Undefined reference to sin

Question

I have the following code  stripped down to the bare basics for this question     include lt stdio h gt   include lt math h gt   double f1 double x        double res   sin x       return 0        The main function    int main void        return 0      When compiling it with gcc test c I get the following error  and I can t work out why    tmp ccOF5bis o  In function  f1   test2 c   text 0x13   undefined reference to  sin  collect2  ld returned 1 exit status   However  I ve written various test programs that call sin from within the main function  and those work perfectly  I must be doing something obviously wrong here - but what is it

User · Answer

You need to link with the math library  libm     gcc -Wall foo c -o foo -lm

User · Answer

I had the same problem  which went away after I listed my library last  gcc prog c -lm

User · Answer

I have the problem anyway with -lm added  gcc -Wall -lm mtest c -o mtest o mtest c  In function  f1   mtest c 6 12  warning  unused variable  res   -Wunused-variable   tmp cc925Nmf o  In function  f1   mtest c   text 0x19   undefined reference to  sin  collect2  ld returned 1 exit status   I discovered recently that it does not work if you first specify -lm  The order matters   gcc mtest c -o mtest o -lm   Just link without problems  So you must specify the libraries after

User · Answer

You have compiled your code with references to the correct math h header file  but when you attempted to link it  you forgot the option to include the math library   As a result  you can compile your  o object files  but not build your executable   As Paul has already mentioned add  -lm  to link with the math library in the step where you are attempting to generate your executable   In the comment  linuxD asks      Why for sin   in  lt math h gt   do we need -lm option explicitly  but    not for printf   in  lt stdio h gt     Because both these functions are implemented as part of the  Single UNIX Specification    This history of this standard is interesting  and is known by many names  IEEE Std 1003 1  X Open Portability Guide  POSIX  Spec 1170      This standard  specifically separates out the  Standard C library  routines from the  Standard C Mathematical Library  routines  page 277    The pertinent passage is copied below      Standard C Library       The Standard C library is automatically searched by   cc to resolve external references  This library supports all of the   interfaces of the Base System  as defined in Volume 1  except for the   Math Routines        Standard C Mathematical Library       This library supports   the Base System math routines  as defined in Volume 1  The cc option   -lm is used to search this library    The reasoning behind this separation was influenced by a number of factors    The UNIX wars led to increasing divergence from the original AT amp T UNIX offering  The number of UNIX platforms added difficulty in developing software for the operating system  An attempt to define the lowest common denominator for software developers was launched  called 1988 POSIX  Software developers programmed against the POSIX standard to provide their software on  POSIX compliant systems  in order to reach more platforms  UNIX customers demanded  POSIX compliant  UNIX systems to run the software    The pressures that fed into the decision to put -lm in a different library probably included  but are not limited to    It seems like a good way to keep the size of libc down  as many applications don t use functions embedded in the math library  It provides flexibility in math library implementation  where some math libraries rely on larger embedded lookup tables while others may rely on smaller lookup tables  computing solutions   For truly size constrained applications  it permits reimplementations of the math library in a non-standard way  like pulling out just sin   and putting it in a custom built library    In any case  it is now part of the standard to not be automatically included as part of the C language  and that s why you must add -lm

[c] Undefined reference to `sin`

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