sqrt() function not working with variable arguments

Question

I don't know if I'm missing something obvious, but it appears that I'm unable to compute square roots of a variable in C; the sqrt() function only seems to work on constants. This is my code:

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

int main()
{
    double a = 2.0;
    double b = sqrt(a);
    printf("%f", b);
    return 0;
}

When I run this program, I get the following error:

gcc -Wall -o "test2" "test2.c" (in directory: /home/eddy/Code/euler)
/tmp/ccVfxkNh.o: In function `main':
test2.c:(.text+0x30): undefined reference to `sqrt'
collect2: ld returned 1 exit status
Compilation failed.

However, if I replace the argument in sqrt() with a constant such as 2.0 for example, (b = sqrt(2.0)), then it works fine. Is sqrt() not supposed to work with variables or something?

Thanks for the help

Answer 1

You need to link with the math library (use a '-lm' on the command line). In the constant case, the compiler is probably being smart and precomputing sqrt(2.0) (so the code that is compiled is essentially 'b = 1.414...;')

Answer 2

You probably need to add -lm when you compile. When you take the square root of a constant, the compiler is optimizing the code by taking the square root while it compiles, so it doesn't use sqrt at all.

Answer 3

Use the command gcc -Wall -o "test2" "test2.c" -lm which will likely fix this.

This includes the math library in addition to the standard C runtime library. On most systems, the math library is historically a separate entity that needs to be explicitly requested.

Answer 4

Compile with:

gcc -Wall -o test2 test2.c -lm

You need to link against the math library.