我试图找出在添加或删除记录后如何更新数据库。我在添加和删除记录函数中使用了fprintf语句，它让我知道元素数据实际被添加，由于某种原因，添加将起作用（删除记录没有），但是当我去打印数据库时，它是不打印新数据库。如何更新数据库并相应打印？我是否需要一个全局变量来接收新数据库？
标题文件：
#ifndef myStruct
#define myStruct
struct creditCard
{
char firstName[100];
char lastName[100];
char cardNumber[17]; //TA advised to use string since 16 digits will overflow in plain integer
char expMonth[10];
};
//function headers below
int printCreditCard(struct creditCard *);
int sizeOfDb(struct creditCard *);
int countRecords(struct creditCard *);
int deleteRecord(struct creditCard *);
int addRecord(struct creditCard *);
#endif
主程序：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "myStruct.h"
int accesses = 0; //number of times database was manipulated
int count = 0; //used to count how many records there are; we always start at 4
//int size = 0; //used to tell us how big the database is
struct creditCard *ptr; //chunk of memory for each record/structure
struct creditCard *headPtr; //chunk of memory for the first records/structure
int main(void)
{
ptr = (struct creditCard *) malloc(4 * sizeof(struct creditCard));
headPtr = ptr;
memcpy(ptr->firstName,"Bob",100);
memcpy(ptr->lastName,"Marley",100);
memcpy(ptr->cardNumber,"0000000000000000",17);
memcpy(ptr->expMonth,"September", 10);
ptr++;
count = count+1;
memcpy(ptr->firstName, "Will", 100);
memcpy(ptr->lastName, "Smith", 100);
memcpy(ptr->cardNumber,"1111111111111111",17);
memcpy(ptr->expMonth, "October", 10);
ptr++;
count = count+1;
memcpy(ptr->firstName, "Bill", 100);
memcpy(ptr->lastName, "Gates", 100);
memcpy(ptr->cardNumber,"2222222222222222",17);
memcpy(ptr->expMonth, "January", 10);
ptr++;
count = count+1;
memcpy(ptr->firstName, "Steve", 100);
memcpy(ptr->lastName, "Jobs", 100);
memcpy(ptr->cardNumber,"3333333333333333",17);
memcpy(ptr->expMonth, "May", 10);
count = count+1;
while (1)
{
// headPtr = ptr; //put the newest database into headPtr so it points to the first record in that database
// ptr = headPtr; //start the database back at the first record
//countRecords(ptr); //update the count of the current database
int sel;
printf("MAIN MENU\n");
printf("=====\n");
printf("1. Select 1 to print all records.\n");
printf("2. Select 2 to print number of records .\n");
printf("3. Select 3 to print size of database.\n");
printf("4. Select 4 to add record.\n");
printf("5. Select 5 to delete record.\n");
printf("6. Select 6 to print number of accesses to database.\n");
printf("7. Select 7 to Exit.\n");
printf("Enter Your Selection: \n");
scanf("%d", &sel); //get user input;
if (sel == 1)
{
printCreditCard(ptr);
accesses++;
}
else if (sel == 2)
{
fprintf(stderr,"Number of records in the database is %d records\n", count); //pulls value of count from global updated variable
accesses++;
}
else if (sel == 3)
{
sizeOfDb(ptr);
accesses++;
}
else if (sel == 4)
{
ptr = headPtr;
addRecord(ptr);
accesses++;
}
else if (sel == 5)
{
deleteRecord(ptr);
accesses++;
}
else if (sel == 6)
{
fprintf(stderr,"Number of accesses to the database is %d\n", accesses);
accesses++;
}
else if (sel == 7)
{
printf("Now Exiting.\n");
return 0;
}
else
{
printf("Invalid input, please select a valid option.\n");
break; //go back to the main menu
}
}
}
//functions defined below
int sizeOfDb(struct creditCard *card2)
{
int size = 0;
int j;
for (j = 1; j <= count; j++)
{
size += sizeof(card2->firstName); //get the size of each element
size += sizeof(card2->lastName);
size += sizeof(card2->cardNumber);
size += sizeof(card2->expMonth);
card2++;
}
//loop through each record and get sizeof() of each record
fprintf(stderr, "Total Size of the Database is %d bytes.\n", size);
return size;
}
int addRecord(struct creditCard *card3)
{
char fName[100];
char lName[100];
char number[17];
char month[10];
count = count+1;
fprintf(stderr, "count is %d \n", count);
int p;
struct creditCard *tempStruct;
struct creditCard *tempHead;
tempStruct = (struct creditCard *) malloc (count * sizeof(struct creditCard)); //allocate memory to a dummy record
tempHead = tempStruct;
card3 = headPtr; //start at the beginning of the old database
for (p = 1; p < count; p++) //copies the old database in the new database up to the record before the newly allocated record
{
memcpy(tempStruct->firstName, card3->firstName, 100);
memcpy(tempStruct->lastName, card3->lastName, 100);
memcpy(tempStruct->cardNumber, card3->cardNumber, 17);
memcpy(tempStruct->expMonth, card3->expMonth, 10);
fprintf(stderr, "first name is %s\n", tempStruct->firstName);
if (p == count-1) //if we are on the record before the last record, then only increment tempStruct, card3 has one less record than the new database has
