Structs

Quick Reference

Declaring and using:

typedef struct {
    char name[50];
    int id;
    double gpa;
} Student;

Student s = {"Alice", 1001, 3.85};    // Initialize
printf("%s: %.2f\n", s.name, s.gpa);  // Access with .

Dot vs arrow:

p->name is shorthand for (*p).name.

Dynamic allocation:

Student *s = calloc(1, sizeof(Student));   // NOT sizeof(Student *)
strcpy(s->name, "Alice");
s->id = 1001;
s->gpa = 3.85;
// ... use s ...
free(s);

Nested free order:

// Struct with pointer field:
typedef struct { char *name; int id; } Record;

// FREE INNER FIRST:
free(r->name);    // 1. Free the string
free(r);          // 2. Free the struct

Common Pitfalls

• sizeof(Student *) vs sizeof(Student) — Pointer is 8 bytes. Struct might be 64 bytes. Wrong sizeof = buffer overflow.
• Shallow copy with pointer fields — s2 = s1 copies the pointer, not the data. Both share the string. Deep copy requires strcpy into new allocation.
• Can’t compare with == — Must compare field by field. No built-in equals.
• Free order — Inner allocations first, outer struct last. Reverse order leaks memory.
• Forgetting -> with pointers — p.name on a pointer is a compile error. Use p->name.
• Uninitialized fields — Struct fields are garbage unless initialized. Use = {0} or set each field.