c - 如何在c中存储具有相同内存位置的值？

Question

如果我有一个包含内容的文件流

123 1234

1223 124235

21432 325

在我的程序中，我逐行读取文件并将每行的第一个目标存储到我的列表中。这些行具有相同的位置，当我运行程序时，它将继续指向最新的数据并将其放入列表中。这意味着如果我在 while 循环中有一个名为 printL() 的函数。它将打印

123/

1223/1223/

21432/21432/21432/

而不是

123/

123/1223/

123/1223/21432

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


typedef struct n{
  char *value;
  struct n *next;
} Node;


void printList(Node *head){
  Node *cur = head;
  while(cur!=NULL){
    printf("%s/", cur->value);
    cur = cur->next;
  }
  printf("\n");
}

void insertIntoList(Node **head, char *data){
  Node *newNode = malloc(sizeof(Node));
  if (newNode == NULL){
    perror("Failed to allocate a new node for the linked list");
    exit(1);
  }
  newNode->value = data;
  newNode->next = NULL;

  Node *currentList = *head;
  if(*head == NULL){    //if the linked list head is null, then add the target into linked list
    *head = newNode;
  }
  else{
    while(currentList->next!=NULL){
      currentList = currentList->next;
    }
    currentList->next = newNode;
  }
}


int main(int argc, char**argv){
  FILE *fileStream;


  size_t len = 0;
  char *line = NULL;
  Node *head = NULL;


  int j;
  for(j=1; j<argc-2;j++){
    fileStream = fopen(argv[j], "r");
    if(fileStream == NULL){
      fprintf(stderr, "could not open");
      continue;
    }
    insertIntoList(&head,"a"); /////////////Line 95
    insertIntoList(&head,"b");
    insertIntoList(&head,"c"); 
    insertIntoList(&head,"d");
    printf("here is a try\n");
    printList(head);
    while(getline(&line, &len, fileStream)!=EOF){ /////////////Line 101
            char *targetNum = strtok(line, " \t\r\n");
            printf("*****%s\n", targetNum);
            insertIntoList(&head, targetNum);
            printf("######print head here is##########\n");
          printList(head);
          printf("######print head here is##########->\n");
      }
      //printList(head);
  }
  return 0;
}

Answer 1

为了保留从strtok()返回的每个加载字段的内容，只需在调用insertIntoList()之前添加一个strdup() > 检查是否不是空指针后。

In your code, if you compare the value of both line and targetNum are the same. If fact, the strtok() function returns a pointer to the input string and keep the pointer for the next argument.

替换以下代码:

    char *targetNum = strtok(line, " \t\r\n");
    printf("*****%s\n", targetNum);
    insertIntoList(&head, targetNum);

由那个人:

    char *targetNum = strtok(line, " \t\r\n");
    if (targetNum != NULL) {
        printf("*****%s\n", targetNum);
        insertIntoList(&head, strdup(targetNum));
    }