Why am I getting undefined reference to sqrt error even though I include math h header

Question

I m very new to C and I have this code     include  lt stdio h gt   include  lt math h gt  int main void      double x   0 5    double result   sqrt x     printf  The square root of  lf is  lf n   x  result     return 0      But when I compile this with   gcc test c -o test   I get an error like this    tmp cc58XvyX o  In function  main   test c   text 0x2f   undefined reference to  sqrt  collect2  ld returned 1 exit status   Why does this happen  Is sqrt   not in the math h header file  I get the same error with cosh and other trigonometric functions  Why

User · Answer

Because you didn t tell the linker about location of math library  Compile with gcc test c -o test -lm

User · Answer

The math library must be linked in when building the executable   How to do this varies by environment  but in Linux Unix  just add -lm to the command   gcc test c -o test -lm   The math library is named libm so  and the -l command option assumes a lib prefix and  a or  so suffix

User · Answer

You need to link the with the -lm linker option  You need to compile as  gcc test c  -o test -lm   gcc  Not g    historically would not by default include the mathematical functions while linking  It has also been separated from libc onto a separate library libm  To link with these functions you have to advise the linker to include the library -l linker option followed by the library name m thus -lm

User · Answer

Add header    include lt math h gt   Note  use abs    sometimes at the time of evaluation sqrt   can take negative values which leave to domain error   abs  - provides absolute values    example  abs -3   3  Include -lm at the end of your command during compilation time   gcc  lt filename extension gt  -lm

User · Answer

This is a likely a linker error  Add the -lm switch to specify that you want to link against the standard C math library  libm  which has the definition for those functions  the header just has the declaration for them - worth looking up the difference

[c] Why am I getting "undefined reference to sqrt" error even though I include math.h header?

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