Writing structure variables into a file, problem

Hi I have to write the contents of a structure variable into a file. I have a working program but the output looks distorted, you can understand when you look at the output. The simple version of the code is as below and the outputs follows.

Code:

#include<stdio.h>
#include <stdlib.开发者_Python百科h>

struct mystruct
{
 char xx[20];
 char yy[20];
 int zz;
};

void filewrite(struct mystruct *myvar)
{
   FILE *f;

   f = fopen("trace.bin","wb");
   if (f == NULL)
   {
     printf("\nUnable to create the file");
     exit(0);
   }

   fwrite(myvar, sizeof(struct mystruct), 1, f);
   fclose(f);

}


void main()
{
    struct mystruct myvar = {"Rambo 1", "Rambo 2", 1234};

    filewrite(&myvar);

}

Output:

(1. Where is the integer '1234'? I need that intact.)

(2. Why does some random character appear here?)

trace.bin

Rambo 1Rambo 2Ò

Your program is correct and the output too...

Your are writing a binary file containing the raw data from memory. The integer zz gets written to disk as 4 bytes (or 2 depending on the size of an int on your system), with the least significant byte first (Intel machine I guess).

1234 (decimal) gets written as 0xD2, 0x04, 0x00, 0x00 to disk. 0xD2 is a Ò when you look at in text form. The 0x04 and the 0x's are non-printable characters so they don't show.

First, in general it's not a good practice to copy non-packed struct-types to files since the compiler can add padding to the struct in order to align it in memory. Thus you will end up with either a non-portable implementation, or some garbled output where someone else tries to read your file, and the bits/bytes are not placed at the correct offset because of the compiler's padding bytes.

Second, I'm not sure how you are reading your file back (it appears you just copied it into a buffer and tried to print that), but the last set of bytes is an int type at the end ... it's not going to be a null-terminated string, so the manner in which it prints will not look "correct" ... printing non-null-terminated strings as strings can also lead to buffer overflows resulting in segmentation faults, etc.

In order to read back the contents of the file in a human-readable format, you would need to open the file and read contents back into the correct data-structures/types, and then appropriately call printf or some other means of converting the binary data to ASCII data for a print-out.

I don't recommend dumping memory directly into file, you should use some serialization method (e.x if you have pointer in the struct, you are doomed). I recommend Google Buffers Protocol if data will be shared between multiple applications.