malloc，重铸和免费 [英] malloc, recasting and free
问题描述
我有一些结构可以存储不同种类的列表:typedef int db_int;typedef char db_string [DB_STRING_LEN];
I have some structures to store different kinds of lists: typedef int db_int; typedef char db_string[DB_STRING_LEN];
struct List_db_int {
struct List_db_int *next;
struct List_db_int *prev;
db_int v;
};
struct List_db_string {
struct List_db_string *next;
struct List_db_string *prev;
db_string v;
};
struct List_db_void {
struct List_db_void *next;
struct List_db_void *prev;
};
我也有一个可以存储任何列表指针的联合体:
I also have an union which can store any of this list pointers:
union Uni_list {
struct List_db_int *db_type_int;
struct List_db_string *db_type_string;
struct List_db_void *db_type_void;
};
我想创建一个函数，该函数将从列表中删除某些元素，但是我希望它与列表类型无关，所以我提供了以下解决方案:
I want to create a function which will delete some elements from lists but I want it to be list type agnostic so I come with following solution:
/*data is an array of pointers to begin of lists*/
union Uni_list *data;
struct List_db_void *last;
for (i = 0; i < ELEMENTS_; i++) {
last = data[i].db_type_void->next;
for (j = 0; j < HOW_MANY_ELEMENTS_REMOVE; j++) {
free_list(last, t->cols[i]/*type of column*/);
last = last->next;
}
data[i].db_type_void->next = last;
}
这是free_list函数:无效
Here is the free_list function: void
free_list(struct List_db_void *elm, enum db_type type) {
switch(type) {
case db_type_int:
free((struct List_db_int*) elm);
break;
case db_type_string:
free((struct List_db_string*) elm);
break;
default:
/*Should not reach*/
return ;
break;
}
}
但是它不能正常工作，当我尝试读取修改后的列表时，我最终得到:
But it does not work correctly and when I try to read modified lists, I end up with:
*** Error in `./ppbase': free(): invalid pointer: 0x00007f84aa47f678 ***
======= Backtrace: =========
/usr/lib/libc.so.6(+0x72ecf)[0x7f84aa14cecf]
/usr/lib/libc.so.6(+0x7869e)[0x7f84aa15269e]
/usr/lib/libc.so.6(+0x79377)[0x7f84aa153377]
./ppbase[0x402001]
./ppbase[0x40217e]
./ppbase[0x40273d]
/usr/lib/libc.so.6(__libc_start_main+0xf5)[0x7f84aa0fbbc5]
./ppbase[0x4009d9]
======= Memory map: ========
00400000-00404000 r-xp 00000000 08:04 10883382 /home/hafron/dev/ppbase/ppbase
00603000-00604000 rw-p 00003000 08:04 10883382 /home/hafron/dev/ppbase/ppbase
01797000-017b8000 rw-p 00000000 00:00 0 [heap]
7f84a9ec4000-7f84a9ed9000 r-xp 00000000 08:03 49858 /usr/lib/libgcc_s.so.1
7f84a9ed9000-7f84aa0d9000 ---p 00015000 08:03 49858 /usr/lib/libgcc_s.so.1
7f84aa0d9000-7f84aa0da000 rw-p 00015000 08:03 49858 /usr/lib/libgcc_s.so.1
7f84aa0da000-7f84aa27c000 r-xp 00000000 08:03 9136 /usr/lib/libc-2.18.so
7f84aa27c000-7f84aa47b000 ---p 001a2000 08:03 9136 /usr/lib/libc-2.18.so
7f84aa47b000-7f84aa47f000 r--p 001a1000 08:03 9136 /usr/lib/libc-2.18.so
7f84aa47f000-7f84aa481000 rw-p 001a5000 08:03 9136 /usr/lib/libc-2.18.so
7f84aa481000-7f84aa485000 rw-p 00000000 00:00 0
7f84aa485000-7f84aa4a5000 r-xp 00000000 08:03 7209 /usr/lib/ld-2.18.so
7f84aa672000-7f84aa675000 rw-p 00000000 00:00 0
7f84aa6a1000-7f84aa6a4000 rw-p 00000000 00:00 0
7f84aa6a4000-7f84aa6a5000 r--p 0001f000 08:03 7209 /usr/lib/ld-2.18.so
7f84aa6a5000-7f84aa6a6000 rw-p 00020000 08:03 7209 /usr/lib/ld-2.18.so
7f84aa6a6000-7f84aa6a7000 rw-p 00000000 00:00 0
7fff249c8000-7fff249e9000 rw-p 00000000 00:00 0 [stack]
7fff249fe000-7fff24a00000 r-xp 00000000 00:00 0 [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
zsh: abort (core dumped) ./ppbase < tests/filter
如何正确使用此代码中的自由功能?我可以将任何列表投射到db_type_void-> next(第二个示例中的第5行)上吗?
How should I use free function in this code correctly? Can I safly cast any of the List into: db_type_void->next (line 5 in second example)?
推荐答案
此功能:
free_list(struct List_db_void *elm, enum db_type type) {
switch(type) {
case db_type_int:
free((struct List_db_int*) elm);
break;
case db_type_string:
free((struct List_db_string*) elm);
break;
default:
/*Should not reach*/
return ;
break;
}
}
与(与您的评论所建议的那样，我们从未经过 default 分支一样)完全相同
is exactly the same as (considering that we never go through the default branch, as your comment suggests)
free_list(struct List_db_void *elm, enum db_type type) {
free(elm);
}
指针类型转换适用于类型系统，并且具有 no 动态语义.可以说，它们在编译后消失了.
Pointer casts are for the type system and have no dynamic semantics. They disappear after compiling, so to speak.
话虽这么说，我正在看看是什么导致了段错误.
This being said I'm taking a look to see what causes the segfault.
编辑:可能是这样！
for (j = 0; j < HOW_MANY_ELEMENTS_REMOVE; j++) {
free_list(last, t->cols[i]/*type of column*/);
last = last->next;
}
释放后，您正在使用 last .做类似的事情
You're using last after freeing it. Do something like
for (j = 0; j < HOW_MANY_ELEMENTS_REMOVE; j++) {
struct List_db_void *temp = last->next;
free_list(last, t->cols[i]/*type of column*/);
last = temp;
}
这篇关于malloc，重铸和免费的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
