将指针变量作为指向C中其他类型的指针进行访问 [英] Accessing pointer variable as a pointer to a different type in C

问题描述

通过取消指向指向不同类型或void的指针的指针来访问指针变量是一种好习惯吗?这可以打破严格的别名规则吗? C和C ++在别名规则上有一些区别.在此问题中，我们重点关注C.可以在

Is it good practice to access a pointer variable by dereferencing a pointer to a pointer, which points to a different type or void? Could this break strict aliasing rules? C and C++ have some differences in aliasing rules. In this question we focus on C. The other question considering C++ can be found here. In the following example a double* is accessed as a void*.

int create_buffer(void** ptr, ...)
{
    *ptr = malloc(...);
    ...
}

int main(void)
{
    double* buffer;

    // The problematic code is here, double**
    // is coerced to void**, which is later
    // dereferenced by the function
    create_buffer((void**)&buffer, ...);
    ...
}

以下各项是否更好:

// keeping void** just as an indicator in the interface
// that the pointer is pointing to a pointer to any type
// it could be replaced by just void*
int create_buffer(void** ptr, ...)
{
    void* result = malloc(...);
    memcpy((void*)ptr, &result, sizeof result);
}

推荐答案

我宁愿这样写，假设您使用"int"向调用者返回一些有用的信息，而这些信息离不开:

I would rather write it like this, assuming you use the "int" to return some useful information to the caller that it can't do without:

void *create_buffer(size_t n, int *otherInfo) {
    void *ptr = malloc(n);
    *otherInfo = 0;
    if (ptr) {
        // Do whatever other initialization is expected
        // memset(ptr, 0, n);
    } else {
        *otherInfo = -1;
    }
    return ptr;
}

int main(void)
{
    double* buffer;
    int code;

    /* cast is not required by language or compiler, 
       but I still find it useful when reading */
    buffer = (double *)create_buffer(SIZE_REQUIRED, &code);

    /* equality testing can be done either way. This way here,
       if you ever forget a =, will never compile anywhere. Nowadays
       many compilers are smart enough to catch it on their own anyway */
    if (NULL == buffer) {
        // Handle out of memory error or other problem
        switch(code) {
            case -1:
                fprintf(stderr, "Out of memory\n");
            ...
            default:
                fprintf(stderr, "Unexpected error %d\n", code);            
                break;
        }
    }
}

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