c - 为结构内的结构分配内存

Question

typedef struct{
    char id[15];
    int count;
}hashtag;

typedef struct node{
    hashtag *hashtag;
    struct node*next;
}*link;

我正在编写一个程序来从句子中读取主题标签，并且我想将它们存储在列表中。我已经定义了这两个结构，我可以读取主题标签并将其传递给下面的函数，但我需要分配内存的帮助才能将字符串复制到列表。

void check_insert(char newh[]){
    link x;
    //malloc to allocate memory for the string I want to copy

    strcpy(x->hashtag->id, newh);
    x->hashtag->count += 1;
    head = insert_end(head, x->hashtag); //head is a global variable that points to the 1st element of the list
}

Answer 1

您应该在 check_insert 中分配并初始化指针 x，在不先分配的情况下取消引用它并访问其成员是未定义的行为:

void check_insert(char newh[]){
    link x = malloc(sizeof *x);
    x->hashtag = malloc(sizeof *x->hashtag);

    // strcpy(x->hashtag->id, newh); <-- UB if newh is larger than 14 in size
    x->hashtag->id[0] = '\0';
    strncat(x->hashtag->id, newh, sizeof(x->hashtag->id));
    x->hashtag->count = 1;
    head = insert_end(head, x->hashtag);
}