使用 math.h 函数时 gdb 给出奇怪的输出 [英] gdb gives strange output when using math.h functions

问题描述

可能重复:
为什么 gdb 将 sqrt(3) 评估为 0?

这里是C新手.必须有一个明显的解释，为什么 gdb 在尝试内联使用 math.h 函数时会给出奇怪的输出.例如，下面的 fabs 函数应该采用绝对值，并返回一个双精度值.

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

我使用的代码包含了 math.h，实际代码似乎可以正确执行，尽管在 gdb 中嵌入的相同代码会产生奇怪的结果.我可以忽略它，但这似乎是一个很好的学习机会.

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.

推荐答案

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

gdb 缺少 cos 函数的调试信息，因此假定它是 int cos(...) 函数，所以没有正确返回值(尤其是在 x86 上，因为存储浮点返回和整数返回的寄存器不同).

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).

这可以通过指定类型来解决:

This could be worked around by specifying the type:

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

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