文件操作 - ax_c_referenceable_passed_va_list.m4
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文件: /usr/share/aclocal/ax_c_referenceable_passed_va_list.m4
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# ====================================================================================== # https://www.gnu.org/software/autoconf-archive/ax_c_referenceable_passed_va_list.html # ====================================================================================== # # SYNOPSIS # # AX_C_REFERENCEABLE_PASSED_VA_LIST # # DESCRIPTION # # Checks whether f(va_list va){ &va; } works as expected. # # This macro uses compile-time detection and so is cross-compile ready. # # C99 mentioned passing a pointer to va_list to other functions (footnote # 212 of "7.15 Variable arguments <stdarg.h>"). However, f(va_list va) { # &va; } produces broken pointer on some environments such as gcc on # x86_64, although { va_list va; &va; } works as expected. See the # detection code of this file and any of pages # https://www.gnu.org/software/autoconf/manual/html_node/Function-Portability.html, # http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14557, and # http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20951 for further # information. # # Although C99 does not define the operations f(va_list va) { &va; } and # &va itself as standard (footnotes are declared as "normative part, # information only"), certain situations need it. This macro provides a # type detection about va_list implementation to deal with the operation. # # Following workaround will probably work on such environments although it # does not ensure to be safe and portable. At least it is working on # x86_64-unknown-linux-gnu: # # f(va_list va) # { # va_list *vap; # # #if HAVE_REFERENCEABLE_PASSED_VA_LIST # vap = &va; # #else # vap = (va_list *)va; # #endif # } # # LICENSE # # Copyright (c) 2008 YAMAMOTO Kengo <yamaken@bp.iij4u.or.jp> # # Copying and distribution of this file, with or without modification, are # permitted in any medium without royalty provided the copyright notice # and this notice are preserved. This file is offered as-is, without any # warranty. #serial 9 AC_DEFUN([AX_C_REFERENCEABLE_PASSED_VA_LIST], [ AC_CACHE_CHECK([whether f(va_list va){ &va; } works as expected], [ax_cv_c_referenceable_passed_va_list], [AC_LINK_IFELSE([[ #include <stdarg.h> volatile va_list g_va; void vf(va_list callee_va) { /* typeof(callee_va) differs from typeof(caller_va) by a compiler trick, if * va_list is implemented as an array. On such environment, this copy * operation fails. */ g_va = callee_va; } void f(int last, ...) { va_list caller_va; va_start(caller_va, last); vf(caller_va); /* passed as &caller_va[0] if va_list is an array type */ va_end(caller_va); } int main(int argc, char *argv[]) { f(0xdeadbeef, 0xfedbeef, 0xfeedee); return 0; } ]], [ax_cv_c_referenceable_passed_va_list=yes], [ax_cv_c_referenceable_passed_va_list=no])]) if test "x$ax_cv_c_referenceable_passed_va_list" = xyes; then AC_DEFINE([HAVE_REFERENCEABLE_PASSED_VA_LIST], [1], [Define to 1 if f(va_list va){ &va; } works as expected.]) fi ])
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