问题
I tried to convert C code that is written under Linux (fedora 9) to assembly x86 code, however, I have problem in a Math.h functions. The functions in this library such as ceil, floor, log, log10, pow are undefined in the assembly x86. Can you please help me to solve this problem?
Thanks.
回答1:
Most library functions won't be defined in assembly language, at least not in the sense of the addition operator directly mapping to the ADD instruction. If you want to re-write the library in assembly, you'll have to implement the function using whatever capabilities that your processor has available. Most library functions will require a separate assembly language subroutine, not just a single operation. The easiest way to approach this is to get the individual library subroutines working in isolation, then incorporate them into the larger program.
You can compile the C code and examine the disassembled output, but beware of compiler optimizations that can make the output hard for a human to follow.
May I ask what the purpose is behind this task? Since a compiler is essentially a C to assembly-language translator, there's rarely a need to do this by hand. Is this homework?
回答2:
The best way to find out what these functions do is to take a look at their implementation in glibc's source. It should give you clear enough insight. Another way would be to take a look at the disassembly of lm.so found in /usr/lib/.
来源：https://stackoverflow.com/questions/10762079/math-h-library-functions-in-assembly-x86