C语言fread函数怎么用？

[dhqiuhihqoqhoqhoqwhf]

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

struct Books{
char name[30];
char author[20];
char press[20];
char presstime[10];
char c;
int number;
float price;
int id;
}bk[200],*bp,*bp1;
int count=20;
void main()
{
FILE *fp;
float f;
if((fp=fopen("tushuxinxi","ab"))==NULL){
printf("Cannot open the file!");
getchar();
exit(0);
}
printf("name:");
gets(bk[count].name);
printf("author:");
gets(bk[count].author);
printf("press:");
gets(bk[count].press);
printf("presstime:");
gets(bk[count].presstime);
printf("class:");
scanf("%c",&bk[count].c);
printf("number:");
scanf("%d",&bk[count].number);
printf("price:");
scanf("%f",&f);
bk[count].price=f;
bk[count].id=count;
bp=&bk[count];
fwrite(bp,sizeof(struct Books),1,fp);
printf("The book is already stored away!\n");
rewind(fp);
fread(bp1,sizeof(struct Books),1,fp);/*读取文件信息*/
puts(bp1->name); /*输出部分信息*/
fclose(fp);
}
编译和连接都没问题，但是当执行到fread语句时提示出错信息：Expression string!=NULL。不知道怎么回事，请各位帮帮忙！

[liutengfeigo]

http://zhidao.baidu.com/question/81314373.html

[wade_2003]

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

struct Books{
	char name[30];
	char author[20];
	char press[20];
	char presstime[10];
	char c;
	int number;
	float price;
	int id;
}bk[200],*bp,*bp1;
int count=20;
void main()
{
	FILE *fp;
	float f;
	if((fp=fopen("tushuxinxi","ab"))==NULL){
		printf("Cannot open the file!");
		getchar();
		exit(0);
	}
	printf("name:");
	gets(bk[count].name);
	printf("author:");
	gets(bk[count].author);
	printf("press:");
	gets(bk[count].press);
	printf("presstime:");
	gets(bk[count].presstime);
	printf("class:");
	scanf("%c",&bk[count].c);
	printf("number:");
	scanf("%d",&bk[count].number);
	printf("price:");
	scanf("%f",&f);
	bk[count].price=f;
	bk[count].id=count;
	bp=&bk[count];
	fwrite(bp,sizeof(struct Books),1,fp);
	printf("The book is already stored away!\n");
	rewind(fp);
	bp1 = (struct Books*)malloc(sizeof(struct Books));//mark
	fread(bp1,sizeof(struct Books),1,fp);/*读取文件信息*/
	puts(bp1->name); /*输出部分信息*/
	fclose(fp);
}

应该是bp1没有分配内存吧

[elegant87]

//MSDN
Reads data from a stream.

size_t fread( void *buffer, size_t size, size_t count, FILE *stream );
buffer

Storage location for data

size

Item size in bytes

count

Maximum number of items to be read

stream

Pointer to FILE structure

#include <stdio.h>

void main( void )
{
   FILE *stream;
   char list[30];
   int  i, numread, numwritten;

   /* Open file in text mode: */
   if( (stream = fopen( "fread.out", "w+t" )) != NULL )
   {
      for ( i = 0; i < 25; i++ )
         list[i] = (char)('z' - i);
      /* Write 25 characters to stream */
      numwritten = fwrite( list, sizeof( char ), 25, stream );
      printf( "Wrote %d items\n", numwritten );
      fclose( stream );

   }
   else
      printf( "Problem opening the file\n" );

   if( (stream = fopen( "fread.out", "r+t" )) != NULL )
   {
      /* Attempt to read in 25 characters */
      numread = fread( list, sizeof( char ), 25, stream );
      printf( "Number of items read = %d\n", numread );
      printf( "Contents of buffer = %.25s\n", list );
      fclose( stream );
   }
   else
      printf( "File could not be opened\n" );
}

[yunyun1886358]

改好了， bp1没有分配内存空间。

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

struct Books{
char name[30];
char author[20];
char press[20];
char presstime[10];
char c;
int number;
float price;
int id;
}bk[200],*bp,*bp1;
int counter=20;
void main()
{
FILE *fp;
float f;
if((fp=fopen("tushuxinxi","ab"))==NULL){
 printf("Cannot open the file!");
 getchar();
 exit(0);
 }
printf("name:");
gets(bk[counter].name);
printf("author:");
gets(bk[counter].author);
printf("press:");
gets(bk[counter].press);
printf("presstime:");
gets(bk[counter].presstime);
printf("class:");
scanf("%c",&bk[counter].c);
printf("number:");
scanf("%d",&bk[counter].number);
printf("price:");
scanf("%f",&f);
bk[counter].price=f;
bk[counter].id=counter;
bp=&bk[counter];
fwrite(bp,sizeof(struct Books),1,fp);
printf("The book is already stored away!\n");
rewind(fp);
bp1 = new Books();
fread(bp1,sizeof(struct Books),1,fp);/*读取文件信息*/
puts(bp1->name); /*输出部分信息*/
fclose(fp);
delete bp1;
}

[dhqiuhihqoqhoqhoqwhf]

[Quote=引用 4 楼 yunyun1886358 的回复:]
改好了， bp1没有分配内存空间。

C/C++ code

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

struct Books{
char name[30];
char author[20];
char press[20];
char presstime[10];
char c;
int number;
float price;
i……
[/Quote]运行没问题，但输出的信息是乱码。