fwrite() not working to write integer in binary file

Question

Can someone tell my why won\'t this function work? I just can\'t get it...

void writeRegister(FILE *arq, Book *L){ //writes in actual file position
  char          


        

Answer 1

You must pass the address of L->YEAR to fwrite

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

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

Answer 2

the following code:

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

Answer 3

The first parameter of fwrite expects a pointer.

Lines such as the following:

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

Should be written as follows:

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

Sames goes for YEAR and PRICE members of that struct

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

Note, you don't need to make the same change for TITLE, PUBLISHER, and AUTHOR because the type of those member fields are already pointers (char*).

Answer 4

The signature of fwrite is

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

The first argument to the function needs to be a pointer. I am surprised you didn't get compiler errors with the following lines.

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

They need to be

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

Also, it is a good practice to check the return values of all IO functions to make sure that they work as you expect them to.

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