结构的静态数组字段的动态内存重新分配

Question

请考虑以下结构：

// A simple structure to hold some information 
struct A {
    unsigned long id; 
    char title[100]; 
};

// A simple database for storing records of B
struct B {
    unsigned int tableSize; // Total available space in the 'table' field (initially, set to 5)
    unsigned int entriesUsed; // Number of used entries of the 'table' field
    struct A table[5]; 
};

假设以下代码中的realloc函数（下面的第5行）正确地定义了table字段的大小，尽管它被定义为静态数组，这是否正确？

void add(struct B* db, struct A* newEntry)
{
    if (db->entriesUsed >= db->tableSize) {
        // Does the following line increase the size of 'table' correctly?
        db = (struct B*)realloc(db, sizeof(db) + 5*sizeof(struct A));
        db->rowsTotal += 5;
    }

    db->table[db->entriesUsed].id = newEntry->id;
    memcpy(db->table[db->entriesUsed].title, table->title, sizeof(newEntry->title));

    db->entriesUsed++;
}

Answer 1

在此示例中，您正在将内存分配给传递给add的struct B指针。这对该结构包含的数组大小没有任何作用。

您尝试执行的操作的实现可能看起来像这样：

// A simple structure to hold some information 
struct A {
    unsigned long id; 
    char title[100]; 
};

// A simple database for storing records of B
struct B {
    unsigned int tableSize; // Total available space in the 'table' field (initially, set to 5)
    unsigned int entriesUsed; // Number of used entries of the 'table' field
    struct A *table; 
};

void add(struct B* db, struct A* newEntry)
{
    if (db->entriesUsed >= db->tableSize) {
        // Add 5 more entries to the table
        db->tableSize += 5
        struct A *temp = malloc(sizeof(struct A) * db->tableSize)
        memcpy(temp, db->table, sizeof(struct A) * db->tableSize)
        free(db->table)
        db->table = temp
    }

    memcpy(db->table[db->entriesUsed], newEntry, sizeof(struct A));

    db->entriesUsed++;
}