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?

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.