xz: Create separate is_tty() function.

The new is_tty() will report if a file descriptor is a terminal or not.
On POSIX systems, it is a wrapper around isatty(). However, the native
Windows implementation of isatty() will return true for all character
devices, not just terminals. So is_tty() has a special case for Windows
so it can use alternative Windows API functions to determine if a file
descriptor is a terminal.

This fixes a bug with MSVC and MinGW-w64 builds that refused to read from
or write to non-terminal character devices because xz thought it was a
terminal. For instance:

    xz foo -c > /dev/null

would fail because /dev/null was assumed to be a terminal.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/xz/util.h b/src/xz/util.h
index 6d7e1481..1da40371 100644
--- a/src/xz/util.h
+++ b/src/xz/util.h
@@ -105,6 +105,20 @@ lzma_attribute((__format__(__printf__, 3, 4)))
 extern void my_snprintf(char **pos, size_t *left, const char *fmt, ...);
 
 
+/// \brief      Test if file descriptor is a terminal
+///
+/// For POSIX systems, this is a simple wrapper around isatty(). However on
+/// Windows, isatty() returns true for all character devices, not just
+/// terminals.
+///
+/// \param      fd    File descriptor to test
+///
+/// \return     bool:
+///             - true if file descriptor is a terminal
+///             - false otherwise
+extern bool is_tty(int fd);
+
+
 /// \brief      Test if stdin is a terminal
 ///
 /// If stdin is a terminal, an error message is printed and exit status set