gh-115119: Fix libmpdec detection when pkg-config files are missing by stratakis · Pull Request #150502 · python/cpython · GitHub

The fallback for systems with no mpdecimal .pc files used AC_LINK_IFELSE directly, which relied on LIBMPDEC_LIBS being set. On failure this was empty so the link test ran without -lmpdec.

Use AC_CHECK_HEADER and AC_CHECK_LIB instead.

Issue: Unvendor libmpdec sources #115119

…sing The fallback for systems with no mpdecimal .pc files used AC_LINK_IFELSE directly, which relied on LIBMPDEC_LIBS being set. On failure this was empty so the link test ran without -lmpdec. Use AC_CHECK_HEADER and AC_CHECK_LIB instead.

Found when trying to test what configure options to add for my buildbots and I saw the _decimal module wasn't actually building on centos stream 9 and 10, because they ship mpdecimal 2.5.1, hence no .pc files.

I followed the convention of other libraries.

Could you please add a news entry?

LGTM (modulo news entry)

Thanks for fixing this, it was my fault to believe, that pkg-config support was introduced in v2.5.0. But it was v4.0.0.

(Tested with v2.5.0)

CC @zware, @vstinner

How can I test this change? I need a system with libmpdec library and header files installed, but where pkg-config 'libmpdec >= 2.5.0' --libs fails? Can I replace libmpdec >= 2.5.0 with libmpdec >= 55.0 to test the change for example? What is the expected behavior?

How can I test this change?

You can install libmpdec from sources for versions < 4.0.0. (./configure && make all install)

I also checked Debian and derivatives, nobody ships *.pc files as a patch :(.

I applied the following change on configure.ac and I ran make regen-configure.

diff --git a/configure.ac b/configure.ac
index c8dce3fd49b..b9020368664 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4376,7 +4376,7 @@ if test "$ac_cv_ffi_complex_double_supported" = "yes"; then
 fi
 
 dnl Check for libmpdec >= 2.5.0
-PKG_CHECK_MODULES([LIBMPDEC], [libmpdec >= 2.5.0], [have_mpdec=yes], [
+PKG_CHECK_MODULES([LIBMPDEC], [libmpdec >= 55.0], [have_mpdec=yes], [
   WITH_SAVE_ENV([
     CPPFLAGS="$CPPFLAGS $LIBMPDEC_CFLAGS"
     LIBS="$LIBS $LIBMPDEC_LIBS"

./configure fails to locate libmpdecimal:

checking for libmpdec >= 55.0... no
...
checking for stdlib extension module _decimal... missing

I downloaded this PR, made a similar ./configure.ac change, and ran make regen-configure. This time, it succeeded:

checking for libmpdec >= 55.0... no
checking for mpdecimal.h... (cached) yes
checking for mpd_version in -lmpdec... (cached) yes
...
checking for stdlib extension module _decimal... yes

I get these flags in Makefile:

$ grep ^MODULE__DECIMAL Makefile
MODULE__DECIMAL_STATE=yes
MODULE__DECIMAL_CFLAGS= -DTEST_COVERAGE
MODULE__DECIMAL_LDFLAGS=-lmpdec -lm

Python builds successfully and I can import the decimal module.

AC_CHECK_HEADER([mpdecimal.h] seems to be redundant to me. Is it really useful? We already check we can build/link a C program which starts with #include <mpdecimal.h> (below).

I think it's good to have it there for the helpful error message, referencing that the header is missing aka "checking for mpdecimal.h... no". In the case the library is there without the header it will move on to compile the program with "checking for mpd_version in -lmpdec... yes" and then fail.

Agreed. These checks are logically distinct: first is for the header, next is for the library version.

Merged, thanks for the fix.

The 3.15 branch has different code in configure.ac, it doesn't seem to be affected.

The 3.15 branch has different code in configure.ac, it doesn't seem to be affected.

Old releases are affected. But in such cases the fallback is using bundled libmpdec, instead of undetected from the system. I think it's less severe issue, probably not worth to be fixed.

The 3.15 branch has different code in configure.ac, it doesn't seem to be affected.

Old releases are affected. But in such cases the fallback is using bundled libmpdec, instead of undetected from the system. I think it's less severe issue, probably not worth to be fixed.

