c - 如何在 C 中返回静态分配的二维数组？

Question

我在 spbox.c 中有以下代码:

#include <stdbool.h>
#include <stdint.h>

typedef struct {
    bool initialized;
    uint32_t Spbox[8][64]; // Combined S and P boxes
} spboxState;

static spboxState stateInstance;

uint32_t ** desGetSpbox(void) {
    if (!(stateInstance.initialized)) {
        // TODO: Compute data to populate Spbox array
        stateInstance.initialized = true;
    }
    return stateInstance.Spbox;
}

我编译它:

clang -c spbox.c

我收到关于不兼容指针返回类型的警告:

spbox.c:16:9: warning: incompatible pointer types returning 'uint32_t [8][64]' from a function with result type 'uint32_t **' (aka 'unsigned int **') [-Wincompatible-pointer-types]
        return stateInstance.Spbox;
               ^~~~~~~~~~~~~~~~~~~
1 warning generated.

如何更改我的代码以使警告消失？这是说 uint32_t ** 与 uint32_t [8][64] 不兼容。但是，如果我尝试使后者成为返回类型，则会出现语法错误。

Answer 1

您不能返回数组。但是您可以返回一个指向数组或其第一个元素的指针。

例如

uint32_t ( * desGetSpbox(void) )[8][64] {
    if (!(stateInstance.initialized)) {
        // TODO: Compute data to populate Spbox array
        stateInstance.initialized = true;
    }
    return &stateInstance.Spbox;
}

或者

uint32_t ( * desGetSpbox(void) )[64] {
    if (!(stateInstance.initialized)) {
        // TODO: Compute data to populate Spbox array
        stateInstance.initialized = true;
    }
    return stateInstance.Spbox;
}

这是一个演示程序

#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>
#include <inttypes.h>

enum { M = 8, N = 64 };

typedef struct {
    bool initialized;
    uint32_t Spbox[M][N]; // Combined S and P boxes
} spboxState;

static spboxState stateInstance;

uint32_t ( *desGetSpbox1( void ) )[M][N] 
{
    stateInstance.Spbox[0][0] = 10;
    return &stateInstance.Spbox;
}

uint32_t ( *desGetSpbox2( void ) )[N] 
{
    stateInstance.Spbox[0][0] = 10;
    return stateInstance.Spbox;
}


int main(void) 
{
    uint32_t ( *p1 )[M][N] = desGetSpbox1();
    
    printf( "%" PRIu32 "\n", ( *p1 )[0][0] );

    uint32_t ( *p2 )[N] = desGetSpbox2();
    
    printf( "%" PRIu32 "\n", ( *p2 )[0] );

    return 0;
}

程序输出为

10
10