Reading a Binary File....

[swetha]

HI Every1,
I have a problem in reading a binary file.
Actually i want a C program which reads in the data from a file which
is in binary format and i want to update values in it.
The file consists of structures of type----

struct record {
int acountnum;
char name[20];
float value;
char phone[10];
}

I wrote a program which reads in these values into a structure which i
created in my program.While printing the values i got the "name" and
"phone" values correctly ( as they are in char) ....but iam not
getting the accountnum and value fields.Iam getting some arbitary
values.SO can any1 help me out in this of how to retrieve the values
correctly ......

My code is...

struct record{
int acctnum;
char name[20];
float value;
char phone[10];
int age;
};

struct record data;
FILE *file = fopen("db07", "rb");
while ( fread(&data, sizeof (struct record), 1, file) == 1 )
{
printf("Name:%s\nAccount Number :%d\nAge:%d\nPhone:%s\nValue:%f\n",
data.name, data.acctnum,data.age,data.phone,data.value);
}

 

[John Gordon]

I wrote a program which reads in these values into a structure which i
created in my program.While printing the values i got the "name" and
"phone" values correctly ( as they are in char) ....but iam not
getting the accountnum and value fields.Iam getting some arbitary
values.SO can any1 help me out in this of how to retrieve the values
correctly ......

Was the original datafile created on a different computer?

 

[Martin Ambuhl]

HI Every1,
I have a problem in reading a binary file.
Actually i want a C program which reads in the data from a file which
is in binary format and i want to update values in it.
The file consists of structures of type----

struct record {
int acountnum;
char name[20];
float value;
char phone[10];
}

I wrote a program which reads in these values into a structure which i
created in my program.While printing the values i got the "name" and
"phone" values correctly ( as they are in char) ....but iam not
getting the accountnum and value fields.Iam getting some arbitary
values.SO can any1 help me out in this of how to retrieve the values
correctly ......

[OP's code is at EOM]

/* You don't indicate how these files are created. In the example
* below I have added the writing of these files. Note that the
* structures in your file are not the same as those that you are using
* in your program. Also note that for 10-digit phone numbers (or
* 20-character names) your members name and phone are not strings,
* since they are not null terminated */

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

struct file_record
{
int acountnum;
char name[20];
float value;
char phone[10];
};

struct internal_record
{
int acctnum;
char name[20];
float value;
char phone[10];
int age;
};

void show_char_array(const char *label, size_t n, const char *s)
{
size_t i;
printf("%s:", label);
for (i = 0; i < n && s; i++)
putchar(s);
putchar('\n');
}

int main(void)
{
struct file_record make[3] = {
{1, "first guy", 72.3, "3123375032"},
{2, "second guy", 333.72, "2128836234"},
{3, "third guy", -62, "6182213"}
};
struct internal_record data;
FILE *file;
size_t written;

if (!(file = fopen("db07", "wb"))) {
fprintf(stderr, "could not open output file\n");
exit(EXIT_FAILURE);
}
written = fwrite(&make, sizeof *make, 3, file);
printf("%zu records written.\n", written);
fclose(file);

if (!(file = fopen("db07", "rb"))) {
fprintf(stderr, "could not open input file\n");
exit(EXIT_FAILURE);
}
printf("Note that the age member is not present in the input.\n"
"I have set it at 26\n\n");
data.age = 26;
while (fread(&data, sizeof *make, 1, file)) {
show_char_array("Name", 20, data.name);
printf("Account Number :%d\nAge:%d\n", data.acctnum, data.age);
show_char_array("Phone", 10, data.phone);
printf("Value:%f\n\n", data.value);
}
fclose(file);
remove("db07");
return 0;
}

[Output]
3 records written.
Note that the age member is not present in the input.
I have set it at 26

Name:first guy
Account Number :1
Age:26
Phone:3123375032
Value:72.300003

Name:second guy
Account Number :2
Age:26
Phone:2128836234
Value:333.720001

Name:third guy
Account Number :3
Age:26
Phone:6182213
Value:-62.000000


[OP's code]

