Why does a program using sqrt compile with gcc without specifying a math library

Question

given the following program:

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

int
main(void)
{
    double x = sqrt(2);
    printf("The square root of two is %f\n", x);
    return 0;
}

and compiling with:

gcc calc.c -o calc

succeeds? why doesn't it require -lm or /usr/lib/blah/libm.so.x

inspecting the binary object with ldd produces:

linux-vdso.so.1 (0x00007fff4f5e5000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007feeffd1b000)
/lib64/ld-linux-x86-64.so.2 (0x00007fef000e1000

No libm is referenced. However, if I look at the libc.so.6 library or the ld-linux-x86-64.so.2 library using nm -D, no sqrt function is in those libraries.

What's going on here? Is gcc magically including a default set of common functions or something else?

Answer 1

No, the gcc compiler knows that sqrt(2) is a constant value and just calculates the value at compile time.

To trigger a use of the sqrt() library function, use code like this:

volatile double y = 2;
double x = sqrt(y);

One could also use the -ffreestanding gcc option, but this is not recommeneded.