C flexible array members

C flexible array members (FAM) allow a struct to have a variable-length array as its last member with size specified at runtime. Declare as type array[] with no size; allocate the struct plus extra space for array data.

Use FAM for variable-sized data structures without separate pointer indirection.

// compile: gcc -o fam fam.c
// run: ./fam
// description: flexible array members for variable-size structs
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
struct Vector {
    int size;
    int capacity;
    int data[];  // Flexible array member
};
 
struct Vector *create_vector(int capacity) {
    struct Vector *v = malloc(sizeof(struct Vector) + capacity * sizeof(int));
    v->size = 0;
    v->capacity = capacity;
    return v;
}
 
void push(struct Vector *v, int value) {
    if (v->size < v->capacity) {
        v->data[v->size++] = value;
    }
}
 
int main() {
    struct Vector *v = create_vector(10);
    push(v, 1);
    push(v, 2);
    push(v, 3);
 
    printf("Vector size: %d\n", v->size);
    printf("Data: %d %d %d\n", v->data[0], v->data[1], v->data[2]);
 
    free(v);
    return 0;
}

Declaration:

• type array[] as last member, no size specified
• Only valid as last member (C99)

Allocation:

• malloc(sizeof(struct) + n * sizeof(type))
• Extra space allocated for array elements
• All in one contiguous allocation

Advantages over separate pointer:

• Single allocation, better cache locality
• No extra pointer dereferencing
• Cleaner semantics for fixed-size arrays

Limitations:

• Must be last member
• Can't be nested in other structs
• Requires careful size calculation