问题
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
回答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.
回答2:
To give you more information on another aspect of your post, to find where the libraries are on your machine will be operating system dependent. As it appears you are using an OS with bash you will find most of your libraries in the /usr or /usr/local folders (maybe I should not say most, but it has been a default install location for many libraries I have used, such as boost). This is not the location c++ libraries tend to be installed however.
My c++ libraries were located in folders labeled lib, which made them easy to search for using the find tool. Something like find / -iname "*lib*" 2>/dev/null should help you find any locations you might have libraries installed that you will need to tell your linker about. (You can ignore the 2>/dev/null if you want to sudo the command).
I had a lot of trouble recently getting my IDE's to play nice with boost, so hopefully this helps any future troubles you might have with linking your libraries.
EDIT: I wanted to add a bit of information about how to tell your IDE's about the libraries. Using the find tool, looking for all folders named include will lead to any folders that should be included in the IDE's compiler Additional Include Directories (not all folders named include, I should clarify, but this will help find any folders with the name and the specific one you want, such as seeing /usr/local/include/boost in the search results). The libraries, which I mentioned how to find earlier, will be added in the linker section under Additional Library Directories so they know where to look for the libraries specifically.
EDIT 2: To give some information on where to find the appropriate files on Windows, the search is a bit harder. There does not seem to really be a "standard" location that developers install to (if anything it might be the home directory of the current user, but that is far from the only place used). Further, the find functionality on Windows is not as simple or useful (in my opinion). On top of that, specifically for c++ files, it will be entirely based on HOW you obtained the files. For instance, if you are using Visual Studio, they are located inside the folder for the c++ portion of Visual Studio (i.e. it is C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\ on my machine) but if you are using the files you can obtain from MinGW it will be in a different location. My only advice here is to create a folder in a location of your choice (I used a folder directly inside C:) to manually install everything to that you add to your system.
Boost, for instance, allows easily changing the install path from the default at the time of install. Most (I am more comfortable saying nearly all here) tools created will have this feature, especially on Windows. Otherwise, searching the default locations specific to each utility you add or learning to use the find utility Windows provides are viable options.
回答3:
Standard C library functions are all defined in libc and you don't have to link it to your program as it is done automatically by your linker at linker time. The only exception to this is math functions. They are put in a separate library called libm. They were put in a separate library for historical reasons related to the Fortran compiler on UNIX (I don't have any source to support this claim).
回答4:
The implementation of the math library is (typically) in two parts.
<math.h> is a header file, typically /usr/include/math.h. It's C source code that tells the compiler how to generate calls to pow and other functions. It should be available by default; all you need is #include <math.h>, and the compiler will know where to find it. (So your title is a little misleading.)
The functions declared in <math.h> are implemented in a library file, which contains precompiled executable code that can be linked into your program (after it's been compiled). That library, for historical reasons, is typically not searched for by default, and you have to specify an additional -lm option to tell the linker where to find it. (This is widely considered a bug; on the other hand, not searching the math library by default can make linking very slightly faster, and most C programs don't use the math library.)
来源：https://stackoverflow.com/questions/19525675/why-does-math-h-need-to-be-linked-in-makefile-but-not-string-h