将元素添加到灵活数组成员

Question

I've read and looked at some example of flexible array members but I am not exactly sure how to add and read elements of this variable length array.

typedef struct School {
    char *name;
    char *courses[]; //Flexible Array Member
} School;

1) Can someone please show me an example of how I can add an element to this Flexible Length Member and print it after it is stored.

2) I also would like to know how to malloc it correctly. Based on what I have read about Flexible Array Members, we would need to add more space for the flexible array member and can't just use sizeof(School); . My only issue is how do I know how much do add for that flexible member.

Answer 1

You should modify the struct to add the number of courses present in the allocated structure:

typedef struct School {
    char *name;
    int ncourses;
    char *courses[]; //Flexible Array Member
} School;

Say you have 2 schools, one with 3 courses, one with 2. You would allocate the structures this way:

School *mc = malloc(offsetof(struct School, courses) + 3 * sizeof(char *));
mc->name = strdup("Math College");
mc->ncourses = 3;
mc->courses[0] = strdup("Math 101");
mc->courses[1] = strdup("Math 102");
mc->courses[2] = strdup("Math 103");

School *ps = malloc(offsetof(struct School, courses) + 2 * sizeof(char *));
ps->name = strdup("Psycho School");
ps->ncourses = 2;
ps->courses[0] = strdup("Psycho 101");
ps->courses[1] = strdup("Unknown 404");

As you can see, elements of the variable array are accessed like any other array elements. The malloc call allocates the appropriate size in bytes for the struct members and the array elements (here char * pointers), that are located at the end of the structure.

You could use a generic function to allocate and initialize such structures:

School create_school(const char *school_name, int ncourses, char *courses[]) {
    School *sp = malloc(offsetof(struct School, courses) + ncourses * sizeof(char *));
    sp->name = strdup(school_name);
    sp->ncourses = ncourses;
    for (int i = 0; i < ncourses; i++) {
        sp->courses[i] = strdup(courses[i]);
    }
    return sp;
}

Answer 2

The exact formula for computing the required size of your structure is:

size_t need = offsetof(struct School, courses) + num_courses * sizeof(char *);

Note the use of offsetof . Some people use sizeof , but this can introduce memory overhead due to structure padding.

Answer 3

Essentially the technique is to dynamically allocate enough memory for the struct, plus the elements of the last array.

School *data = malloc(sizeof(*data) + number * sizeof(*(data->courses)));

for (i = 0; i < number; ++i)
{
     const char hello[] = "Hello";
     data->courses[i] = malloc(strlen(hello) + 1));   /* sizeof char is 1 by definition */
     strcpy(courses[i], hello);
}