My code is...

struct record{
int acctnum;
char name[20];
float value;
char phone[10];
int age;
};

struct record data;
FILE *file = fopen("db07", "rb");
while ( fread(&data, sizeof (struct record), 1, file) == 1 )
{
printf("Name:%s\nAccount Number :%d\nAge:%d\nPhone:%s\nValue:%f\n",
data.name, data.acctnum,data.age,data.phone,data.value);
}

 

[Eric Sosman]

swetha wrote On 11/19/07 16:24,:

HI Every1,
I have a problem in reading a binary file.
Actually i want a C program which reads in the data from a file which
is in binary format and i want to update values in it.
The file consists of structures of type----

struct record {
int acountnum;
char name[20];
float value;
char phone[10];
}

I wrote a program which reads in these values into a structure which i
created in my program.While printing the values i got the "name" and
"phone" values correctly ( as they are in char) ....but iam not
getting the accountnum and value fields.Iam getting some arbitary
values.SO can any1 help me out in this of how to retrieve the values
correctly ......

My code is...

struct record{
int acctnum;
char name[20];
float value;
char phone[10];
int age;
};

struct record data;
FILE *file = fopen("db07", "rb");
while ( fread(&data, sizeof (struct record), 1, file) == 1 )
{
printf("Name:%s\nAccount Number :%d\nAge:%d\nPhone:%s\nValue:%f\n",
data.name, data.acctnum,data.age,data.phone,data.value);
}

Any of several things -- or a combination -- could be
the problem. Here are a few areas you should probably take
a look at:

1) Why does your program try to read an "age" element
that's not mentioned in your description of the file format?
If you read into a struct whose layout doesn't match that of
the file, you can't expect anything useful to happen.

2) Was the file written on a system compatible with the
one that's trying to read it? Systems differ in many ways,
such as the way numbers (int, float) are represented, the
amount and placement of padding within structs, and so on.
Padding differences would produce problems like those of (1),
and representation differences (e.g., "endianness") can be
even thornier: Something that means 123 to one machine might
easily mean -2231369728 to another.

3) The "name" and "phone" elements: Are all 20 or 10
characters actually present in the file, even if the strings
are as short as "Zaphod" and "n/a"? Or does the file hold
only the "significant" characters? (And see Martin Ambuhl's
warning about the possible non-string-ness of these items.)

A hex dump of the first couple hundred bytes of the file,
along with what they're "supposed to" represent and what you
actually get from your program, might be helpful.

 

[swetha]

HI Every1,
I have a problem in reading a binary file.
Actually i want a C program which reads in the data from a file which
is in binary format and i want to update values in it.
The file consists of structures of type----

struct record {
int acountnum;
char name[20];
float value;
char phone[10];
}

I wrote a program which reads in these values into a structure which i
created in my program.While printing the values i got the "name" and
"phone" values correctly ( as they are in char) ....but iam not
getting the accountnum and value fields.Iam getting some arbitary
values.SO can any1 help me out in this of how to retrieve the values
correctly ......

[OP's code is at EOM]

/* You don't indicate how these files are created. In the example
* below I have added the writing of these files. Note that the
* structures in your file are not the same as those that you are using
* in your program. Also note that for 10-digit phone numbers (or
* 20-character names) your members name and phone are not strings,
* since they are not null terminated */

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

struct file_record
{
int acountnum;
char name[20];
float value;
char phone[10];

};

struct internal_record
{
int acctnum;
char name[20];
float value;
char phone[10];
int age;

};

void show_char_array(const char *label, size_t n, const char *s)
{
size_t i;
printf("%s:", label);
for (i = 0; i < n && s; i++)
putchar(s);
putchar('\n');

