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Correctly link libfastjson against libm on glibc ≥ 2.42 #171

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Correctly link libfastjson against libm on glibc ≥ 2.42 #171

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24e7150
Revert "build: link libfastjson against libm for modf()"
Whissi Dec 5, 2025
2e7330e
build: Use AC_CHECK_LIB to ensure libfastjson links against libm
Whissi Dec 5, 2025
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5 changes: 2 additions & 3 deletions configure.ac
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Original file line number Diff line number Diff line change
Expand Up @@ -105,9 +105,8 @@ AC_FUNC_VPRINTF
AC_FUNC_MEMCMP
AC_CHECK_FUNCS(strcasecmp strdup strerror snprintf vsnprintf vasprintf open vsyslog strncasecmp setlocale)

if test "$ac_cv_have_decl_isnan" = "yes" ; then
AC_TRY_LINK([#include <math.h>], [float f = 0.0; return isnan(f)], [], [LIBS="$LIBS -lm"])
fi
# Force linking against libm (needed for modf on glibc >= 2.42)
AC_CHECK_LIB([m], [modf], [LIBS="$LIBS -lm"],[AC_MSG_ERROR([libm (math library) is required for modf()])])
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@gemini-code-assist gemini-code-assist bot Dec 5, 2025

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high

This change is insufficient to link libfastjson.so against libm. The LIBS variable is not used by libtool for linking shared libraries; instead, dependencies should be added to the _LIBADD variable in Makefile.am.

To correctly link libfastjson.so against libm, you'll need to modify both configure.ac and Makefile.am.

  1. In configure.ac, create and substitute a variable for the math library. You can replace the current line with:

    LIBM=""
AC_CHECK_LIB([m], [modf], [LIBM="-lm"], [AC_MSG_ERROR([libm (math library) is required for modf()])])
AC_SUBST([LIBM])
LIBS="$LIBS $LIBM"

    This creates a LIBM variable for Makefile.am and also updates LIBS for pkg-config.

  2. In Makefile.am (which you'll need to add to this PR), use this new variable:

    libfastjson_la_LIBADD = libfastjson-internal.la $(LIBM)

This will ensure libfastjson.so is correctly linked against libm, achieving the goal of this pull request.

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@Whissi Whissi Dec 5, 2025

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I believe this is a false positive. I am matching existing style which already writes directly into $LIBS variable and I verified that the resulting libfastjson.so is now linked against libm:

$ objdump -x /usr/lib64/libfastjson.so.4.3.0 | grep NEEDED | grep libm
  NEEDED               libm.so.6

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@mbiebl mbiebl Dec 5, 2025

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my autotools knowledge is a bit rusty at this point but his looks correct to me. Thanks @Whissi !

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@mbiebl mbiebl Dec 6, 2025

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I believe this is a false positive. I am matching existing style which already writes directly into $LIBS variable and I verified that the resulting libfastjson.so is now linked against libm:

The generated Makefile.in looks like this:

Makefile.in:    $(AM_V_CCLD)$(libfastjson_internal_la_LINK)  $(libfastjson_internal_la_OBJECTS) $(libfastjson_internal_la_LIBADD) $(LIBS)

Which would confirm that LIBS is actually passed to the linker flags.


#check if .section.gnu.warning accepts long strings (for __warn_references)
AC_LANG_PUSH([C])
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2 changes: 1 addition & 1 deletion libfastjson.pc.in
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Expand Up @@ -8,5 +8,5 @@ Description: a fast JSON implementation in C
Version: @VERSION@
Requires:
Libs.private: @LIBS@
Libs: -L${libdir} -lfastjson -lm
Libs: -L${libdir} -lfastjson
Cflags: -I${includedir}/libfastjson