C - fwrite()不能在二进制文件中写入整数

Question

有人能告诉我为什么这个函数不起作用吗?我就是搞不懂……

void writeRegister(FILE *arq, Book *L){ //writes in actual file position
  char c = '|';
  int sizeRegWrite = reglen(L); //reglen() returns the size of Book
  fwrite(&sizeRegWrite, sizeof(int), 1, arq);
  fwrite(L->TITLE, sizeof(char), strlen(L->TITLE), arq);
  fwrite(&c, sizeof(char), 1, arq); //writing delimiter
  fwrite(L->AUTHOR, sizeof(char), strlen(L->AUTHOR), arq);
  fwrite(&c, sizeof(char), 1, arq); //writing delimiter
  fwrite(L->PUBLISHER, sizeof(char), strlen(L->PUBLISHER), arq);
  fwrite(&c, sizeof(char), 1, arq); //writing delimiter
  fwrite(L->YEAR, sizeof(int), 1, arq);
  fwrite(&c, sizeof(char), 1, arq); //writing delimiter
  fwrite(L->LANGUAGE, sizeof(char), strlen(L->LANGUAGE), arq);
  fwrite(&c, sizeof(char), 1, arq); //writing delimiter
  fwrite(L->PAGES, sizeof(int), 1, arq);
  fwrite(&c, sizeof(char), 1, arq); //writing delimiter
  fwrite(L->PRICE, sizeof(float), 1, arq);
  fwrite(&c, sizeof(char), 1, arq); //writing delimiter
        return;
}  

结构体Book的声明如下:

typedef struct {
    char *TITLE;
    char *AUTHOR;
    char *PUBLISHER;
    int YEAR;
    char *LANGUAGE;
    int PAGES;
float PRICE;
} Book;

主要

int main() {
FILE *arq = fopen("BD_books2.bin", "rb+");
if(arq == NULL)
    printf("Error while opening file!!!");
Book L;
readData(&L); //Reads all fields from keyboard and places in Book. Working properly
writeRegister(arq, &L);
system("pause");
return 0;

}

必须在结构体内部使用这些指针，所以不能删除它们。顺便说一下，我的问题只是那些integers和float。

这个函数只有在我用fprintf()写所有的整数和浮点数(YEAR, PAGES和PRICE)时才能正常工作，但是我把它写在二进制文件中，当然我想用二进制来写，所以我试图使用fwrite()。

另一件事是:编译器将incompatible type for argument 1 of 'fwrite'指向这一行:fwrite(L->PRICE, sizeof(float), 1, arq);

谁能给我解释一下发生了什么事?我的程序在试图写入文件时崩溃了

Answer 1

fwrite的第一个参数需要一个指针

像下面这样的行:

fwrite(L->PAGES, sizeof(int), 1, arq);

应该这样写:

fwrite(&(L->PAGES), sizeof(int), 1, arq);

对于该结构体的YEAR和PRICE成员也是如此

fwrite(&(L->YEAR), sizeof(int), 1, arq);
...
fwrite(&(L->PRICE), sizeof(float), 1, arq);

注意，您不需要对TITLE、PUBLISHER和AUTHOR进行相同的更改，因为这些成员字段的类型已经是指针(char*)。

Answer 2

fwrite的签名为

std::size_t fwrite( const void* buffer, std::size_t size, std::size_t count, std::FILE* stream );

函数的第一个实参必须是指针。我很惊讶你没有得到编译错误的以下行。

fwrite(L->YEAR, sizeof(int), 1, arq);
fwrite(L->PAGES, sizeof(int), 1, arq);
fwrite(L->PRICE, sizeof(float), 1, arq);

它们需要

fwrite(&(L->YEAR), sizeof(int), 1, arq);
fwrite(&(L->PAGES), sizeof(int), 1, arq);
fwrite(&(L->PRICE), sizeof(float), 1, arq);

此外，检查所有IO函数的返回值以确保它们按您的期望工作是一个很好的做法。

if ( fwrite(&(L->YEAR), sizeof(int), 1, arq) != 1 )
{
   // Deal with the error condition.
}

Answer 3

必须将L->YEAR的地址传递给fwrite

fwrite(L->YEAR, sizeof(int), 1, arq);
fwrite(&(L->YEAR), sizeof(int), 1, arq);

Answer 4

以下代码:

1) eliminates the bad practice of typedef'ing a struct
2) removed the mis-leading all caps of the struct field names
3) contains the needed #includes
4) contains the needed prototypes for the external (in another file) functions
5) checks for the worst of the many possible runtime errors
6) replaces the mis-leading 'L' with a meaningful name
7) modifies the file pointer variable to a commonly known name
suggest compiling with all warnings enabled (and fix the warnings)
for gcc, at a minimum, use '-Wall -Wextra -pednatic'

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

struct Book
{
    char *title;
    char *author;
    char *publisher;
    int   year;
    char *language;
    int   pages;
    float price;
};
int reglen( struct Book * );
int readData( struct Book * );

void writeRegister(FILE *fp, struct Book *myBook)
{ //writes in actual file position
    char c = '|';
    int sizeRegWrite = reglen(myBook); //reglen() returns the size of Book
    fwrite(&sizeRegWrite, sizeof(int), 1, fp);
    fwrite(myBook->title, sizeof(char), strlen(myBook->title), fp);
    fwrite(&c, sizeof(char), 1, fp); //writing delimiter
    fwrite(myBook->author, sizeof(char), strlen(myBook->author), fp);
    fwrite(&c, sizeof(char), 1, fp); //writing delimiter
    fwrite(myBook->publisher, sizeof(char), strlen(myBook->publisher), fp);
    fwrite(&c, sizeof(char), 1, fp); //writing delimiter
    fwrite(&(myBook->year), sizeof(int), 1, fp);
    fwrite(&c, sizeof(char), 1, fp); //writing delimiter
    fwrite(myBook->language, sizeof(char), strlen(myBook->language), fp);
    fwrite(&c, sizeof(char), 1, fp); //writing delimiter
    fwrite(&(myBook->pages), sizeof(int), 1, fp);
    fwrite(&c, sizeof(char), 1, fp); //writing delimiter
    fwrite(&(myBook->price), sizeof(float), 1, fp);
    fwrite(&c, sizeof(char), 1, fp); //writing delimiter
    return;
} // end function: writeRegister

int main( void )
{
    FILE *fp = fopen("BD_books2.bin", "rb");
    if(fp == NULL)
    {
        perror( "fopen for BD_books2.bin failed" );
        exit( EXIT_FAILURE );
    }
    // implied else, fopen successful
    struct Book myBook;
    int goodRead = readData(&myBook); //Reads all fields from keyboard and places in Book. Working properly
    if( goodRead )
    {
        writeRegister(fp, &myBook);
    }
    system("pause");
    return 0;
} // end function: main