C语言 为什么当我试图写入文件时出现奇怪的字符

Question

在这个程序中，我尝试编写一个文件，该文件显示用户输入的未知数量的记录。每条记录有以下字段:名、姓、地址、城市、州、邮政编码和电话号码。我认为最好的方法是用上面的字段定义一个结构体Record，然后声明一个Records数组，其中包含用户输入的记录数量。为了实现这一点，我将使用一个循环来获取每个记录的每个字段的输入，然后如果用户想要继续动态地在record数组中分配额外的空间，并继续直到用户输入no。程序编译并创建了文件，但是所写的字符完全是奇怪的。是什么导致了这种情况的发生?

#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 %sn", filename);
            exit(1);
        }
        fclose(fileWriter);
    }
    else
    {
        printf("Error opening file.");
    }
}
}

<<p> 编辑版本/strong>我已经修改了代码并修复了许多正在发生的错误，但现在我目前在scanf("%c"，answer) 处获得访问冲突。

#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,"%sn",records[i].fname);
            fprintf(fileWriter,"%sn",records[i].lname);
            fprintf(fileWriter,"%sn",records[i].address);
            fprintf(fileWriter,"%sn",records[i].city);
            fprintf(fileWriter,"%sn",records[i].state);
            fprintf(fileWriter,"%dn",records[i].zipcode);
            fprintf(fileWriter,"%dn",records[i].phoneNumber);
        }
        fclose(fileWriter);
    }
    else
    {
        printf("Error opening file.");   
    }
}

Answer 1

如果分配内存，则不清除该内存。

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

为100个字符分配内存。但是内存可以包含随机数据。如果你从这个地址写，你可能会把随机数据写进文件。

清除内存，确保没有随机数据。

memset(data, 0, 100);

对于struct:

struct Demo {
    int a;
    int b;
};
Demo *demo = (Demo*)malloc(sizeof(Demo));
memset(demo, 0, sizeof(Demo));

你的代码已更正

#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 %sn", filename);
            exit(1);
        }
        fclose(fileWriter);
    } else {
        printf("Error opening file.");
    }
}

Answer 2

回答OP评论的问题，询问为什么程序会自动退出，我建议这样做(通过示例)。注意%c前面的' '空格，它清除了前面的空白。

#include<stdio.h>
int main(void) {
    int phone;
    char answer;
    scanf("%d", &phone);        // leaves newline in buffer
    //answer  = getchar();      // reads the newline still in buffer
    scanf(" %c", &answer);      // skips preceding white-space
    return 0;
}

Answer 3

fwrite(records,sizeof(*records),size,fileWriter)有几个问题:

• struct Record包含5个以空结尾的字符串，所以你的文件将包含5个空字符分隔这些字符串，你可能想要换行字符。

• struct Record包含2个整数，它们以二进制数据的形式写入文件，并且可以肯定地符合"文件中出现的奇怪字符"。

---

您可以通过根据其特定类型编写每个字段来修复它:

fprintf(fileWriter,"%sn",records->fname);
fprintf(fileWriter,"%sn",records->lname);
fprintf(fileWriter,"%sn",records->address);
fprintf(fileWriter,"%sn",records->city);
fprintf(fileWriter,"%sn",records->state);
fprintf(fileWriter,"%dn",records->zipcode);
fprintf(fileWriter,"%dn",records->phoneNumber);