Why do you have to link the math library in C

Question

If I include  lt stdlib h gt  or  lt stdio h gt  in a C program I don t have to link these when compiling but I do have to link to  lt math h gt   using -lm with gcc  for example   gcc test c -o test -lm   What is the reason for this  Why do I have to explicitly link the math library but not the other libraries

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An explanation is given here      So if your program is using math functions and including math h  then you need to explicitly link the math library by passing the -lm flag  The reason for this particular separation is that mathematicians are very picky about the way their math is being computed and they may want to use their own implementation of the math functions instead of the standard implementation  If the math functions were lumped into libc a it wouldn t be possible to do that     Edit   I m not sure I agree with this  though  If you have a library which provides  say  sqrt    and you pass it before the standard library  a Unix linker will take your version  right

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I would guess that it is a way to make apps which don t use it at all perform slightly better  Here s my thinking on this   x86 OSes  and I imagine others  need to store FPU state on context switch  However  most OSes only bother to save restore this state after the app attempts to use the FPU for the first time   In addition to this  there is probably some basic code in the math library which will set the FPU to a sane base state when the library is loaded   So  if you don t link in any math code at all  none of this will happen  therefore the OS doesn t have to save restore any FPU state at all  making context switches slightly more efficient   Just a guess though   EDIT  in response to some of the comments  the same base premise still applies to non-FPU cases  the premise being that it was to make apps which didn t make use libm perform slightly better    For example  if there is a soft-FPU which was likley in the early days of C  Then having libm separate could prevent a lot of large  and slow if it was used  code from unnecessarily being linked in   In addition  if there is only static linking available  then a similar argument applies that it would keep executable sizes and compile times down

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Note that -lm may not always need to be specified even if you use some C math functions  For example  the following simple program   include  lt stdio h gt   include  lt math h gt   int main          printf  quot output   f n quot   sqrt 2 0        return 0     can be compiled and run successfully with the following command  gcc test c -o test  Tested on gcc 7 5 0  on Ubuntu 16 04  and gcc 4 8 0  on CentOS 7   The post here gives some explanations   The math functions you call are implemented by compiler built-in functions  See also   https   gcc gnu org onlinedocs gcc Other-Builtins html https   stackoverflow com a 57062793 6064933

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Because of ridiculous historical practice that nobody is willing to fix  Consolidating all of the functions required by C and POSIX into a single library file would not only avoid this question getting asked over and over  but would also save a significant amount of time and memory when dynamic linking  since each  so file linked requires the filesystem operations to locate and find it  and a few pages for its static variables  relocations  etc   An implementation where all functions are in one library and the -lm  -lpthread  -lrt  etc  options are all no-ops  or link to empty  a files  is perfectly POSIX conformant and certainly preferable   Note  I m talking about POSIX because C itself does not specify anything about how the compiler is invoked  Thus you can just treat gcc -std c99 -lm as the implementation-specific way the compiler must be invoked for conformant behavior

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Because time   and some other functions are builtin defined in the C library  libc  itself and GCC always links to libc unless you use the -ffreestanding compile option  However math functions live in libm which is not implicitly linked by gcc

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If I put stdlib h or stdio h  I don t have to link those but I have to link when I compile    stdlib h  stdio h are the header files  You include them for your convenience  They only forecast what symbols will become available if you link in the proper library  The implementations are in the library files  that s where the functions really live   Including math h is only the first step to gaining access to all the math functions   Also  you don t have to link against libm if you don t use it s functions  even if you do a  include  lt math h gt  which is only an informational step for you  for the compiler about the symbols   stdlib h  stdio h refer to functions available in libc  which happens to be always linked in so that the user doesn t have to do it himself

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All libraries like stdio h and stdlib h have their implementation in libc so or libc a and get linked by the linker by default  The libraries for libc so are automatically linked while compiling and is included in the executable file   But math h has its implementations in libm so or libm a which is seperate from libc so and it does not get linked by default and you have to manually link it while compiling your program in gcc by using -lm flag   The gnu gcc team designed it to be seperate from the other header files  while the other header files get linked by default but math h file doesn t   Here read the item no 14 3  you could read it all if you wish  Reason why math h is needs to be linked  Look at this article  why we have to link math h in gcc   Have a look at the usage  using the library

