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Ticket UUID:	9087fae43f3ae496a4151ca15a9dd37d697d6f1e
Title:	configure: X11 header check always fails because CPP is not set ("couldn't find any!")
Type:	Bug	Version:	9.0.4, 9.1b1
Submitter:	serhiy.storchaka	Created on:	2026-09-25 11:35:05
Subsystem:	84. Unix Build	Assigned To:	jan.nijtmans
Priority:	5 Medium	Severity:	Minor
Status:	Closed	Last Modified:	2026-09-25 13:05:48
Resolution:	Fixed	Closed By:	jan.nijtmans
		Closed on:	2026-09-25 13:05:48
Description:	Since Tk 9.0 every Unix configure run prints checking for X11 header files... couldn't find any! although the X11 headers are installed and the build works. config.log shows ./configure: line 1818: conftest.c: command not found `SC_PATH_X` checks for `<X11/Xlib.h>` with `AC_PREPROC_IFELSE`, but `$CPP` is empty. The only expansion of `AC_PROG_CPP` is inside `if test $tk_ok = no; then` (the `fd_set` check in `unix/configure.ac`, via `AC_EGREP_HEADER`), so it is skipped wherever `fd_set` is in `<sys/types.h>`. Before [b830ddb20c] removed the `stdlib.h` checks, they expanded it unconditionally earlier; 8.6 still has them and is not affected. Consequences: the header check never succeeds when `AC_PATH_X` finds X in the default paths, and `SC_PATH_X` falls back to its directory list, adding `-I` for the first of those directories that contains X11/Xlib.h.
User Comments:	jan.nijtmans added on 2026-09-25 13:05:48: Fixed [bf45f48c4500fe1f\|here] and in core-9-0-branch. Dang... I noticed this already for a long time, but it didn't seem to hurt and I had no idea how to approach this. Many thanks! serhiy.storchaka added on 2026-09-25 11:38:31: Proposed fix in [78b2122ad8] (branch configure-prog-cpp): call `AC_PROG_CPP` right after `AC_PROG_CC`. With it CPP is set, the spurious message is gone, and XINCLUDES/XLIBSW are unchanged. 9.0 needs the same change.