Writing a structure into a file

I am using the following code

typedef struct A {
int x;
int y;
int z;
};

int main(void) {

struct A A;
struct A B;
FILE *fp;
A.x = 80000;
A.y = 40000;
A.z = 12345;

printf("%d %d %d\n", A.x, A.y, A.z);
fp = fopen("file.txt", "wb");
fwrite(&A, sizeof A, 3, fp);
fclose(fp);
fp = fopen("file.txt", "rb");
fread(&B, sizeof B, 1, fp);
fclose(fp);
printf("%d %d %d\n", B.x, B.y, B.z);
return 0;
}


But the file.txt is containing junk values when i open it . Please let
me know what is wrong in my program.

Thanks,
Zaheer

re: Writing a structure into a file

zaheer031@gmail.com wrote:
Don't forget <stdio.h>
What's the (3) for?
You don't have three A's


--
pete

re: Writing a structure into a file

On Jun 24, 7:36 pm, pete <pfil...@mindspring.comwrote:Sorry for that it should have been 1

re: Writing a structure into a file

zaheer031@gmail.com wrote:Either provide a name for the "new type", or (better, I think) don't
typedef.

typedef <somethingNewTypeName;

Your <somethingis the whole struct A above. You do not have a name
for the 'new type'.
My compiler emitted a warning for the construction without a name for
the typedef:
"warning: useless storage class specifier in empty declaration"

After removing the typedef, keeping only "struct A { ... }" and
#includeing <stdio.h>, the compilation worked fine and the program
output was the expected one ... but see below
What if "file.txt" cannot be opened? You should test fp.
if (fp == NULL) { /* file.txt could not be opened for writing */ }
Now you're trying to access 3 elements of size "sizeof A".
You have no guarantee that the second element (or the third) can be
read.
Verify that the fopen() call didn't fail.
What do you mean 'junk values'?
If your program didn't fail at writing 3 (inexistent) elements at the
fwrite() call, file.txt will have 3 * sizeof(struct A) bytes.
On my computer that is 3 * 12 bytes = 36 bytes.
ON MY COMPUTER, the first 12 bytes of the file correspond to the
members x, y, and z.
I have absolutely no idea what the other 24 bytes are.
1) Failure to #include <stdio.h>
2) accessing inexistent data
3) writing more than you'd like to the file
4)? Misinterpretation of 'junk values'?

Why don't you write to the file in text mode using fprintf? and read
with fgets()?

fprintf(fp, "struct A: x = %d; y = %d; z = %d\n", A.x, A.y, A.z);

and

#define MAXLINE_LENGTH 80

char buf[MAXLINE_LENGTH];

if (fgets(buf, sizeof buf, fp) != NULL) {
/* parse buf to extract B.x, B.y, and B.z */
}

re: Writing a structure into a file

zaheer031@gmail.com wrote:My second problem is that this is the output on my machine:

C:\Program Files\DevStudio\SharedIDE\bin\Debug>new
80000 40000 12345
80000 40000 12345

C:\Program Files\DevStudio\SharedIDE\bin\Debug>

Intermittent problems tend to be a little tougher to diagnose.

I convert the program to a correct C program and see what turns up.

--
pete

re: Writing a structure into a file

badc0de4@gmail.com wrote:That's the first thing that popped into my mind.

/* BEGIN new.c */

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

int main(void)
{
struct A {
int x;
int y;
int z;
};
struct A A;
struct A B = {0};
FILE *fp;

A.x = 80000;
A.y = 40000;
A.z = 12345;
printf("%d %d %d\n", A.x, A.y, A.z);
fp = fopen("file.txt", "w");
if (fp == NULL) {
puts("fp == NULL #1");
exit(EXIT_FAILURE);
}
fprintf(fp, "%d %d %d\n", A.x, A.y, A.z);
fclose(fp);
fp = fopen("file.txt", "r");
if (fp == NULL) {
puts("fp == NULL #2");
exit(EXIT_FAILURE);
}
if (fscanf(fp, "%d %d %d", &B.x, &B.y, &B.z) != 3) {
puts("fscanf problem");
fclose(fp);
exit(EXIT_FAILURE);
}
fclose(fp);
printf("%d %d %d\n", B.x, B.y, B.z);
return 0;
}

