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2.1 Build Options

All the usual autoconf configure options are available, run ‘./configure --help’ for a summary. The file INSTALL.autoconf has some generic installation information too.


configure’ requires various Unix-like tools. See Notes for Particular Systems, for some options on non-Unix systems.

It might be possible to build without the help of ‘configure’, certainly all the code is there, but unfortunately you’ll be on your own.

Build Directory

To compile in a separate build directory, cd to that directory, and prefix the configure command with the path to the GMP source directory. For example

cd /my/build/dir

Not all ‘make’ programs have the necessary features (VPATH) to support this. In particular, SunOS and Slowaris make have bugs that make them unable to build in a separate directory. Use GNU make instead.

--prefix and --exec-prefix

The --prefix option can be used in the normal way to direct GMP to install under a particular tree. The default is ‘/usr/local’.

--exec-prefix can be used to direct architecture-dependent files like libgmp.a to a different location. This can be used to share architecture-independent parts like the documentation, but separate the dependent parts. Note however that gmp.h is architecture-dependent since it encodes certain aspects of libgmp, so it will be necessary to ensure both $prefix/include and $exec_prefix/include are available to the compiler.

--disable-shared, --disable-static

By default both shared and static libraries are built (where possible), but one or other can be disabled. Shared libraries result in smaller executables and permit code sharing between separate running processes, but on some CPUs are slightly slower, having a small cost on each function call.

Native Compilation, --build=CPU-VENDOR-OS

For normal native compilation, the system can be specified with ‘--build’. By default ‘./configure’ uses the output from running ‘./config.guess’. On some systems ‘./config.guess’ can determine the exact CPU type, on others it will be necessary to give it explicitly. For example,

./configure --build=ultrasparc-sun-solaris2.7

In all cases the ‘OS’ part is important, since it controls how libtool generates shared libraries. Running ‘./config.guess’ is the simplest way to see what it should be, if you don’t know already.

Cross Compilation, --host=CPU-VENDOR-OS

When cross-compiling, the system used for compiling is given by ‘--build’ and the system where the library will run is given by ‘--host’. For example when using a FreeBSD Athlon system to build GNU/Linux m68k binaries,

./configure --build=athlon-pc-freebsd3.5 --host=m68k-mac-linux-gnu

Compiler tools are sought first with the host system type as a prefix. For example m68k-mac-linux-gnu-ranlib is tried, then plain ranlib. This makes it possible for a set of cross-compiling tools to co-exist with native tools. The prefix is the argument to ‘--host’, and this can be an alias, such as ‘m68k-linux’. But note that tools don’t have to be setup this way, it’s enough to just have a PATH with a suitable cross-compiling cc etc.

Compiling for a different CPU in the same family as the build system is a form of cross-compilation, though very possibly this would merely be special options on a native compiler. In any case ‘./configure’ avoids depending on being able to run code on the build system, which is important when creating binaries for a newer CPU since they very possibly won’t run on the build system.

In all cases the compiler must be able to produce an executable (of whatever format) from a standard C main. Although only object files will go to make up libgmp, ‘./configure’ uses linking tests for various purposes, such as determining what functions are available on the host system.

Currently a warning is given unless an explicit ‘--build’ is used when cross-compiling, because it may not be possible to correctly guess the build system type if the PATH has only a cross-compiling cc.

Note that the ‘--target’ option is not appropriate for GMP. It’s for use when building compiler tools, with ‘--host’ being where they will run, and ‘--target’ what they’ll produce code for. Ordinary programs or libraries like GMP are only interested in the ‘--host’ part, being where they’ll run. (Some past versions of GMP used ‘--target’ incorrectly.)

CPU types

In general, if you want a library that runs as fast as possible, you should configure GMP for the exact CPU type your system uses. However, this may mean the binaries won’t run on older members of the family, and might run slower on other members, older or newer. The best idea is always to build GMP for the exact machine type you intend to run it on.

The following CPUs have specific support. See for details of what code and compiler options they select.

CPUs not listed will use generic C code.

Generic C Build

If some of the assembly code causes problems, or if otherwise desired, the generic C code can be selected with the configure --disable-assembly.

Note that this will run quite slowly, but it should be portable and should at least make it possible to get something running if all else fails.

Fat binary, --enable-fat

Using --enable-fat selects a “fat binary” build on x86, where optimized low level subroutines are chosen at runtime according to the CPU detected. This means more code, but gives good performance on all x86 chips. (This option might become available for more architectures in the future.)


On some systems GMP supports multiple ABIs (application binary interfaces), meaning data type sizes and calling conventions. By default GMP chooses the best ABI available, but a particular ABI can be selected. For example

./configure --host=mips64-sgi-irix6 ABI=n32

See ABI and ISA, for the available choices on relevant CPUs, and what applications need to do.


By default the C compiler used is chosen from among some likely candidates, with gcc normally preferred if it’s present. The usual ‘CC=whatever’ can be passed to ‘./configure’ to choose something different.

For various systems, default compiler flags are set based on the CPU and compiler. The usual ‘CFLAGS="-whatever"’ can be passed to ‘./configure’ to use something different or to set good flags for systems GMP doesn’t otherwise know.

The ‘CC’ and ‘CFLAGS’ used are printed during ‘./configure’, and can be found in each generated Makefile. This is the easiest way to check the defaults when considering changing or adding something.