}

int main(void)
{
struct file_record make[3] = {
{1, "first guy", 72.3, "3123375032"},
{2, "second guy", 333.72, "2128836234"},
{3, "third guy", -62, "6182213"}
};
struct internal_record data;
FILE *file;
size_t written;

if (!(file = fopen("db07", "wb"))) {
fprintf(stderr, "could not open output file\n");
exit(EXIT_FAILURE);
}
written = fwrite(&make, sizeof *make, 3, file);
printf("%zu records written.\n", written);
fclose(file);

if (!(file = fopen("db07", "rb"))) {
fprintf(stderr, "could not open input file\n");
exit(EXIT_FAILURE);
}
printf("Note that the age member is not present in the input.\n"
"I have set it at 26\n\n");
data.age = 26;
while (fread(&data, sizeof *make, 1, file)) {
show_char_array("Name", 20, data.name);
printf("Account Number :%d\nAge:%d\n", data.acctnum, data.age);
show_char_array("Phone", 10, data.phone);
printf("Value:%f\n\n", data.value);
}
fclose(file);
remove("db07");
return 0;

}

[Output]
3 records written.
Note that the age member is not present in the input.
I have set it at 26

Name:first guy
Account Number :1
Age:26
Phone:3123375032
Value:72.300003

Name:second guy
Account Number :2
Age:26
Phone:2128836234
Value:333.720001

Name:third guy
Account Number :3
Age:26
Phone:6182213
Value:-62.000000

[OP's code]

My code is...

struct record{
int acctnum;
char name[20];
float value;
char phone[10];
int age;
};

struct record data;
FILE *file = fopen("db07", "rb");
while ( fread(&data, sizeof (struct record), 1, file) == 1 )
{
printf("Name:%s\nAccount Number :%d\nAge:%d\nPhone:%s\nValue:%f\n",
data.name, data.acctnum,data.age,data.phone,data.value);
}- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

Hi.....actually i have forgot age in the previous structre...age is
present in both....and there is no need to put data into that
file.....already i have a file which consists of data in the binary
format.....i wanted to read that data ...while doing so i could easily
i could retrieve the char values correctly but i have a problem with
the float and int values .....for float and int what should i do ???
i dont have any idea wher that file is from ..is ther a way to know
that ???

 

[Ben Bacarisse]

HI Every1,
I have a problem in reading a binary file.
Actually i want a C program which reads in the data from a file which
is in binary format and i want to update values in it.
The file consists of structures of type----

struct record {
int acountnum;
char name[20];
float value;
char phone[10];
}

.i wanted to read that data ...while doing so i could easily
i could retrieve the char values correctly but i have a problem with
the float and int values .....for float and int what should i do ???
i dont have any idea wher that file is from ..is ther a way to know
that ???

You need to look at the data and see what format it is in. A hex dump
program like od or hd (on *nix systems) will show you exactly what is
in the file. There are so many possible formats that guessing is
probably pointless. Posting a small extract in hex somewhere topical
(maybe comp.programming) might help.

 

[santosh]

HI Every1,
I have a problem in reading a binary file.
Actually i want a C program which reads in the data from a file which
is in binary format and i want to update values in it.

If this file has been created on a different system to the one in which
it is being read, or if a different program has created it than the one
doing the reading, or if your program does not know the exact details
of the file's format, subtle errors may occur.

This is why it is often better to deal with text files unless there are
compelling reasons for doing otherwise. For many programs on modern
desktop systems, which need not be performance critical, it is just
fine to store data in text files.

The file consists of structures of type----

struct record {
int acountnum;
char name[20];
float value;
char phone[10];
}

You are missing a semi-colon to terminate the structure declaration.
Please copy and paste code. Retyping has been proven, time and again,
to result in annoying typos and incomplete code.

I wrote a program which reads in these values into a structure which i
created in my program. While printing the values i got the "name" and
"phone" values correctly ( as they are in char) ....but iam not
getting the accountnum and value fields.Iam getting some arbitary
values.SO can any1 help me out in this of how to retrieve the values
correctly ......

