为什么gdb tdump会将某些值显示为<unavailable>？

Question

我正在尝试调试单元测试;在5943号呼叫memcpy_c时，它应该是恐慌，但不是。为了调试这个，我使用gdb的tracepoint机制来记录来自所有这些调用的数据。

这是我的调试会话：

设定目标＆amp;跟踪

(gdb) target remote :53268
Remote debugging using :53268
0x00000000004068de in _start ()
(gdb) trace memcpy_c
Tracepoint 1 at 0x402718: file memory.c, line 49.
(gdb) actions
Enter actions for tracepoint 1, one per line.
End with a line saying just "end".
>collect dst, bufsize, src, count
>end
(gdb) break lsc_exit
Breakpoint 2 at 0x406a00

运行程序

(gdb) tstart
(gdb) continue
Continuing.

Breakpoint 2, 0x0000000000406a00 in lsc_exit ()
(gdb) tstop

分析追踪

我可以获取快照5900的数据：

(gdb) tfind 5900
Found trace frame 5900, tracepoint 1
#0  memcpy_c (dst=0x7fffffffe539, dst@entry=<error reading variable: PC not available>, bufsize=785, bufsize@entry=<error reading variable: PC not available>, src=0x7fffffffe4c8, 
    src@entry=<error reading variable: PC not available>, count=5, count@entry=<error reading variable: PC not available>) at memory.c:49
49      if (dst == NULL) {
(gdb) tdump
Data collected at tracepoint 1, trace frame 5900:
dst = (void * restrict) 0x7fffffffe539
bufsize = 785
src = (const void * restrict) 0x7fffffffe4c8
count = 5

我无法获取快照5943的数据（我正在尝试调试的最后一个快照）：

(gdb) tfind 5943
Found trace frame 5943, tracepoint 1
#0  memcpy_c (dst=<unavailable>, bufsize=<unavailable>, src=<unavailable>, count=<unavailable>) at memory.c:49
49      if (dst == NULL) {
(gdb) tdump
Data collected at tracepoint 1, trace frame 5943:
dst = <unavailable>
bufsize = <unavailable>
src = <unavailable>
count = <unavailable>

可能导致参数显示为<unavailable>的原因是什么？

供参考，这里是memcpy_c：

void *memcpy_c(void *restrict dst, ulen bufsize, const void *restrict src, ulen count) {
    if (dst == NULL) {
            panic_static("memcpy_c destination pointer is NULL");
    }
    if (src == NULL) {
            panic_static("memcpy_c source pointer is NULL");
    }
    if (count > bufsize) {
            panic_static("memcpy_c caught overflow");
    }
    if (dst + count >= src && dst < src + count) {
            panic_static("memcpy_c attempted to use overlapping regions");
    }
    return memcpy(dst, src, count);
}