Note that when ‘CC’ and ‘CFLAGS’ are specified on a system supporting multiple ABIs it’s important to give an explicit ‘ABI=whatever’, since GMP can’t determine the ABI just from the flags and won’t be able to select the correct assembly code.

If just ‘CC’ is selected then normal default ‘CFLAGS’ for that compiler will be used (if GMP recognises it). For example ‘CC=gcc’ can be used to force the use of GCC, with default flags (and default ABI).


Any flags like ‘-D’ defines or ‘-I’ includes required by the preprocessor should be set in ‘CPPFLAGS’ rather than ‘CFLAGS’. Compiling is done with both ‘CPPFLAGS’ and ‘CFLAGS’, but preprocessing uses just ‘CPPFLAGS’. This distinction is because most preprocessors won’t accept all the flags the compiler does. Preprocessing is done separately in some configure tests.


Some build-time programs are compiled and run to generate host-specific data tables. ‘CC_FOR_BUILD’ is the compiler used for this. It doesn’t need to be in any particular ABI or mode, it merely needs to generate executables that can run. The default is to try the selected ‘CC’ and some likely candidates such as ‘cc’ and ‘gcc’, looking for something that works.

No flags are used with ‘CC_FOR_BUILD’ because a simple invocation like ‘cc foo.c’ should be enough. If some particular options are required they can be included as for instance ‘CC_FOR_BUILD="cc -whatever"’.

C++ Support, --enable-cxx

C++ support in GMP can be enabled with ‘--enable-cxx’, in which case a C++ compiler will be required. As a convenience ‘--enable-cxx=detect’ can be used to enable C++ support only if a compiler can be found. The C++ support consists of a library and header file gmpxx.h (see Headers and Libraries).

A separate has been adopted rather than having C++ objects within in order to ensure dynamic linked C programs aren’t bloated by a dependency on the C++ standard library, and to avoid any chance that the C++ compiler could be required when linking plain C programs. will use certain internals from and can only be expected to work with from the same GMP version. Future changes to the relevant internals will be accompanied by renaming, so a mismatch will cause unresolved symbols rather than perhaps mysterious misbehaviour.

In general will be usable only with the C++ compiler that built it, since name mangling and runtime support are usually incompatible between different compilers.


When C++ support is enabled, the C++ compiler and its flags can be set with variables ‘CXX’ and ‘CXXFLAGS’ in the usual way. The default for ‘CXX’ is the first compiler that works from a list of likely candidates, with g++ normally preferred when available. The default for ‘CXXFLAGS’ is to try ‘CFLAGS’, ‘CFLAGS’ without ‘-g’, then for g++ either ‘-g -O2’ or ‘-O2’, or for other compilers ‘-g’ or nothing. Trying ‘CFLAGS’ this way is convenient when using ‘gcc’ and ‘g++’ together, since the flags for ‘gcc’ will usually suit ‘g++’.

It’s important that the C and C++ compilers match, meaning their startup and runtime support routines are compatible and that they generate code in the same ABI (if there’s a choice of ABIs on the system). ‘./configure’ isn’t currently able to check these things very well itself, so for that reason ‘--disable-cxx’ is the default, to avoid a build failure due to a compiler mismatch. Perhaps this will change in the future.

Incidentally, it’s normally not good enough to set ‘CXX’ to the same as ‘CC’. Although gcc for instance recognises as C++ code, only g++ will invoke the linker the right way when building an executable or shared library from C++ object files.

Temporary Memory, --enable-alloca=<choice>

GMP allocates temporary workspace using one of the following three methods, which can be selected with for instance ‘--enable-alloca=malloc-reentrant’.

For convenience, the following choices are also available. ‘--disable-alloca’ is the same as ‘no’.

alloca is reentrant and fast, and is recommended. It actually allocates just small blocks on the stack; larger ones use malloc-reentrant.

malloc-reentrant’ is, as the name suggests, reentrant and thread safe, but ‘malloc-notreentrant’ is faster and should be used if reentrancy is not required.

The two malloc methods in fact use the memory allocation functions selected by mp_set_memory_functions, these being malloc and friends by default. See Custom Allocation.

An additional choice ‘--enable-alloca=debug’ is available, to help when debugging memory related problems (see Debugging).

FFT Multiplication, --disable-fft

By default multiplications are done using Karatsuba, 3-way Toom, higher degree Toom, and Fermat FFT. The FFT is only used on large to very large operands and can be disabled to save code size if desired.

Assertion Checking, --enable-assert

This option enables some consistency checking within the library. This can be of use while debugging, see Debugging.

Execution Profiling, --enable-profiling=prof/gprof/instrument

Enable profiling support, in one of various styles, see Profiling.


Various assembly versions of each mpn subroutines are provided. For a given CPU, a search is made though a path to choose a version of each. For example ‘sparcv8’ has

MPN_PATH="sparc32/v8 sparc32 generic"

which means look first for v8 code, then plain sparc32 (which is v7), and finally fall back on generic C. Knowledgeable users with special requirements can specify a different path. Normally this is completely unnecessary.


The source for the document you’re now reading is doc/gmp.texi, in Texinfo format, see Texinfo in Texinfo.

Info format ‘doc/’ is included in the distribution. The usual automake targets are available to make PostScript, DVI, PDF and HTML (these will require various TeX and Texinfo tools).

DocBook and XML can be generated by the Texinfo makeinfo program too, see Options for makeinfo in Texinfo.

Some supplementary notes can also be found in the doc subdirectory.

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