{
tempStruct++;
}
else
{
tempStruct++;
card3++;
}
}
printf("Please enter your first name.\n");
scanf("%s", fName);
memcpy(tempStruct->firstName, fName, 100);
printf("Please enter your last name.\n");
scanf("%s", lName);
memcpy(tempStruct->lastName, lName, 100);//put first name in struct
printf("Please enter your 16 digit credit card number with no spaces.\n");
scanf("%s", number);
memcpy(tempStruct->cardNumber, number, 17);//put first name in struct
printf("Please enter the month in which your credit card expires.\n");
scanf("%s", month);
memcpy(tempStruct->expMonth, month, 10);//put first name in struct
fprintf(stderr, "tempStruct first name is %s\n", tempStruct->firstName);
fprintf(stderr, "tempStruct last name is %s\n", tempStruct->lastName);
fprintf(stderr, "tempStruct card number is %s\n", tempStruct->cardNumber);
fprintf(stderr, "tempStruct exp month is %s\n", tempStruct->expMonth);
tempStruct = tempHead;
card3 = tempStruct; //put the new database in place of the old database
//free(tempStruct); //clear memory for the dummy record because we don't need it anymore
return 0;
}
int deleteRecord(struct creditCard *card4) //goes to the last record in the database and clears the memory for it, essentially deleting it
{
int b;
count = count-1;
int l;
struct creditCard *newDb; //will hold the new database with one less record at the end
struct creditCard *newDbHead;
newDb = (struct creditCard *) malloc(count * sizeof(struct creditCard));
newDbHead = newDb;
for (l = 1; l <= count; l++)
{
memcpy(newDb->firstName,card4->firstName,100);
memcpy(newDb->lastName,card4->lastName,100);
memcpy(newDb->cardNumber,card4->cardNumber,17);
memcpy(newDb->expMonth,card4->expMonth,10);
if (l == count)
{
continue;
}
else
{
card4++;
newDb++;
}
}
newDb = newDbHead; //start back at the first record
for (b = 1; b <= count; b++)
{
fprintf(stderr, "first name for the current record is %s\n", newDb->firstName);
newDb++;
}
card4 = newDb; //put the new database into ptr to hold as the new database
return 0;
}
int printCreditCard(struct creditCard *card)
{
card = headPtr; //start at the beginning of the database
int i;
if (count == 0)
{
printf("The database is empty\n");
return 0;
}
else
{
for (i = 1; i <= count; i++)
{
printf("Credit Card Record %d\n", i);
fprintf(stderr, "First Name = \%s\n", card-> firstName);
fprintf(stderr, "Last Name = \%s\n", card-> lastName);
fprintf(stderr, "Card Number = \%s\n", card-> cardNumber);
fprintf(stderr, "Expiration Date = \%s\n\n", card-> expMonth);
card++; //go to the next record to print
}
}
return 1; //we have now printed all records, go to main menu.
}
答案 0 :(得分：4)
您提供了相当大的样本，但仅举一例：
memcpy(ptr->firstName, "Bill", 100);
对我来说似乎没问题。你指定大小100，但字符数组＆＃34;比尔＆＃34;
是5个字节长（1为空终止符）。所以我认为它会尝试阅读
过了阵列。在这种情况下，您可以尝试strcpy，这将完全复制
5个字节。
同样如Serge Ballesta所指出，sizeofDb内部存在类似问题
方法。您应该将指针设置为数据库的开头：
int sizeOfDb(struct creditCard *card2)
{
card2=headPtr;
...
否则不清楚你之后解除了什么。实际上，它之后在sizeofDb内没有崩溃的原因是由于：Is dereferencing null pointer valid in sizeof operation。
但从逻辑上讲，它应该仍然指向开头。
答案 1 :(得分：2)
如果我已正确理解您的逻辑，数据库是一个struct数组，当您添加或删除记录时，您将分配一个新数组，将相关记录复制到新数组，以及添加复制新记录。
一切都很好（即使效率不高），但似乎你永远不会让全局变量headPtr指向新数组。您只需释放旧数据库并使headPtr指向新数据库。
addRecord和deleteRecord：
free(headPtr);
headPtr = tempHead; /* for addRecord */
headPtr = newDbHead; /* for deleteRecord */
独立于georgem注意到的问题（不应该是致命的），您的deleteRecord函数是错误的：您从旧数据库复制记录而不首先指向第一条记录。你应该至少使用：
int deleteRecord(struct creditCard *card4) //goes to the last record in the database and clears the memory for it, essentially deleting it
{
int b;
count = count-1;
int l;
struct creditCard *newDb; //will hold the new database with one less record at the end
struct creditCard *newDbHead;
newDb = (struct creditCard *) malloc(count * sizeof(struct creditCard));
newDbHead = newDb;
ptr = headPtr;
for (l = 1; l <= count; l++)
{
if (ptr == card4) ptr++;
/* no use to copy element by element : just copy full struct
memcpy(newDb->firstName,ptr->firstName,100);
memcpy(newDb->lastName,ptr->lastName,100);
memcpy(newDb->cardNumber,ptr->cardNumber,17);
memcpy(newDb->expMonth,ptr->expMonth,10); */
/* memcpy(newDb, ptr, sizeof(creditCard)); */
/* even better, thanks to Jonathan Leffler */
*newDb++ = *ptr++;
}
newDb = newDbHead; //start back at the first record
for (b = 1; b <= count; b++)
{
fprintf(stderr, "first name for the current record is %s\n", newDb->firstName);
newDb++;
}
card4 = newDb; //put the new database into ptr to hold as the new database
free(headPtr);
headPtr = newDbHead;
return 0;
}