Why does math.h need to be linked in makefile but not string.h? [duplicate]

Question

I've been including <string.h> all over the place and the moment I go to include <math.h> for pow() I discover that I need to modify my makefile to get access to the definition of pow.

I also do not know where I can look up more libraries if I need to include them. I randomly read that -lm flag (or at least the m of it) indicates the standard c math library, but I have no idea what any of the other standard c libraries might be called.

I just looked in user/local/lib and all I see is:

$ cd /usr/local/lib
$ ls -al
drwxr-xr-x  3 root root  4096 Apr 25  2012 .
drwxr-xr-x 10 root root  4096 Apr 25  2012 ..
drwxrwsr-x  4 root staff 4096 Oct 14 10:19 python2.7

Answer 1

string.h is a header reference to a module of the standard library, but due to historical reasons math.h is not a header reference to a module of the standard library. Only the standard library is typically included by default in the linker when compiling programs.

At one point in time, one might substitute libm.so math.h implementations with other implementations that were better optimized for memory, CPU performance, etc. Effectively there was not a single libm.so "default" implementation. However, most systems at least provided an implementation. That implementation is in the default library location as libm.so and would be linked in with -lm.

In the event that you had a faster (perhaps even at the expense of less accurate) library that you understood you could use, you had the ability to override the system-provided math.h libm.so implementation.

The early CRAY systems (I didn't work on one) did optimize a few math implementations to not do proper "full" math operations, with the understanding that to get a 100% correct answer, you would "finish" the assembly operations with code that often wasn't important due to the number of significant digits in the computation. (From my understanding of a Cryptology Museum display in Washsington D.C.)

The problem with multiple implementations is that you now have a choice, and the standard C library is not structured to provide you with a choice of implementation.