“ free：在tcache 2中检测到双重释放”是什么意思？

如何解决“ free：在tcache 2中检测到双重释放”是什么意思？

我正在编写一个用C进行多项式加法的程序。我在使用free()时遇到了一些问题。我想出了一个解决方案，但我不知道后台发生了什么。

我正在上传完整的文件（而不是代码段）。这是GitHub repository。我提供了正确的版本为polynomial.c和错误的版本为polynomial_uncorrected.c。问题出在polynomial_uncorrected.c中的第41-42行。

我运行程序。 如果p1和p2都不是0多项式，则程序正在正确运行。如果p1或p2中的任何一个是零多项式（所有元素都输入为0），则我遇到了以下问题。

free(): double free detected in tcache 2
Aborted (core dumped)

Valgrind显示以下输出

==11720== Memcheck,a memory error detector
==11720== Copyright (C) 2002-2017,and GNU GPL'd,by Julian Seward et al.
==11720== Using Valgrind-3.16.1 and LibVEX; rerun with -h for copyright info
==11720== Command: ./p_c.out
==11720== 
FIRST POLYNOMIAL :
Enter degree of polynomial : 2
Enter coefficient of x^2 : 1
Enter coefficient of x^1 : 2
Enter coefficient of x^0 : 3

SECOND POLYNOMIAL :
Enter degree of polynomial : 0
Enter coefficient of x^0 : 0

FIRST POLYNOMIAL :
1x^2 + 2x + 3

SECOND POLYNOMIAL :
0

SUM :
1x^2 + 2x + 3
==11720== Invalid free() / delete / delete[] / realloc()
==11720==    at 0x483A9AB: free (vg_replace_malloc.c:538)
==11720==    by 0x1092F2: main (in /<path>/p_c.out)
==11720==  Address 0x4a4b960 is 0 bytes inside a block of size 24 free'd
==11720==    at 0x483A9AB: free (vg_replace_malloc.c:538)
==11720==    by 0x1092CC: main (in /<path>/p_c.out)
==11720==  Block was alloc'd at
==11720==    at 0x483BD7B: realloc (vg_replace_malloc.c:834)
==11720==    by 0x1093D8: inp_pol (in /<path>/p_c.out)
==11720==    by 0x10920B: main (in /<path>/p_c.out)
==11720== 
==11720== 
==11720== HEAP SUMMARY:
==11720==     in use at exit: 0 bytes in 0 blocks
==11720==   total heap usage: 5 allocs,6 frees,2,096 bytes allocated
==11720== 
==11720== All heap blocks were freed -- no leaks are possible
==11720== 
==11720== For lists of detected and suppressed errors,rerun with: -s
==11720== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

我以为p3没有分配，所以我重新分配了

p3 = realloc (p3,(*size3)*sizeof(element));

这些行仍然在代码（polynomial_uncorrected.c）中作为注释出现。但是我遇到了同样的错误。

然后我意识到只有free(p3)和p1都不为NULL（即非零多项式）时，才应该p2。它奏效了，但我不知道为什么。

为什么要说在tcache 2中检测到双重免费？