On Fri, 2 May 2025 at 02:30, Bruno Haible <br...@clisp.org> wrote:
I don't think requiring -ldl is a "small" price to pay. It increases the
> startup times of the programs linked to that library. Therefore how about
> using dladdr only on those platforms where it is in libc? This would cover
> all the platform that you mentioned, and on Cygwin it would be a speedup
> (likely no system calls instead of reading from the /proc file system).
>
I just re-read this, and realised that presumably this means that using
dladdr would also be an improvement on Linux with glibc or uClibc, where
currently we also read /proc? Which means that the /proc-reading code can
be removed, as it is only used on the above-mentioned combinations plus
Cygwin, which you already said would be faster with dladdr.
I attach an updated patch. I have dealt with the shortcomings I previously
identified: I check whether the path returned by dladdr is NULL before
attempting to strdup it, and I add the same support to the relocatable-lib
module. I have removed the previous Cygwin/Linux code in favour of this
simpler, faster method.
--
https://rrt.sc3d.org
From 68b2eb373be7a39f0ad6c025a988bfe5c9f6e96b Mon Sep 17 00:00:00 2001
From: Reuben Thomas <r...@sc3d.org>
Date: Thu, 1 May 2025 20:45:19 +0100
Subject: [PATCH] relocatable-lib{,-lgpl}: use dladdr to get path to shared
library
This is simpler (and probably faster) than using /proc, and works on many
more systems (dladdr is already implemented on most BSDs, for which
previously we had no solution, and is even part of POSIX:2024[1]).
[1] https://pubs.opengroup.org/onlinepubs/9799919799/functions/dladdr.html
---
lib/relocatable.c | 64 +++++++-----------------------------
modules/relocatable-lib | 3 ++
modules/relocatable-lib-lgpl | 3 ++
3 files changed, 18 insertions(+), 52 deletions(-)
diff --git a/lib/relocatable.c b/lib/relocatable.c
index 15b3bc442d..5a23a39100 100644
--- a/lib/relocatable.c
+++ b/lib/relocatable.c
@@ -71,6 +71,10 @@
# define GetModuleFileName GetModuleFileNameA
#endif
+#if !(defined _WIN32 && !defined __CYGWIN__) && !(defined __EMX__)
+#include <dlfcn.h>
+#endif
+
/* Faked cheap 'bool'. */
#undef bool
#undef false
@@ -318,10 +322,10 @@ static char *shared_library_fullname;
#if defined _WIN32 && !defined __CYGWIN__
/* Native Windows only.
- On Cygwin, it is better to use the Cygwin provided /proc interface, than
- to use native Windows API and cygwin_conv_to_posix_path, because it
- supports longer file names
- (see <https://cygwin.com/ml/cygwin/2011-01/msg00410.html>). */
+
+ On Cygwin, it is better to use dladdr, than to use native Windows
+ API and cygwin_conv_to_posix_path, because it supports longer file
+ names (see <https://cygwin.com/ml/cygwin/2011-01/msg00410.html>). */
/* Determine the full pathname of the shared library when it is loaded.
@@ -402,54 +406,10 @@ _DLL_InitTerm (unsigned long hModule, unsigned long ulFlag)
static void
find_shared_library_fullname ()
{
-#if (defined __linux__ && (__GLIBC__ >= 2 || defined __UCLIBC__)) || defined __CYGWIN__
- /* Linux has /proc/self/maps. glibc 2 and uClibc have the getline()
- function.
- Cygwin >= 1.5 has /proc/self/maps and the getline() function too.
- But it is costly: ca. 0.3 ms on Linux, 3 ms on Cygwin 1.5, and 5 ms on
- Cygwin 1.7. */
- FILE *fp;
-
- /* Open the current process' maps file. It describes one VMA per line. */
- fp = fopen ("/proc/self/maps", "r");
- if (fp)
- {
- unsigned long address = (unsigned long) &find_shared_library_fullname;
- for (;;)
- {
- unsigned long start, end;
- int c;
-
- if (fscanf (fp, "%lx-%lx", &start, &end) != 2)
- break;
- if (address >= start && address <= end - 1)
- {
- /* Found it. Now see if this line contains a filename. */
- while (c = getc (fp), c != EOF && c != '\n' && c != '/')
- continue;
- if (c == '/')
- {
- size_t size;
- int len;
-
- ungetc (c, fp);
- shared_library_fullname = NULL; size = 0;
- len = getline (&shared_library_fullname, &size, fp);
- if (len >= 0)
- {
- /* Success: filled shared_library_fullname. */
- if (len > 0 && shared_library_fullname[len - 1] == '\n')
- shared_library_fullname[len - 1] = '\0';
- }
- }
- break;
- }
- while (c = getc (fp), c != EOF && c != '\n')
- continue;
- }
- fclose (fp);
- }
-#endif
+ Dl_info info;
+ dladdr (find_shared_library_fullname, &info);
+ if (info.dli_fname != NULL)
+ shared_library_fullname = strdup (info.dli_fname);
}
#endif /* Native Windows / EMX / Unix */
diff --git a/modules/relocatable-lib b/modules/relocatable-lib
index 7cdca5ce0a..f3f8e15dff 100644
--- a/modules/relocatable-lib
+++ b/modules/relocatable-lib
@@ -7,6 +7,7 @@ doc/relocatable.texi
lib/relocatable.h
lib/relocatable.c
lib/relocatable.valgrind
+m4/libdl.m4
m4/relocatable-lib.m4
m4/build-to-host.m4
@@ -19,8 +20,10 @@ gl_RELOCATABLE_LIBRARY
if test $RELOCATABLE = yes; then
AC_LIBOBJ([relocatable])
fi
+gl_LIBDL
Makefile.am:
+lib_LDFLAGS += $(LIBDL)
Include:
"relocatable.h"
diff --git a/modules/relocatable-lib-lgpl b/modules/relocatable-lib-lgpl
index b8ecb51ef8..da905347cd 100644
--- a/modules/relocatable-lib-lgpl
+++ b/modules/relocatable-lib-lgpl
@@ -7,6 +7,7 @@ doc/relocatable.texi
lib/relocatable.h
lib/relocatable.c
lib/relocatable.valgrind
+m4/libdl.m4
m4/relocatable-lib.m4
m4/build-to-host.m4
@@ -18,9 +19,11 @@ gl_RELOCATABLE_LIBRARY
if test $RELOCATABLE = yes; then
AC_LIBOBJ([relocatable])
fi
+gl_LIBDL
Makefile.am:
DEFS += -DNO_XMALLOC
+lib_LDFLAGS += $(LIBDL)
Include:
"relocatable.h"
--
2.43.0
