the same physical file with two
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independent FILE objects. */
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int outer_fd, inner_fd;
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FILE *fp;
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support_openpty (&outer_fd, &inner_fd, 0, 0, 0);
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fp = fdopen (inner_fd, "r+");
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if (!fp)
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{
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perror ("fdopen");
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return 1;
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}
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XWRITE (outer_fd, "abc\n\004", "first line + EOF");
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TEST_COMPARE (fgetc (fp), 'a');
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TEST_COMPARE (fgetc (fp), 'b');
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TEST_COMPARE (fgetc (fp), 'c');
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TEST_COMPARE (fgetc (fp), '\n');
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TEST_COMPARE (fgetc (fp), EOF);
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TEST_VERIFY_EXIT (feof (fp));
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TEST_VERIFY_EXIT (!ferror (fp));
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XWRITE (outer_fd, "d\n", "second line");
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/* At this point, there is a new full line of input waiting in the
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kernelside input buffer, but we should still observe EOF from
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stdio, because the end-of-file indicator has not been cleared. */
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TEST_COMPARE (fgetc (fp), EOF);
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/* Clearing EOF should reveal the next line of input. */
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clearerr (fp);
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TEST_COMPARE (fgetc (fp), 'd');
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TEST_COMPARE (fgetc (fp), '\n');
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fclose (fp);
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close (outer_fd);
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return 0;
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}
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#include <support/test-driver.c>
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