c - 为什么当我尝试写入文件时会出现奇怪的字符？

Question

在这个程序中，我试图编写一个文件来显示用户输入的未知数量的记录。每条记录都有以下字段:名字、姓氏、地址、城市、州、邮政编码和电话号码。我假设最好的方法是用上面的字段定义一个结构 Record，然后声明一个 Records 数组，其中包含与用户输入的记录一样多的记录。为了实现这一点，我将使用一个循环来获取每条记录的每个字段的输入，然后如果用户想继续动态地在 Record 数组中分配一个额外的空间并继续，直到用户输入 no。程序编译并创建了文件，但是写的字符很奇怪。是什么导致了这种情况？

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

struct Record
    {
        char fname[51];
        char lname[51];
        char address[51];
        char city[51];
        char state[51];
        int zipcode;
        int phoneNumber;
    };




int main()
{
    FILE *fileWriter;
    const char filename[] = "data.txt";
    char answer = 'y';
    int size = 1;
    int i = 0;
    struct Record *records = NULL;
    struct Record *records_temp;




    while(answer == 'y' || answer == 'Y')
    {
        struct Record *records_temp = realloc(records,(size)*sizeof(*records));

        if(records_temp == NULL)  
        {
            free(records); 

        }
        records = records_temp;
        printf("First Name: \n");
        scanf("%s", records[i].fname);
        printf("Last Name: \n");
        scanf("%s", records[i].lname);

        printf("Address: \n");
        scanf(" %[^\n]", records[i].address);

        printf("City: \n");
        scanf("%s", records[i].city);

        printf("State: \n");
        scanf("%s", records[i].state);

        printf("Zipcode: \n");
        scanf("%d", &records[i].zipcode);

        printf("Phone Number: \n");
        scanf("%d", &records[i].phoneNumber);
        //stores all record info

        printf("Are there anymore records? [y/n] ");
        answer = getchar();
        if(answer == 'y' || answer == 'Y')
        {
            size++;
            records[i++];
            printf("\n");
        }

        //open file

    fileWriter = fopen(filename,"wb");

    if(fileWriter != NULL)
    {
        if(fwrite(records,sizeof(*records),size,fileWriter) != 1)
        {
            fprintf(stderr, "Failed to write to %s\n", filename);
            exit(1);
        }
        fclose(fileWriter);
    }
    else
    {
        printf("Error opening file.");
    }
}
}

编辑版本我修改了代码并修复了许多发生的错误，但现在我在 scanf( "%c", answer) 处遇到访问冲突

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

struct Record
    {
        char fname[51];
        char lname[51];
        char address[51];
        char city[51];
        char state[51];
        char zipcode[51];
        char phoneNumber[51];
    };




int main()
{
    FILE *fileWriter;
    const char filename[] = "data.txt";
    char answer = 'y';
    char a = 'n';
    int size = 1;
    int i = 0;
    struct Record *records;
    struct Record *records_temp = NULL;




    while(answer == 'y' || answer == 'Y')
    {
        struct Record *records_temp = calloc((size),sizeof(*records));  

        records = records_temp;
        printf("First Name: \n");
        scanf("%s", records[size-1].fname);

        printf("Last Name: \n");
        scanf("%s", records[size-1].lname);

        printf("Address: \n");
        scanf(" %[^\n]", records[size-1].address);

        printf("City: \n");
        scanf("%s", records[size-1].city);

        printf("State: \n");
        scanf("%s", records[size-1].state);

        printf("Zipcode: \n");
        scanf("%s", records[size-1].zipcode);

        printf("Phone Number: \n");
        scanf("%s", records[size-1].phoneNumber);
        //stores all record info

        printf("Are there anymore records? [y/n] ");
        scanf(" %c", answer);
        if(answer == 'y' || answer == 'Y')
        {
            size++;
            printf("\n");
        }
    }
        //open file

    fileWriter = fopen(filename,"wb");

    if(fileWriter != NULL)
    {
        for(;i< size; i++)
        {
            fprintf(fileWriter,"%s\n",records[i].fname);
            fprintf(fileWriter,"%s\n",records[i].lname);
            fprintf(fileWriter,"%s\n",records[i].address);
            fprintf(fileWriter,"%s\n",records[i].city);
            fprintf(fileWriter,"%s\n",records[i].state);
            fprintf(fileWriter,"%d\n",records[i].zipcode);
            fprintf(fileWriter,"%d\n",records[i].phoneNumber);
        }
        fclose(fileWriter);
    }
    else
    {
        printf("Error opening file.");   
    }

}

Answer 1

如果分配内存，则不会清除此内存。

char *data = (char*)malloc(100);

这确实为 100 个字符分配了内存。但是内存可以包含随机数据。如果您从此地址写入，您最终可能会将随机数据写入文件。

清除内存以确保没有随机数据。

memset(data, 0, 100);

或者对于结构:

struct Demo {
    int a;
    int b;
};

Demo *demo = (Demo*)malloc(sizeof(Demo));
memset(demo, 0, sizeof(Demo));

您的代码已更正

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


struct Record
{
    char fname[51];
    char lname[51];
    char address[51];
    char city[51];
    char state[51];
    int zipcode;
    int phoneNumber;
};


int main()
{
    char answer = 'y';
    int size = 0;
    Record *records = NULL;

    do {
        ++size;
        // resize records array.
        if (records == NULL) {
            records = (Record*)malloc(sizeof(Record)*size);
        } else {
            records = (Record*)realloc(records, sizeof(Record)*size);
        }
        // Get a pointer to the current record.
        Record *currentRecord = &records[size-1];
        // Clear the record
        memset(currentRecord, 0, sizeof(Record));

        // Ask for all data.
        printf("First Name: \n");
        scanf("%s", currentRecord->fname);
        printf("Last Name: \n");
        scanf("%s", currentRecord->lname);
        printf("Address: \n");
        scanf(" %[^\n]", currentRecord->address);
        printf("City: \n");
        scanf("%s", currentRecord->city);
        printf("State: \n");
        scanf("%s", currentRecord->state);
        printf("Zipcode: \n");
        scanf("%d", &(currentRecord->zipcode));
        printf("Phone Number: \n");
        scanf("%d", &(currentRecord->phoneNumber));

        // Ask for more.
        printf("Are there anymore records? [y/n] ");
        answer = getchar();
        printf("\n");
    } while (answer == 'y'||answer == 'Y');

    // Write all records
    const char filename[] = "data.txt";
    FILE *fileWriter = fopen(filename, "wb");
    if (fileWriter != NULL) {
        if (fwrite(records, sizeof(Record), size, fileWriter) != size) {
            fprintf(stderr, "Failed to write to %s\n", filename);
            exit(1);
        }
        fclose(fileWriter);
    } else {
        printf("Error opening file.");
    }
}