/* END new.c */

--
pete

re: Writing a structure into a file

zaheer031@gmail.com wrote:This indicates that you've included <stdio.h>, which is a
good thing, but that you've omitted the inclusion from what
you've posted. What else did you leave out? Is the code I'm
trying to debug in any way to the code that's actually giving
you problems?
Did the fopen() call succeed? Probably, or you'd most
likely see a different sort of error. Still, it *never* hurts
to check.
Even under the most favorable of circumstances, your program
goes off the rails here and its behavior becomes unpredictable.
You have asked fwrite() to output the contents of three consecutive
`struct A' instances, but only one instance exists at the memory
location you've indicated. When fwrite() trustingly tries to output
the other two, anything can happen.

Oh, yes: Did the fwrite() succeed or fail, or succeed partially?
Again, it wouldn't hurt to check ...
Success? Failure?
Success? Failure?
Success? Failure? This time, at least, you're not asking for
more data than you've got memory to store it in.
Fix the obvious blunder in fwrite(), add success/failure
checks, re-run, and come back again if you're still having
trouble.

--
Eric.Sosman@sun.com

re: Writing a structure into a file

pete wrote:Is it the case that by "when i open it"
you are referring to a text editor,
and not the output of your program?

If so, then that's kind of what binary mode is all about.
There's no way that only knowing this program's source code,
will tell you which bytes are going to wind up in that file.

--
pete

re: Writing a structure into a file

On Jun 24, 7:13 pm, "zaheer...@gmail.com" <zaheer...@gmail.comwrote:
with stdio.h included, I am getting the output you expected.
80000 40000 12345
80000 40000 12345
Your issue is already addressed. Your code appears to be broken in
more than one ways but the issue of seeing junk is that you are using
a text editor to analyze the file. Use a binary/hexadecimal editor and
you can check that you are getting the right values and some junk
because of your 3 in fwrite. (If you haven't used any binary/hex
editor, it will be all greek and latin with all the endian-ness and
stuff to worry about)

My compiler warns about your typedef:

bin.c:10: warning: useless storage class specifier in empty
declaration

Using a static code checker can let you know other issues which you
are ignoring ( not checking the return value for fopen, fread, fwrite
etc). In my case, lint has got this to say.

Splint 3.1.1 --- 19 Jul 2006

bin.c: (in function main)
bin.c:25:28: Possibly null storage fp passed as non-null param:
fwrite (..., fp)
A possibly null pointer is passed as a parameter corresponding to a
formal
parameter with no /*@null@*/ annotation. If NULL may be used for
this
parameter, add a /*@null@*/ annotation to the function parameter
declaration.
(Use -nullpass to inhibit warning)
bin.c:24:8: Storage fp may become null
bin.c:25:3: Return value (type size_t) ignored: fwrite(&A, sizeo...
Result returned by function call is not used. If this is intended,
can cast
result to (void) to eliminate message. (Use -retvalother to inhibit
warning)
bin.c:26:3: Return value (type int) ignored: fclose(fp)
Result returned by function call is not used. If this is intended,
can cast
result to (void) to eliminate message. (Use -retvalint to inhibit
warning)
bin.c:28:27: Possibly null storage fp passed as non-null param: fread
(..., fp)
bin.c:27:8: Storage fp may become null
bin.c:28:3: Return value (type size_t) ignored: fread(&B, sizeof...
bin.c:29:3: Return value (type int) ignored: fclose(fp)


lint is not related to the language C, but the output I have posted is
related to your C problem.