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

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Solution 1

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.

Solution 2

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.

Solution 3

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.)

Solution 4

Add header:

#include<math.h>

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 <filename.extension> -lm

Solution 5

Because you didn't tell the linker about location of math library. Compile with gcc test.c -o test -lm

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Ant's
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Updated on February 07, 2020

Comments

  • Ant's
    Ant's over 4 years

    I'm very new to C and I have this code:

    #include <stdio.h>
    #include <math.h>
    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?