重新分配结构体数组

发布于 2024-11-10 12:19:12 字数 962 浏览 2 评论 0原文

我正在尝试为从文件读取或向文件读取或由用户输入的结构数组（实际上是 2 个结构体中的每一个，但为了简单起见，此处包含 1 个结构体）动态重新分配内存。

typedef Struct
{
    char surname[21];
    char firstname[21];
    char username[21];
...
} User;

...在 main() 中：

int size = 0; /* stores no. of structs */
User* user_array = (User *) calloc(1, sizeof(User));
if(user_array == NULL)
{
    printf("Cannot allocate initial memory for data\n");
    exit(1);
}
else
    size++;

然后，我尝试在需要时使用函数调用来增加数组：

int growArray(User user_array*, int size)
{
    User *temp;
    size++;
    temp = (User *) realloc(user_array, (size * sizeof(User));
    if(temp == NULL)
    {
        printf("Cannot allocate more memory.\n");
        exit(1);
    }
    else
        user_array = temp;
    return size;
}

不幸的是，realloc 永远不会工作。这两个结构每个实例只有大约 200 字节，将初始大小设置为 10 就可以正常工作，因此我尝试使用 realloc 的方式一定有问题。

系统是 Win 7 64，Core i5，4GB，运行 Quincy（MinGW GUI）。

原文

I am trying to dynamically reallocate memory for an array of structs (actually an array each of 2 structs but 1 included here for simplicity) that is being read from/to a file or inputted by the user.

typedef Struct
{
    char surname[21];
    char firstname[21];
    char username[21];
...
} User;

...in main():

int size = 0; /* stores no. of structs */
User* user_array = (User *) calloc(1, sizeof(User));
if(user_array == NULL)
{
    printf("Cannot allocate initial memory for data\n");
    exit(1);
}
else
    size++;

I am then trying to use a function call to increase the array when needed:

int growArray(User user_array*, int size)
{
    User *temp;
    size++;
    temp = (User *) realloc(user_array, (size * sizeof(User));
    if(temp == NULL)
    {
        printf("Cannot allocate more memory.\n");
        exit(1);
    }
    else
        user_array = temp;
    return size;
}

Unfortunately the realloc never works. Both structs are only about 200 bytes per instance and setting the initial size to say 10 will work fine, so there must be something wrong with the way I am trying to use realloc.

System is Win 7 64, on Core i5 with 4GB, running Quincy (a MinGW GUI).

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爱你不解释 2024-11-17 12:19:12

realloc 将 user_array 指向的内存大小更改为指定大小，但不会增加大小。鉴于您的函数名为 growArray，我假设您希望它将数组的大小增加 size，在这种情况下您需要：

int growArray(User **user_array, int currentSize, int numNewElems)
{
    const int totalSize = currentSize + numNewElems;
    User *temp = (User*)realloc(*user_array, (totalSize * sizeof(User)));

    if (temp == NULL)
    {
        printf("Cannot allocate more memory.\n");
        return 0;
    }
    else
    {
        *user_array = temp;
    }

    return totalSize;
}

请注意 growArray 获取 user_array 的地址，原因是 realloc 如果无法将现有块扩展到所需的大小，可能会移动内存。

使用方法：

int size = 0;
User* user_array = (User *) calloc(1, sizeof(User));
if(user_array == NULL)
{
    printf("Cannot allocate initial memory for data\n");
    exit(1);
}

/* add 10 new elements to the array */
size = growArray(&user_array, size, 10);

realloc changes the size of the memory pointed to by user_array to the specified size, it doesn't increase it by size. Seeing as your function is called growArray, i'd presume you want it to increase the size of the array by size, in which case you need to:

int growArray(User **user_array, int currentSize, int numNewElems)
{
    const int totalSize = currentSize + numNewElems;
    User *temp = (User*)realloc(*user_array, (totalSize * sizeof(User)));

    if (temp == NULL)
    {
        printf("Cannot allocate more memory.\n");
        return 0;
    }
    else
    {
        *user_array = temp;
    }

    return totalSize;
}

Note that growArray takes the address of user_array, the reason for this is that realloc might move the memory if it couldn't extend the existing block to the required size.

To use it:

int size = 0;
User* user_array = (User *) calloc(1, sizeof(User));
if(user_array == NULL)
{
    printf("Cannot allocate initial memory for data\n");
    exit(1);
}

/* add 10 new elements to the array */
size = growArray(&user_array, size, 10);

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