My code is...

struct record{
int acctnum;
char name[20];
float value;
char phone[10];
int age;
};

Which structure is the one you are actually using? This one or the one
previous? This once again illustrates why code should be, wherever
possible, cut and pasted.

struct record data;
FILE *file = fopen("db07", "rb");

You might use a name other than 'file' for readability reasons. Have two
identifiers differring only in case, though legal in C, is not a very
good programming practise. And it might lead to conflicts if the
identifiers happen to have external linkage _and_ you happen to use a
very old or primitive linker. Better to be safe and avoid the
possibility altogether.

while ( fread(&data, sizeof (struct record), 1, file) == 1 )
{
printf("Name:%s\nAccount Number :%d\nAge:%d\nPhone:%s\nValue:%f\n",
data.name, data.acctnum,data.age,data.phone,data.value);
}

A likely possibility is that the data file was created on a system other
than the one it is under now, or that it was created by a program
compiled with certain compiler options that this program (i.e., the one
reading the file) has not taken into account.

If it is indeed one of these possibilities then you need to know the
exact layout of the file's structure objects before you can read it in
correctly.

 

[Keith Thompson]

I have a problem in reading a binary file.
Actually i want a C program which reads in the data from a file which
is in binary format and i want to update values in it.
The file consists of structures of type----

struct record {
int acountnum;
char name[20];
float value;
char phone[10];
}

I wrote a program which reads in these values into a structure which i
created in my program.While printing the values i got the "name" and
"phone" values correctly ( as they are in char) ....but iam not
getting the accountnum and value fields.Iam getting some arbitary
values.SO can any1 help me out in this of how to retrieve the values
correctly ......

My code is...

struct record{
int acctnum;
char name[20];
float value;
char phone[10];
int age;
};

struct record data;
FILE *file = fopen("db07", "rb");
while ( fread(&data, sizeof (struct record), 1, file) == 1 )
{
printf("Name:%s\nAccount Number :%d\nAge:%d\nPhone:%s\nValue:%f\n",
data.name, data.acctnum,data.age,data.phone,data.value);
}

You've shown us two different struct declaration. (Is it spelled
"acountnum" or "acctnum"? Is there an "age" member or not? We don't
care one way or the other, but you need to.) As santosh advised in
another followup, you should *always* copy-and-paste your exact code.
If you attempt to re-type it or summarize it, you'll inevitably
introduce errors that will mask whatever your real problem is. Don't
make us waste our time correcting your typos rather than helping you
with your actual code.

Reading binary files is tricky, since binary formats (sizes of types,
representations of floating-point types, byte ordering, structure
layout, and a bunch of other things) can vary widely from one system to
another.

The comp.lang.c is at <http://www.c-faq.com/>. Read section 12,
particularly the questions containing the word "binary".

If the file you're trying to read was written on a different system, you
may have a lot of difficulty reading it. It may be possible to analyze
the file using hex dumps, and figure out how to do any necessary
translation, but it's a tough job.

If you can guarantee that the writer and reader programs will run on the
same or identical systems, you might be ok. But in that case the first
thing you should do is write a separate program to write some sample
records to a file, to make sure you can read them.

You use the "%s" format to print the name and phone members. This only
works if they contain proper strings with null-character ('\0')
terminators. Is this guaranteed?

If you can use a text format rather than binary, sharing these file
across different systems is going to be a lot easier. Is that a
possibility?

 

[spacecriter \(Bill C\)]

HI Every1,
I have a problem in reading a binary file.
Actually i want a C program which reads in the data from a file which
is in binary format and i want to update values in it.
The file consists of structures of type----

struct record {
int acountnum;
char name[20];
float value;
char phone[10];
}

I wrote a program which reads in these values into a structure which i
created in my program.While printing the values i got the "name" and
"phone" values correctly ( as they are in char) ....but iam not
getting the accountnum and value fields.Iam getting some arbitary
values.SO can any1 help me out in this of how to retrieve the values
correctly ......

