[C言語] 構造体を関数の引数として渡す方法と注意点

#include <stdio.h>
typedef struct {
    int x;
    int y;
} Point;
// 構造体を値渡しする関数
void printPoint(Point p) {
    printf("Point: (%d, %d)\n", p.x, p.y);
}
int main() {
    Point p1 = {10, 20};
    printPoint(p1);
    return 0;
}

Point: (10, 20)

#include <stdio.h>
typedef struct {
    int x;
    int y;
} Point;
// 構造体をポインタ渡しする関数
void modifyPoint(Point *p) {
    p->x = 30;
    p->y = 40;
}
int main() {
    Point p1 = {10, 20};
    modifyPoint(&p1);
    printf("Modified Point: (%d, %d)\n", p1.x, p1.y);
    return 0;
}

Modified Point: (30, 40)

#include <stdio.h>
typedef struct {
    int width;
    int height;
} Rectangle;
// 構造体を値渡しする関数
void printRectangle(Rectangle r) {
    printf("Rectangle: width = %d, height = %d\n", r.width, r.height);
}
int main() {
    Rectangle rect = {5, 10};
    printRectangle(rect);
    printf("Original Rectangle: width = %d, height = %d\n", rect.width, rect.height);
    return 0;
}

Rectangle: width = 5, height = 10
Original Rectangle: width = 5, height = 10

#include <stdio.h>
typedef struct {
    int width;
    int height;
} Rectangle;
// 構造体をポインタ渡しする関数
void modifyRectangle(Rectangle *r) {
    r->width = 15;
    r->height = 20;
}
int main() {
    Rectangle rect = {5, 10};
    modifyRectangle(&rect);
    printf("Modified Rectangle: width = %d, height = %d\n", rect.width, rect.height);
    return 0;
}

Modified Rectangle: width = 15, height = 20

#include <stdio.h>
typedef struct {
    int width;
    int height;
} Rectangle;
// 構造体をポインタ渡しする関数
void safeModifyRectangle(Rectangle *r) {
    if (r == NULL) {
        printf("Error: NULL pointer received.\n");
        return;
    }
    r->width = 15;
    r->height = 20;
}
int main() {
    Rectangle *rect = NULL;
    safeModifyRectangle(rect);
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
typedef struct {
    int width;
    int height;
} Rectangle;
int main() {
    Rectangle *rect = (Rectangle *)malloc(sizeof(Rectangle));
    if (rect == NULL) {
        printf("Memory allocation failed.\n");
        return 1;
    }
    rect->width = 5;
    rect->height = 10;
    printf("Rectangle: width = %d, height = %d\n", rect->width, rect->height);
    free(rect); // メモリを解放
    return 0;
}

#include <stdio.h>
typedef struct {
    int width;
    int height;
} Rectangle;
void printRectangle(Rectangle *r) {
    if (r == NULL) {
        printf("Error: NULL pointer received.\n");
        return;
    }
    printf("Rectangle: width = %d, height = %d\n", r->width, r->height);
}
int main() {
    Rectangle rect = {0}; // 構造体を初期化
    printRectangle(&rect);
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
// ノードを表す構造体
typedef struct Node {
    int data;
    struct Node *next;
} Node;
// リストに新しいノードを追加する関数
void append(Node **head, int newData) {
    Node *newNode = (Node *)malloc(sizeof(Node));
    Node *last = *head;
    newNode->data = newData;
    newNode->next = NULL;
    if (*head == NULL) {
        *head = newNode;
        return;
    }
    while (last->next != NULL) {
        last = last->next;
    }
    last->next = newNode;
}
// リストを表示する関数
void printList(Node *node) {
    while (node != NULL) {
        printf("%d -> ", node->data);
        node = node->next;
    }
    printf("NULL\n");
}
int main() {
    Node *head = NULL;
    append(&head, 1);
    append(&head, 2);
    append(&head, 3);
    printList(head);
    return 0;
}

#include <stdio.h>
typedef struct {
    char name[50];
    int age;
} Person;
int main() {
    Person person = {"Taro", 30};
    FILE *file = fopen("person.dat", "wb");
    if (file != NULL) {
        fwrite(&person, sizeof(Person), 1, file);
        fclose(file);
    }
    Person readPerson;
    file = fopen("person.dat", "rb");
    if (file != NULL) {
        fread(&readPerson, sizeof(Person), 1, file);
        fclose(file);
        printf("Name: %s, Age: %d\n", readPerson.name, readPerson.age);
    }
    return 0;
}

// このサンプルはWindows系では動きません
#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>
typedef struct {
    int id;
    char message[100];
} Packet;
int main() {
    Packet packet = {1, "Hello, Network!"};
    char buffer[sizeof(Packet)];
    memcpy(buffer, &packet, sizeof(Packet));
    // ソケット通信の例(送信側)
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    struct sockaddr_in serverAddr;
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_port = htons(8080);
    serverAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
    sendto(sockfd, buffer, sizeof(Packet), 0, (struct sockaddr *)&serverAddr, sizeof(serverAddr));
    close(sockfd);
    return 0;
}