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diff --git a/m4/unlocked-io.m4 b/m4/unlocked-io.m4
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+#serial 8 -*- autoconf -*-
+
+dnl From Jim Meyering.
+dnl
+dnl See if the glibc *_unlocked I/O macros or functions are available.
+dnl Use only those *_unlocked macros or functions that are declared
+dnl (because some of them were declared in Solaris 2.5.1 but were removed
+dnl in Solaris 2.6, whereas we want binaries built on Solaris 2.5.1 to run
+dnl on Solaris 2.6).
+
+AC_DEFUN([jm_FUNC_GLIBC_UNLOCKED_IO],
+[
+  dnl Persuade glibc and Solaris <stdio.h> to declare
+  dnl fgets_unlocked(), fputs_unlocked() etc.
+  AC_REQUIRE([gl_USE_SYSTEM_EXTENSIONS])
+
+  AC_CHECK_DECLS_ONCE(
+     [clearerr_unlocked feof_unlocked ferror_unlocked
+      fflush_unlocked fgets_unlocked fputc_unlocked fputs_unlocked
+      fread_unlocked fwrite_unlocked getc_unlocked
+      getchar_unlocked putc_unlocked putchar_unlocked])
+])