Writing to a Binary file in C, interpreting output

Question

I'm trying to test writing to a binary file in c, and just want to make sense of my output.

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

void writeToFile();

int main(void) {

    writeToFile();

    return(0);
}

void writeToFile() {
    FILE * file = fopen("file.bin","w");
    char * string;

    if(file == NULL)
        printf("Problem with file\n");

    string = malloc(sizeof(char)*6);

    strcpy(string,"Hello");

    fwrite(&string,sizeof(string),strlen(string)+1,file);

    fclose(file);

}

I'm interpreting my results using the commands:

od -c file.bin

Which displays octal output. And gives me this:

0000000   @ 022   # 001  \0  \0  \0  \0 020 020   # 001  \0  \0  \0  \0                                                                                         
0000020 300 016 374   ? 377 177  \0  \0 264 006   @  \0  \0  \0  \0  \0                                                                                         
0000040   @  \a   @  \0  \0  \0  \0  \0 200 365   c 274 020 177  \0  \0                                                                                         
0000060

I'm not sure how to interpret this output, I know it's in octal, but how do I know my string "Hello" was written correctly?

I was thinking I could convert the output to ascii using an ascii table, but I'm not sure if that's doable here? Is there a simple way I can check if string "Hello" was written correctly?

Maybe I could read the output back in, and somehow check if the string "Hello" exists within it?

Any help would be much appreciated.

You are passing the address of 'string' to fwrite. You should pass its value. — Mick
– Mick, Commented Sep 20, 2016 at 23:45
It might be easier to display the output in hex (I use od -Ax -t x1 — M.M
– M.M, Commented Sep 20, 2016 at 23:54
You are writing the pointer string (not the thing it points to, which is the string), and a bunch of garbage that happens to be after it. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Sep 20, 2016 at 23:54
You are supposed to open your file with mode wb instead of w if you wish to have complete control of what is read and written. Using fopen() with w only is not suitable for binary mode. — Havenard
– Havenard, Commented Sep 20, 2016 at 23:58

Ed Heal · Accepted Answer · 2016-09-20 23:53:19Z

3

The line

 fwrite(&string,sizeof(string),strlen(string)+1,file);

should be

 fwrite(string, strlen(string)+1, 1, file);

Because

You already have a pointer to the item - do not need the pointer to that pointer - hence string and not &string
The size of the item is strlen(string) +
You have one item

This is why you are getting output that does not make sense as it is random stuff in memory.

EDIT

ALso

if(file == NULL)
    printf("Problem with file\n");

should be

if(file == NULL) {
    printf("Problem with file\n");
    return;
 }

as there is little that can be done with a null file in the rest of the function

edited Sep 20, 2016 at 23:53

answered Sep 20, 2016 at 23:45

Ed Heal

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4 Comments

eefefefe Over a year ago

Oh okay, and if i were to have a struct that has a char *, I would write it the same way? Also what does the first column of numbers represent?

Ed Heal Over a year ago

If you have a struct you would need to serialise it in some way (google serialisation). Why not read the manual page for fwrite to find out about the numbers

eefefefe Over a year ago

Care to link a page about serialization for structs in regards to C? Googling it myself seems to yield minimal results.

Ed Heal Over a year ago

encrypted.google.com/search?q=serialization+struct+C+example seems to have a few good starting points

Openeye · Accepted Answer · 2016-09-20 23:50:42Z

1

To write to file in binary you should use the "wb" option, for write byte.

FILE * file = fopen("file.bin","wb");

As for knowing if the file is written correctly, the bulletproof way, (allthough probably it exits a better one) is simply to read the file back in with "rb" and print it in normal format.

answered Sep 20, 2016 at 23:50

Openeye

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