c - 为什么读取结构体指针字段无效？

Question

运行Valgrind中的程序，它表示结构的转换指针处存在“大小为 8 的无效读取”。和calloc有关系吗？如果按原样读取，则为 (nil)。

有一个结构体(称为 trie)，其用法如下:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>

const int MAX_SIZE = 20;

struct _trie {
    int maxNode;
    int nextNode;
    int** transition;
    char* fin;
};

typedef struct _trie * Trie;

Trie createTrie (int maxNode){

    Trie trie;

    trie = (Trie) malloc(sizeof(Trie));

    printf("size of trie: %lu, size of the struct: %lu, size of _trie: %lu\n",sizeof(trie),sizeof(Trie), sizeof(struct _trie));

    trie->maxNode = maxNode;

    printf("maxNode = %d, size of maxNode: %lu\n",trie->maxNode,sizeof(trie->maxNode));
    printf("size of nextNode : %lu, size of transition: %lu, size of fin: %lu\n",
           sizeof(trie->nextNode),sizeof(trie->transition),sizeof(trie->fin));

这里，当 valgrid 尝试读取时，它显示“无效读取大小 8”:

    //invalid read
    printf("transitions points to: %p, address: %p\n",trie->transition,&trie->transition);

来自 char * fin 的相同消息:

    //invalid read
    printf("fin points to: %p, address: %p\n",trie->fin,&trie->fin);

    getchar();

    trie->transition = (int**)calloc(maxNode,sizeof(int*));

    printf("trie->transition done.\n");
    printf("transitions points to: %p, address: %p\n",trie->transition,&trie->transition);

    if(trie->transition == NULL){
        printf("null for trie->transition\n");
        exit(0);
    }

    printf("Size of transition: %lu, size of int:%lu, pointer: %p\n\n",sizeof(trie->transition),sizeof(int),trie->transition);

    for(int counter = 0; counter < maxNode; ++counter){

        trie->transition[counter] = calloc(UCHAR_MAX,sizeof(int));

        if(trie->transition[counter] == NULL){
            printf("null for trie->transition[%d]\n",counter);
            exit(0);
        }

        //printf("size of transition[%d]: %lu\n",counter,sizeof(trie->transition[counter]));

    }


    printf("\nFilling up trie->transition\n");


    for(int counter = 0; counter < maxNode; ++counter){


        for(int counter2 = 0; counter2 < UCHAR_MAX; ++counter2){

            trie->transition[counter][counter2] = -1;

            //printf("size of transition[%d][%d]: %lu, value: %d\n",counter,counter2,sizeof(trie->transition[counter]),trie->transition[counter][counter2]);

        }

        //getchar();
    }

    return (trie);
}

void free_all(Trie trie){

    for(int counter = 0; counter < trie->maxNode; ++counter){

        free(trie->transition[counter]);

    }

    free(trie->transition);
    free(trie);
}

int main(int argc, char *argv[]){

    Trie trie = createTrie(MAX_SIZE);

    free_all(trie);
    return (0);
}

Valgrind 输出:

==3079== Memcheck, a memory error detector
==3079== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==3079== Using Valgrind-3.12.0 and LibVEX; rerun with -h for copyright info
==3079== Command: ./debug_test
==3079== 
size of trie: 8, size of the struct: 8, size of _trie: 24
maxNode = 20, size of maxNode: 4
size of nextNode : 4, size of transition: 8, size of fin: 8
==3079== Invalid read of size 8
==3079==    at 0x1088AD: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 
transitions points to: (nil), address: 0x5201048
==3079== Invalid read of size 8
==3079==    at 0x1088D1: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201050 is 8 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 
fin points to: (nil), address: 0x5201050

==3079== Invalid write of size 8
==3079==    at 0x108907: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 
trie->transition done.
==3079== Invalid read of size 8
==3079==    at 0x108923: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 
transitions points to: 0x5201910, address: 0x5201048
==3079== Invalid read of size 8
==3079==    at 0x10893F: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 
==3079== Invalid read of size 8
==3079==    at 0x108962: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 
Size of transition: 8, size of int:4, pointer: 0x5201910

==3079== Invalid read of size 8
==3079==    at 0x108991: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 
==3079== Invalid read of size 8
==3079==    at 0x1089B9: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 

Filling up trie->transition
==3079== Invalid read of size 8
==3079==    at 0x108A20: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 
==3079== Invalid read of size 8
==3079==    at 0x108A84: free_all (in /projects/trie/debug_test)
==3079==    by 0x108AF8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 
==3079== Invalid read of size 8
==3079==    at 0x108AB3: free_all (in /projects/trie/debug_test)
==3079==    by 0x108AF8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079== 
==3079== 
==3079== HEAP SUMMARY:
==3079==     in use at exit: 0 bytes in 0 blocks
==3079==   total heap usage: 24 allocs, 24 frees, 22,616 bytes allocated
==3079== 
==3079== All heap blocks were freed -- no leaks are possible
==3079== 
==3079== For counts of detected and suppressed errors, rerun with: -v
==3079== ERROR SUMMARY: 5167 errors from 11 contexts (suppressed: 0 from 0)

Answer 1

==3079== Invalid read of size 8
==3079==    at 0x1088AD: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)
==3079==  Address 0x5201048 is 0 bytes after a block of size 8 alloc'd
==3079==    at 0x4C2DB2F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3079==    by 0x108835: createTrie (in /projects/trie/debug_test)
==3079==    by 0x108AE8: main (in /projects/trie/debug_test)

这表示您的代码正在尝试从无效地址读取 8 字节值。

该地址位于通过 createTrie 中的 malloc 分配的 8 字节 block 之后。换句话说，这一行:

    trie = (Trie) malloc(sizeof(Trie));

为什么它认为trie只指向8字节内存？因为您分配了 sizeof (Trie) 字节，并且 Trie 为

typedef struct _trie * Trie;

或者换句话说，当您打算为整个结构分配足够的内存时，您为指针分配了内存。

正是出于这个原因，强烈建议不要将指针隐藏在 typedef 后面。

建议修复:

typedef struct Trie Trie;
struct Trie {
    int maxNode;
    int nextNode;
    int** transition;
    char* fin;
};

Trie *createTrie(int maxNode) {

    Trie *trie;

    trie = malloc(sizeof *trie);

注释:

• 我们对 struct Trie 和(裸)Trie 使用相同的名称 (Trie)，因为其他任何内容都会造成不必要的混淆。<
• 所有指针都明显声明为指针，带有 *。
• 我们不会强制转换 malloc 的返回值，因为这将是另一个潜在的错误来源。
• 我们使用 sizeof *trie 来获取 trie 指向的类型中的正确字节数，无论 trie 是如何声明的.