Home/ Questions/Q 7729365
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-06-01T05:56:58+00:00

So, I know that in C you need to link the code to the

  • 0

So, I know that in C you need to link the code to the math library, libm, to be able to use its functions. Today, while I was trying to demonstrate this to a friend, and explain why you need to do this, I came across the following situation that I do not understand.

Consider the following code:

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

/* #define VARIABLE */

int main(void)
{
#ifdef VARIABLE
    double a = 2.0;
    double b = sqrt(a);
    printf("b = %lf\n",b);
#else
    double b = sqrt(2.0);
    printf("b = %lf\n",b);
#endif
    return 0;
}

If VARIABLE is defined, you need to link against libm as you would normally expect; otherwise you get the usual main.c:(.text+0x29): undefined reference to sqrt linking error indicating that the compiler cannot find the definition for the function sqrt. I was surprised to see that if I comment #define VARIABLE, the code runs fine and the result is correct!

Why is it that I need to link to libm when variables are used but I don’t need to do so when literal constants are used? How does the compiler find the definition of sqrt when the library is not linked? I’m using gcc 4.4.5 under linux.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    As everyone mentions, yes it has to do with constant folding.

    With optimizations off, GCC only seems to do it when sqrt(2.0) is used. Here’s the evidence:

    Case 1: With the variable.

        .file   "main.c"
    .section    .rodata
.LC1:
    .string "b = %lf\n"
    .text
.globl main
    .type   main, @function
main:
    pushl   %ebp
    movl    %esp, %ebp
    andl    $-16, %esp
    subl    $32, %esp
    fldl    .LC0
    fstpl   24(%esp)
    fldl    24(%esp)
    fsqrt
    fucom   %st(0)
    fnstsw  %ax
    sahf
    jp  .L5
    je  .L2
    fstp    %st(0)
    jmp .L4
.L5:
    fstp    %st(0)
.L4:
    fldl    24(%esp)
    fstpl   (%esp)
    call    sqrt
.L2:
    fstpl   16(%esp)
    movl    $.LC1, %eax
    fldl    16(%esp)
    fstpl   4(%esp)
    movl    %eax, (%esp)
    call    printf
    movl    $0, %eax
    leave
    ret
    .size   main, .-main
    .section    .rodata
    .align 8
.LC0:
    .long   0
    .long   1073741824
    .ident  "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
    .section    .note.GNU-stack,"",@progbits

    You can see that it emits a call to the sqrt function. So you’ll get a linker error if you don’t link the math library.

    Case 2: With the Literal.

        .file   "main.c"
    .section    .rodata
.LC1:
    .string "b = %lf\n"
    .text
.globl main
    .type   main, @function
main:
    pushl   %ebp
    movl    %esp, %ebp
    andl    $-16, %esp
    subl    $32, %esp
    fldl    .LC0
    fstpl   24(%esp)
    movl    $.LC1, %eax
    fldl    24(%esp)
    fstpl   4(%esp)
    movl    %eax, (%esp)
    call    printf
    movl    $0, %eax
    leave
    ret
    .size   main, .-main
    .section    .rodata
    .align 8
.LC0:
    .long   1719614413
    .long   1073127582
    .ident  "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
    .section    .note.GNU-stack,"",@progbits

    There’s no call to sqrt. Hence no linker error.

    With optimizations on, GCC will do constant propagation in both cases. So no linker error in either case.

    $ gcc main.c -save-temps
main.o: In function `main':
main.c:(.text+0x30): undefined reference to `sqrt'
collect2: ld returned 1 exit status
$ gcc main.c -save-temps -O2
$
    • 0