从文件读取到链表 C

Question

>我正在尝试读取一个txt文件，然后将其保存到C语言中的链表中。我已经尝试了很多，但我没有找到解决方案。

这是我所拥有的：

typedef struct contact{
    char name[50];
    char number[10];
    char bithday[15];
    struct contact *next;
}contact;
struct contact *head;
FILE *fp;

这部分将我已经拥有的联系人保存到txt文件中。

void backup(){
    struct contact *ap;
    fp=fopen("save_contacts.txt", "w");
    if(fp==NULL){
        printf(" Errorn");
        system("pause");
        return 1;
    }
    for (ap=head;ap!=NULL;ap=ap->next){
        fprintf(fp,"%s ",ap->name);
        //fprintf(fp,", ");
        fprintf(fp,"%s ",ap->number);
        //fprintf(fp,", ");
        fprintf(fp,"%s ",ap->birthday);
        //fprintf(fp,", ");
        fprintf(fp,"; ");
    }
    fprintf(fp, "End");
}

所以我的问题出在这部分，它不读取 txt 文件。

void read() {
    struct contact *ap;
    int i;
    head=NULL;
    char aux[30];
    FILE *fp=fopen("save_contacts.txt","r");
    do{
        head=NULL;
        ap=(contact*)malloc(sizeof(contact));
        fscanf(fp,"%s ",&ap->name);
        fscanf(fp,"%s ",&ap->number);
        fscanf(fp,"%s ",&ap->birthday);
        fscanf(fp,"%s ",aux);
        if(strcmp(aux,";")==0){
            ap=ap->next;
        }
    }while(strcmp(aux,"End")!=0);
}

Answer 1

正在执行

   ap=(contact*)malloc(sizeof(contact));
   fscanf(fp,"%s ",&ap->name);
   fscanf(fp,"%s ",&ap->number);
   fscanf(fp,"%s ",&ap->birthday);
   fscanf(fp,"%s ",aux);
   if(strcmp(aux,";")==0){
       ap=ap->next;

未设置ap->next，因此第二轮 ap 具有未定义的值，因此当您取消引用它时行为未定义

请注意您的保存方式：

 for (ap=head;ap!=NULL;ap=ap->next){
   ...
   fprintf(fp,"; ");
 }
 fprintf(fp, "End");

与您的阅读方式不兼容，其中最后一个元素的";"被"End">替换

你可以这样做(我想最后一个元素没有';'(：

int read() {      
  FILE *fp=fopen("save_contacts.txt","r");
  if (fp == NULL) {
    fprintf(stderr, "cannot open 'save_contacts.txt'n");
    head = NULL;
    return 0;
  }
  contact ** pp = &head;
  char aux[30];
  int result;
  for (;;) {
    *pp = malloc(sizeof(contact));
    if (fscanf(fp,"%49s %9s %14s %29s", (*pp)->name, (*pp)->number, (*pp)->birthday, aux) != 4) {
      fputs("invalid file, cannot read the 3 fields then ;/Endn", stderr);
      result = 0;
      free(*pp);
      break;
    }
    pp = &(*pp)->next;
    if (strcmp(aux,"End") == 0) {
      result = 1;
      break;
    }
    if (strcmp(aux, ";") != 0) {
      fputs("invalid file, expected ; or Endn", stderr);
      result = 0;
      break;
    }
  }
  *pp = NULL;
  fclose(fp);
  return result;
}

出现问题时返回 0，否则返回 1

注意我保护字符串的读取不写出它们，并检查文件是否正确

---

例如，拥有完整的程序

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct contact{
  char name[50];
  char number[10];
  char birthday[15]; /* not bithday */
  struct contact *next;
}contact;
struct contact *head;
int read() {      
  FILE *fp=fopen("save_contacts.txt","r");
  if (fp == NULL) {
    fprintf(stderr, "cannot open 'save_contacts.txt'n");
    head = NULL;
    return 0;
  }
  contact ** pp = &head;
  char aux[30];
  int result;
  for (;;) {
    *pp = malloc(sizeof(contact));
    if (fscanf(fp,"%49s %9s %14s %29s", (*pp)->name, (*pp)->number, (*pp)->birthday, aux) != 4) {
      fputs("invalid file, cannot read the 3 fields then ;/Endn", stderr);
      result = 0;
      free(*pp);
      break;
    }
    pp = &(*pp)->next;
    if (strcmp(aux,"End") == 0) {
      result = 1;
      break;
    }
    if (strcmp(aux, ";") != 0) {
      fputs("invalid file, expected ; or Endn", stderr);
      result = 0;
      break;
    }
  }
  *pp = NULL;
  fclose(fp);
  return result;
}
int main()
{
  if (read()) {
    /* debug */
    for (contact * p = head; p != NULL; p = p->next)
      printf("%s %s %sn", p->name, p->number, p->birthday);
  }
  /* free resources */
  while (head != NULL) {
    contact * p = head;
    head = head->next;
    free(p);
  }
  return 0;
}

编译和执行：

pi@raspberrypi:/tmp $ gcc -pedantic -Wextra -Wall l.c
pi@raspberrypi:/tmp $ cat save_contacts.txt 
bruno 007 19/02/1960 ;
you 001 01/01/2010 ;
bar 123 31/12/1999 End
pi@raspberrypi:/tmp $ ./a.out
bruno 007 19/02/1960
you 001 01/01/2010
bar 123 31/12/1999
pi@raspberrypi:/tmp $ 

瓦尔格林德的处决：

pi@raspberrypi:/tmp $ valgrind ./a.out
==2334== Memcheck, a memory error detector
==2334== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==2334== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==2334== Command: ./a.out
==2334== 
bruno 007 19/02/1960
you 001 01/01/2010
bar 123 31/12/1999
==2334== 
==2334== HEAP SUMMARY:
==2334==     in use at exit: 0 bytes in 0 blocks
==2334==   total heap usage: 6 allocs, 6 frees, 5,712 bytes allocated
==2334== 
==2334== All heap blocks were freed -- no leaks are possible
==2334== 
==2334== For counts of detected and suppressed errors, rerun with: -v
==2334== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)
pi@raspberrypi:/tmp $