Question

如何释放void双指针？

deallocate(void** ptr)

这是我试图反对的情况：

char* allocated = (char*)allocate_array(sizeof(char), BUFSIZ,0);

deallocate_array((void**)&allocated);

我需要检查ptr是否为NULL，所以这是我的尝试：

if(ptr != NULL && *ptr != NULL){
      free(*ptr);
}
ptr = NULL;
return;

Answer 1

你的尝试非常好。你想要这样的东西：

void deallocate(void **ptr) {
    char *allocated = NULL;
    if (ptr) {
        allocated = *ptr;
    }
    if (allocated) {
        free(allocated);
        *ptr = NULL;
    }
}

你会这样称呼：

deallocate(&allocated);

传入指针地址的原因是，您可以在重新分配后将指针设置回null。

Answer 2

你可能会使这比需要更复杂。没有理由将的地址（例如&allocated）传递给deallocate函数（除非将原始指针的值设置为NULL而不使用回报）。你可以简单地传递一个指针。是的，该函数将接收该指针的副本，但它仍将包含要释放的内存块的起始地址。 e.g：

void deallocate (void *ptr) { if (!ptr) return; free (ptr); }

如果您的意图是解除分配并将原始指针指向的值设置为NULL函数中的deallocate，那么是的，您需要传递原始地址以便设置指向NULL的指针会反映在调用函数中（或者您可以创建函数void *并返回指向值设置为null的指针）。将参数作为deallocate指针传递给void，您可以执行以下操作：

void deallocate_doubleptr (void *ptr) { if (!ptr || !*(void **)ptr) return; free (*(void **)ptr); *(void **)ptr = NULL; }

（注意：，你没有传递void **参数，你只需传递一个void *（void）指针并根据需要在函数中进行转换。参数是仍然是void指针。）

一个简短的例子说明两者都是等价的，除了NULL的明确赋值，并猜测你的allocate_array会是什么：

#include <stdio.h> #include <stdlib.h> #include <string.h> void *allocate_array (size_t size, size_t nmemb, int set); void deallocate (void *ptr); void deallocate_doubleptr (void *ptr); int main (int argc, char **argv) { size_t size = argc > 1 ? (size_t)strtoul (argv[1], NULL, 10) : 1; size_t nmemb = argc > 2 ? (size_t)strtoul (argv[2], NULL, 10) : 128; int set = argc > 3 ? (int)strtol (argv[3], NULL, 10) : 0; char *str = "The quick brown fox jumps over a lazy dog."; char *allocated = NULL; if ((allocated = allocate_array (size, nmemb, set))) printf ("\nsuccessfully allocated '%zu' bytes initialized to '%d' ('%c').\n", size * nmemb, set, set); if (31 < set && set < 127) { /* if filled with printable ASCII */ allocated[strlen (str)] = 0; /* nul-terminate */ printf ("allocated : '%s'\n", allocated); /* output array */ } strncpy (allocated, str, size * nmemb - 1); /* copy str to array */ allocated[size * nmemb - 1] = 0; /* nul-terminate */ printf ("allocated : '%s'\n", allocated); /* output */ #ifdef DEALLOCDBL deallocate_doubleptr (&allocated); printf ("deallocated all memeory, pointer reinitialized to 'NULL'\n"); #else deallocate (allocated); printf ("deallocated all memeory.\n"); #endif return 0; } void *allocate_array (size_t size, size_t nmemb, int set) { if (!size || !nmemb) { fprintf (stderr, "error: invalid size or number of members.\n"); return NULL; } void *memptr = NULL; if (set != 0) { memptr = malloc (nmemb * size); memset (memptr, set, nmemb * size); } else memptr = calloc (nmemb, size); if (!memptr) { fprintf (stderr, "error: virtual memory exhausted.\n"); exit (EXIT_FAILURE); } return memptr; } void deallocate (void *ptr) { if (!ptr) return; free (ptr); ptr = NULL; } void deallocate_doubleptr (void *ptr) { if (!ptr || !*(void **)ptr) return; free (*(void **)ptr); *(void **)ptr = NULL; }

<强>编译

gcc -Wall -Wextra -o bin/allocate_deallocate allocate_deallocate.c

或

gcc -Wall -Wextra -DDEALLOCDBL -o bin/allocate_deallocate_dbl allocate_deallocate.c

（大多数编译器应采用相同或非常相似的选项）

<强>输出

无论调用的实现如何，输出都是相同的。（例如allocate_deallocate或allocate_deallocate_dbl）：

$ ./bin/allocate_deallocate 1 64 65 successfully allocated '64' bytes initialized to '65' ('A'). allocated : 'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA' allocated : 'The quick brown fox jumps over a lazy dog.' deallocated all memeory.

内存/错误检查

$ valgrind ./bin/allocate_deallocate 1 64 65

或

$ valgrind ./bin/allocate_deallocate_dbl 1 64 65 ==13759== Memcheck, a memory error detector ==13759== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al. ==13759== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info ==13759== Command: ./bin/allocate_deallocate ==13759== successfully allocated '64' bytes initialized to '65' ('A'). allocated : 'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA' allocated : 'The quick brown fox jumps over a lazy dog.' deallocated all memeory, pointer reinitialized to 'NULL' ==13759== ==13759== HEAP SUMMARY: ==13759== in use at exit: 0 bytes in 0 blocks ==13759== total heap usage: 1 allocs, 1 frees, 64 bytes allocated ==13759== ==13759== All heap blocks were freed -- no leaks are possible ==13759== ==13759== For counts of detected and suppressed errors, rerun with: -v ==13759== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)

仔细看看，如果您有任何问题，请告诉我。

取消分配void双指针

2 个答案: