Where is pow function defined and implemented in C

Question

I read that the pow(double, double) function is defined in "math.h" but I can't find its declaration.

Does anybody know where this function declared? And where is it implemented in C?

Reference:

http://publications.gbdirect.co.uk/c_book/chapter9/maths_functions.html

Answer 1

I’s really defined in math.h. Have you tried including math.h and simply using pow? What do you mean by “can't find it”?

Answer 2

If you are seeking how the calculation is implemented, you can find it here: http://fossies.org/dox/gcc-4.5.3/e__pow_8c_source.html The name of the function is __ieee754_pow which is called by pow function.

Answer 3

declared: in the include directory of your system/SDK (e.g.: /usr/include;/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS3.2.sdk/usr/include/architecture/arm/math.h)

defined (implemented):

• as library (compiled, binary code): in the library directory of your system/SDK (e.g.: /usr/lib (in case of the math library it's libm.dylib)
• as source (program code): this is the interesting part. I work on a Mac OS X 10.6.x right now. The sources for the functions declared in math.h (e.g.: extern double pow ( double, double ); ) are not shipped with the installation (at least I couldn't find it). You are likely to find those sources in your system/SDK's C library. In my case the math library (libm) is a separate project, some of its sources are provided by Apple: http://www.opensource.apple.com/tarballs/Libm/Libm-315.tar.gz

The extern keyword in the function declaration of pow means, that it's defined somewhere else. Math functions are low-level high-performance implementations mostly done in assembly code (*.s). The assembly routines (taking the arguments/giving the parameters via registers/stack) are linked with the rest of the C library. The linking/exporting of the function/routine names is platform specific and doesn't really matter if ones goal is not dive into assembly coding.

I hope this helped, Raphael

Answer 4

Here's a C implementation for fdlibm: http://www.netlib.org/fdlibm/e_pow.c

For what it's worth, when v8 dropped its cos/sine tables, it pulled from fdlibm's implementation to do so: https://code.google.com/p/v8/source/detail?r=22918

From the change commit comments: "Implement trigonometric functions using a fdlibm port."

Mozilla on the other hand calls the cstdlib math functions, which will have variable performance by build and system (ex: may or may not invoke the chip-level implementations of transcendental functions). While C# bytecode seems to make explicit references to chip-level functions when it can. However, "pow" is not one of those, iirc (doesn't seem to have an chip-level function) and is implemented elsewhere.

See also: https://bugzilla.mozilla.org/show_bug.cgi?id=967709

For a cos/sine discussion in the Mozilla community, comparison of Mozilla's implementation vs old v8 implementation.

See also: How is Math.Pow() implemented in .NET Framework?

Intrinsic functions are chip-level, actually implemented on the processor. (We don't necessarily need lookup tables any more.)

Answer 5

Quite often, an include file such as <math.h> will include other header files that actually declare the functions you would expect to see in <math.h>. The idea is that the program gets what it expects when it includes <math.h>, even if the actual function definitions are in some other header file.

Finding the implementation of a standard library function such as pow() is quite another matter. You will have to dig up the source code to your C standard runtime library and find the implementation in there.

Answer 6

Where it's defined depends on your environment. The code is inside a compiled C standard library somewhere.

Its "definition" is in the source code for your c standard library distribution. One such distribution is eglibc. This is browsable online, or in a source distribution:

w_pow.c

math_private.h

Short answer: In the C standard library source code.