Why does gdb evaluate sqrt(3) to 0?

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The problem is not the missing function declaration (which isn’t missing, since you did include <math.h>).

The problem is missing debug info for the sqrt you are actually using. Without that debug info, GDB has no clue what parameter type to pass to sqrt(), and what it returns.

You can get the required debug info on many Linux distributions by installing libc-debuginfo package. Here is what I see on such a system:

gdb -q ./a.out
Reading symbols from /tmp/a.out...done.
(gdb) b main
Breakpoint 1 at 0x400558: file t.c, line 6.
(gdb) r

Breakpoint 1, main () at t.c:6
6     printf("sqrt(3): %f\n", sqrt(3));
(gdb) p sqrt
$1 = {<text variable, no debug info>} 0x7ffff7b7fb50 <__sqrt>

Note: “no debug info”

(gdb) p sqrt(3)
$2 = 0
(gdb) p sqrt(3.0)
$3 = 0

Note: matches your behavior.
What sqrt functions do have debug info?

(gdb) info func sqrt
All functions matching regular expression "sqrt":

File ../sysdeps/x86_64/fpu/e_sqrt.c:
double __ieee754_sqrt(double);

File s_csqrt.c:
complex double __csqrt(complex double);

File ../sysdeps/x86_64/fpu/e_sqrtf.c:
float __ieee754_sqrtf(float);

File w_sqrtf.c:
float __sqrtf(float);

File s_csqrtf.c:
complex float __csqrtf(complex float);

File ../sysdeps/i386/fpu/e_sqrtl.c:
long double __ieee754_sqrtl(long double);

File w_sqrtl.c:
long double __sqrtl(long double);

File s_csqrtl.c:
complex long double __csqrtl(complex long double);

File ../sysdeps/ieee754/dbl-64/mpsqrt.c:
void __mpsqrt(mp_no *, mp_no *, int);

File w_sqrt.c:
double __sqrt(double);

(gdb) p __sqrt
$4 = {double (double)} 0x7ffff7b7fb50 <__sqrt>

Note: __sqrt is at the same address as sqrt, but GDB knows its type!

(gdb) p __sqrt(3)
$5 = 1.7320508075688772
(gdb) p __sqrt(3.0)
$6 = 1.7320508075688772

One can reasonably argue this is a bug in GDB. Feel free to create one in GDB bugzilla.

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