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The functions in stdlib h and stdio h have implementations in libc so  or libc a for static linking   which is linked into your executable by default  as if -lc were specified    GCC can be instructed to avoid this automatic link with the -nostdlib or -nodefaultlibs options   The math functions in math h have implementations in libm so  or libm a for static linking   and libm is not linked in by default   There are historical reasons for this libm libc split  none of them very convincing   Interestingly  the C   runtime libstdc   requires libm  so if you compile a C   program with GCC  g     you will automatically get libm linked in

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stdio is part of the standard C library which  by default  gcc will link against   The math function implementations are in a separate libm file that is not linked to by default so you have to specify it -lm  By the way  there is no relation between those header files and library files

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There s a thorough discussion of linking to external libraries in An Introduction to GCC - Linking with external libraries   If a library is a member of the standard libraries  like stdio   then you don t need to specify to the compiler  really the linker  to link them   EDIT  After reading some of the other answers and comments  I think the libc a reference and the libm reference that it links to both have a lot to say about why the two are separate      Note that many of the functions in  libm a   the math library  are defined in  math h  but are not present in libc a  Some are  which may get confusing  but the rule of thumb is this--the C library contains those functions that ANSI dictates must exist  so that you don t need the -lm if you only use ANSI functions  In contrast   libm a  contains more functions and supports additional functionality such as the matherr call-back and compliance to several alternative standards of behavior in case of FP errors  See section libm  for more details

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As ephemient said  the C library libc is linked by default and this library contains the implementations of stdlib h  stdio h and several other standard header files  Just to add to it  according to  An Introduction to GCC  the linker command for a basic  Hello World  program in C is as below   ld -dynamic-linker  lib ld-linux so 2  usr lib crt1 o   usr lib crti o  usr libgcc-lib  i686 3 3 1 crtbegin o -L usr lib gcc-lib i686 3 3 1 hello o -lgcc -lgcc eh -lc  -lgcc -lgcc eh  usr lib gcc-lib i686 3 3 1 crtend o  usr lib crtn o   Notice the option -lc in the third line that links the C library

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Remember that C is an old language and that FPUs are a relatively recent phenomenon  I first saw C on 8-bit processors where it was a lot of work to do even 32-bit integer arithmetic  Many of these implementations didn t even have a floating point math library available   Even on the first 68000 machines  Mac  Atari ST  Amiga   floating point coprocessors were often expensive add-ons   To do all that floating point math  you needed a pretty sizable library  And the math was going to be slow  So you rarely used floats  You tried to do everything with integers or scaled integers  When you had to include math h  you gritted your teeth  Often  you d write your own approximations and lookup tables to avoid it   Trade-offs existed for a long time  Sometimes there were competing math packages called  fastmath  or such  What s the best solution for math  Really accurate but slow stuff  Inaccurate but fast  Big tables for trig functions  It wasn t until coprocessors were guaranteed to be in the computer that most implementations became obvious  I imagine that there s some programmer out there somewhere right now  working on an embedded chip  trying to decide whether to bring in the math library to handle some math problem   That s why math wasn t standard  Many or maybe most programs didn t use a single float  If FPUs had always been around and floats and doubles were always cheap to operate on  no doubt there would have been a  stdmath

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I think it s kind of arbitrary   You have to draw a line somewhere  which libraries are default and which need to be specified    It gives you the opportunity to replace it with a different one that has the same functions  but I don t think it s very common to do so   EDIT   from my own comments   I think gcc does this to maintain backwards compatibility with the original cc   My guess for why cc does this is because of build time -- cc was written for machines with far less power than we have now   A lot of programs have no floating-point math and they probably took every library that wasn t commonly used out of the default   I m guessing that the build time of the UNIX OS and the tools that go along with it were the driving force

[c] Why do you have to link the math library in C?

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