My code is...

struct record{
int acctnum;
char name[20];
float value;
char phone[10];
int age;
};

struct record data;
FILE *file = fopen("db07", "rb");
while ( fread(&data, sizeof (struct record), 1, file) == 1 )
{
printf("Name:%s\nAccount Number :%d\nAge:%d\nPhone:%s\nValue:%f\n",
data.name, data.acctnum,data.age,data.phone,data.value);
}

You should be using write to output binary values. printf converts the
output to characters.

 

[Willem]

swetha wrote:
) I wrote a program which reads in these values into a structure which i
) created in my program.While printing the values i got the "name" and
) "phone" values correctly ( as they are in char) ....but iam not
) getting the accountnum and value fields.Iam getting some arbitary
) values.SO can any1 help me out in this of how to retrieve the values
) correctly ......
)
) My code is...
)
) struct record{
) int acctnum;
) char name[20];
) float value;
) char phone[10];
) int age;
) };
)
) struct record data;
) FILE *file = fopen("db07", "rb");
) while ( fread(&data, sizeof (struct record), 1, file) == 1 )
) {
) printf("Name:%s\nAccount Number :%d\nAge:%d\nPhone:%s\nValue:%f\n",
) data.name, data.acctnum,data.age,data.phone,data.value);
) }

In C, a struct might be padded. That is, there may be some empty space
between, say, 'name' and 'value'. If you want maximal robustness in
reading a file, you should be reading it element by element instead.

Also, and presumably the cause of your problem, there could be an endianess
issue. Can you give some examples of expected and actual values ?


SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

 

[Keith Thompson]

spacecriter (Bill C) wrote:
[...]

You should be using write to output binary values. printf converts the
output to characters.

You probably mean (or you *should* mean) fwrite rather than write. The
write function is non-standard (it's defined by POSIX, not by the C
standard).

Both fwrite and printf are defined in terms of character output. It
*usually* makes more sense to use fwrite for binary files and printf (or
fputs, etc) for text files, but both will work equally well for either
text or binary.

 

[Old Wolf]

Both fwrite and printf are defined in terms of character output. It
*usually* makes more sense to use fwrite for binary files and printf (or
fputs, etc) for text files, but both will work equally well for either
text or binary.

If you have a block of "binary data", that could contain
null characters, then the printf family isn't a suitable
choice for output.

 

[CBFalconer]

swetha wrote:
.... snip ...

You should be using write to output binary values. printf
converts the output to characters.

Why? He is simply making the output legible on other systems.
Text is portable, binary isn't.

 

[Keith Thompson]

If you have a block of "binary data", that could contain
null characters, then the printf family isn't a suitable
choice for output.

Not if you use the "%s" format, but this:
printf("%c", '\0');
is well defined.

But you're right, printf wouldn't be a *suitable* choice, even though you could
make it work with some effort.

 

[Barry Schwarz]

HI Every1,
I have a problem in reading a binary file.
Actually i want a C program which reads in the data from a file which
is in binary format and i want to update values in it.
The file consists of structures of type----

struct record {
int acountnum;
char name[20];
float value;
char phone[10];
}

I wrote a program which reads in these values into a structure which i
created in my program.While printing the values i got the "name" and
"phone" values correctly ( as they are in char) ....but iam not
getting the accountnum and value fields.Iam getting some arbitary
values.SO can any1 help me out in this of how to retrieve the values
correctly ......

My code is...

struct record{
int acctnum;
char name[20];
float value;
char phone[10];
int age;
};

struct record data;
FILE *file = fopen("db07", "rb");
while ( fread(&data, sizeof (struct record), 1, file) == 1 )
{
printf("Name:%s\nAccount Number :%d\nAge:%d\nPhone:%s\nValue:%f\n",
data.name, data.acctnum,data.age,data.phone,data.value);
}

You should be using write to output binary values. printf converts the
output to characters.

He is not using printf to output binary values. He is using it to
print data (suitably converted to readable characters) that he read
from a binary file.

Since write is not part of the standard library, fwrite would be a
better recommendation in this group if he wanted to output binary
data.


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