I don't see any issues via testing on previous branches, some stale makefile rules but if the system libmpdec option is used then it will properly link to that one without building the bundled one.

if the system libmpdec option is used then it will properly link to that one without building the bundled one.

I don't understand how this could be. See code e.g. in the v3.14:

cpython/configure.ac

Lines 4218 to 4232 in 7756f59

    
           AC_DEFUN([USE_BUNDLED_LIBMPDEC], 
        
                    [LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec" 
        
                     LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)" 
        
                     LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)" 
        
                     have_mpdec=yes 
        
                     with_system_libmpdec=no]) 
        
           AS_VAR_IF( 
        
             [with_system_libmpdec], [yes], 
        
             [PKG_CHECK_MODULES( 
        
               [LIBMPDEC], [libmpdec >= 2.5.0], [], 
        
               [LIBMPDEC_CFLAGS=${LIBMPDEC_CFLAGS-""} 
        
                LIBMPDEC_LIBS=${LIBMPDEC_LIBS-"-lmpdec -lm"} 
        
                LIBMPDEC_INTERNAL=])], 
        
             [USE_BUNDLED_LIBMPDEC()])

If PKG_CHECK_MODULES() fails - USE_BUNDLED_LIBMPDEC() is used with with_system_libmpdec=no.

if the system libmpdec option is used then it will properly link to that one without building the bundled one.

I don't understand how this could be. See code e.g. in the v3.14:

cpython/configure.ac

Lines 4218 to 4232 in 7756f59

AC_DEFUN([USE_BUNDLED_LIBMPDEC],

[LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"

LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"

LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"

have_mpdec=yes

with_system_libmpdec=no])

AS_VAR_IF(

[with_system_libmpdec], [yes],

[PKG_CHECK_MODULES(

[LIBMPDEC], [libmpdec >= 2.5.0], [],

[LIBMPDEC_CFLAGS=${LIBMPDEC_CFLAGS-""}

LIBMPDEC_LIBS=${LIBMPDEC_LIBS-"-lmpdec -lm"}

LIBMPDEC_INTERNAL=])],

[USE_BUNDLED_LIBMPDEC()])

If PKG_CHECK_MODULES() fails - USE_BUNDLED_LIBMPDEC() is used with with_system_libmpdec=no.

USE_BUNDLED_LIBMPDEC() is the argument to AS_VAR_IF() so it won't fall through if PKG_CHECK_MODULES() fails. I've also verified this on a Centos Stream 9 machine on the 3.14 branch, no bundled object files and _decimal.so is linked against the system library (assuming of course I tested properly).

stratakis requested review from AA-Turner, corona10, emmatyping and erlend-aasland as code owners May 27, 2026 02:33

bedevere-app Bot mentioned this pull request May 27, 2026

Unvendor libmpdec sources #115119

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bedevere-app Bot added the awaiting review label May 27, 2026

skirpichev self-requested a review May 27, 2026 02:44

skirpichev approved these changes May 27, 2026

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bedevere-app Bot added awaiting core review and removed awaiting review labels May 27, 2026

Add news entry

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vstinner reviewed Jun 1, 2026

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vstinner merged commit 025cc26 into python:main Jun 2, 2026
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bedevere-app Bot removed the awaiting core review label Jun 2, 2026

-                    ], [const char *x = mpd_version();])],
-                    [have_mpdec=yes],
-                    [have_mpdec=no])
+                  AC_CHECK_HEADER([mpdecimal.h], [

gh-115119: Fix libmpdec detection when pkg-config files are missing by stratakis · Pull Request #150502 · python/cpython · GitHub

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gh-115119: Fix libmpdec detection when pkg-config files are missing - #150502

gh-115119: Fix libmpdec detection when pkg-config files are missing#150502vstinner merged 2 commits intopython:mainpython/cpython:mainfrom stratakis:libmpdec_liiiiinkstratakis/cpython:libmpdec_liiiiinkCopy head branch name to clipboard

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gh-115119: Fix libmpdec detection when pkg-config files are missing#150502
vstinner merged 2 commits into
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stratakis:libmpdec_liiiiink

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