Question

Possible Duplicate:
Why does gdb evaluate sqrt(3) to 0?

C newbie here. There must be an obvious explanation why gdb gives strange outputs when trying to use math.h functions in-line. For example, the fabs function below is supposed to take the absolute value, and return a double.

(gdb) p cos(2*3.141/4)
$13 = 1073291460
(gdb) p fabs(-3)    
$14 = 0
(gdb) p fabs(3)
$15 = 0
(gdb) p fabs(3.333)
$16 = 1
(gdb) p (double) fabs(-3.234)
$17 = 1
(gdb) p (double) fabs((double)-3.234)
$18 = 1
(gdb) p ((double(*)(double))fabs)(-3)
$19 = 682945

The code I’m using has included math.h, and the actual code appears to execute correctly, although the same code placed in-line in gdb produces strange results. I could ignore it, but it seems a good learning opportunity.

Answer 1

(Ref: http://lists.gnu.org/archive/html/gdb/2009-12/msg00004.html)

gdb is missing the debug information of the cos function, and therefore assume it is an int cos(...) function, so the values are not returned correctly (esp. on x86 as the registers to store floating point return and integer return are different).

This could be worked around by specifying the type:

(gdb) p ((double(*)(double))cos) (1.0)
$18 = 0.54030230586813977