📘 Week 4：動態記憶體管理

重點：記憶體配置與釋放、記憶體洩漏
練習:檢查記憶體管理是否正確

Day 1：malloc 配置與使用

#include <stdio.h>
#include <stdlib.h>

int main() {
  int *ptr = (int *)malloc(sizeof(int));
  if (ptr == NULL) {
    printf("記憶體配置失敗\n");
    return 1;
  }

  *ptr = 100;
  printf("儲存的值為 %d\n", *ptr);
  free(ptr);
  return 0;
}
#include <stdio.h>
#include <stdlib.h>

int main() {
  int n = 5;
  int *arr = (int *)malloc(n * sizeof(int));

  for (int i = 0; i < n; i++) {
    arr[i] = i * 10;
    printf("arr[%d] = %d\n", i, arr[i]);
  }

  free(arr);
  return 0;
}
#include <stdio.h>
#include <stdlib.h>

int main() {
  int *data = (int *)malloc(sizeof(int) * 3);
  if (data != NULL) {
    data[0] = 7;
    data[1] = 14;
    data[2] = 21;
    printf("資料：%d, %d, %d\n", data[0], data[1], data[2]);
    free(data);
  }
  return 0;
}
#include <stdio.h>
#include <stdlib.h>

int main() {
  int *ptr = malloc(sizeof(int));
  if (ptr) {
    *ptr = 42;
    printf("值為 %d\n", *ptr);
    free(ptr);
  }
  return 0;
}

Day 2：calloc 配置與初始化

#include <stdio.h>
#include <stdlib.h>

int main() {
  int *arr = (int *)calloc(4, sizeof(int));
  for (int i = 0; i < 4; i++) {
    printf("arr[%d] = %d\n", i, arr[i]);
  }
  free(arr);
  return 0;
}
#include <stdio.h>
#include <stdlib.h>

int main() {
  float *data = (float *)calloc(3, sizeof(float));
  for (int i = 0; i < 3; i++) {
    data[i] = i * 1.5;
    printf("data[%d] = %.2f\n", i, data[i]);
  }
  free(data);
  return 0;
}
#include <stdio.h>
#include <stdlib.h>

int main() {
  char *letters = (char *)calloc(5, sizeof(char));
  letters[0] = 'A';
  letters[1] = 'B';
  printf("字元：%c %c\n", letters[0], letters[1]);
  free(letters);
  return 0;
}
#include <stdio.h>
#include <stdlib.h>

int main() {
  int *ptr = calloc(2, sizeof(int));
  if (ptr) {
    ptr[0] = 11;
    ptr[1] = 22;
    printf("值：%d, %d\n", ptr[0], ptr[1]);
    free(ptr);
  }
  return 0;
}

Day 3：realloc 記憶體擴充

#include <stdio.h>
#include <stdlib.h>

int main() {
  int *arr = malloc(2 * sizeof(int));
  arr[0] = 1;
  arr[1] = 2;

  arr = realloc(arr, 4 * sizeof(int));
  arr[2] = 3;
  arr[3] = 4;

  for (int i = 0; i < 4; i++) {
    printf("%d ", arr[i]);
  }
  printf("\n");
  free(arr);
  return 0;
}
#include <stdio.h>
#include <stdlib.h>

int main() {
  char *text = malloc(5);
  sprintf(text, "Hi");

  text = realloc(text, 10);
  sprintf(text + 2, " there");

  printf("訊息：%s\n", text);
  free(text);
  return 0;
}
#include <stdio.h>
#include <stdlib.h>

int main() {
  int *data = malloc(3 * sizeof(int));
  for (int i = 0; i < 3; i++) data[i] = i;

  data = realloc(data, 6 * sizeof(int));
  for (int i = 3; i < 6; i++) data[i] = i;

  for (int i = 0; i < 6; i++) printf("%d ", data[i]);
  printf("\n");
  free(data);
  return 0;
}
#include <stdio.h>
#include <stdlib.h>

int main() {
  float *nums = malloc(2 * sizeof(float));
  nums[0] = 1.1;
  nums[1] = 2.2;

  nums = realloc(nums, 3 * sizeof(float));
  nums[2] = 3.3;

  for (int i = 0; i < 3; i++) {
    printf("%.1f ", nums[i]);
  }
  printf("\n");
  free(nums);
  return 0;
}

Day 4：free 的正確釋放與陷阱

#include <stdlib.h>

int main() {
  int *ptr = malloc(sizeof(int));
  *ptr = 123;
  free(ptr);
  // free(ptr); // 錯誤：重複釋放
  return 0;
}
#include <stdlib.h>

int main() {
  int *arr = malloc(3 * sizeof(int));
  if (arr != NULL) {
    free(arr);
    arr = NULL; // 避免懸空指標
  }
  return 0;
}
#include <stdlib.h>

void release(int *p) {
  free(p);
  // p = NULL; // 無效：只在函數內改變
}

int main() {
  int *data = malloc(sizeof(int));
  release(data);
  // printf("%d", *data); // 錯誤：使用已釋放記憶體
  return 0;
}
#include <stdlib.h>

int main() {
  int *ptr = malloc(sizeof(int));
  *ptr = 77;
  free(ptr);
  ptr = malloc(sizeof(int)); // 重新配置
  *ptr = 88;
  free(ptr);
  return 0;
}

Day 5：記憶體洩漏示範與修正

#include <stdlib.h>

int main() {
  int *leak = malloc(sizeof(int));
  *leak = 999;
  // 忘記 free(leak); // 洩漏
  return 0;
}
#include <stdlib.h>

int main() {
  int *safe = malloc(sizeof(int));
  *safe = 123;
  free(safe); // 修正洩漏
  return 0;
}
#include <stdlib.h>

void create() {
  int *temp = malloc(10 * sizeof(int));
  // 沒有 free(temp); // 洩漏
}

int main() {
  create();
  return 0;
}
#include <stdlib.h>

int main() {
  for (int i = 0; i < 5; i++) {
    int *block = malloc(100);
    free(block); // 正確釋放
  }
  return 0;
}

Day 6：動態字串儲存與釋放

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

int main() {
  char *name = malloc(20);
  strcpy(name, "會安");
  printf("名字：%s\n", name);
  free(name);
  return 0;
}
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

int main() {
  char *msg = calloc(30, sizeof(char));
  strcat(msg, "Hello ");
  strcat(msg, "World");
  printf("訊息：%s\n", msg);
  free(msg);
  return 0;
}
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

char* createMessage(const char *name) {
  char *msg = malloc(50);
  sprintf(msg, "歡迎 %s 來到 C 語言世界！", name);
  return msg;
}

int main() {
  char *welcome = createMessage("會安");
  printf("%s\n", welcome);
  free(welcome);
  return 0;
}