write a binary file?

[cylin]

Dear all,

I open a binary file and want to write 0x00040700 to this file.
how can I set write buffer?
---------------------------------------------------
typedef unsigned char UCHAR;
int iFD=open(szFileName,O_CREAT|O_BINARY|O_TRUNC|O_WRONLY,S_IREAD|S_IWRITE);
UCHAR buffer[5]; //???????????
write(iFD,buffer,5);
---------------------------------------------------

Thanks.

Regards,
cylin.

 

[Richard Bos]

I open a binary file and want to write 0x00040700 to this file.
how can I set write buffer?

Not like this, in ISO C. What you've got is system-specific, either
POSIX or not-quite-POSIX-M$.

typedef unsigned char UCHAR;
Yeugh.

int iFD=open(szFileName,O_CREAT|O_BINARY|O_TRUNC|O_WRONLY,S_IREAD|S_IWRITE);

Hungarian notation. Double yeugh. HN used to declare what everybody
reading your code, including the compiler, already knows: the types of
your identifiers. Triple yeugh, and go to the self-confidence shop and
buy some.

UCHAR buffer[5]; //???????????

Mixing declarations and executable statements is legal in C99 and C++,
but not in C89. Beware the snark.

write(iFD,buffer,5);

No idea how to solve this using your low-level, unlikely-to-port
functions. In _real_ C, you'd do something like this:

#include <stdio.h>

unsigned char buf[5];
FILE *outfile;

if (!(outfile=fopen(filename, "wb")) {
puts("This is the place where you'd handle a file open error.");
} else {
if (fwrite(buf, sizeof *buf, sizeof buf/sizeof *buf, outfile) !=
sizeof buf/sizeof *buf) {
puts("Handle a write error here.");
}
/* Or, since you know that buf is an unsigned char array and
therefore sizeof *buf must be 1:
fwrite(buffer, 1, sizeof buf, outfile);
*/
fclose(outfile);
/* For critical applications, you should even check the return value
of fclose(), but I rarely do this. */
}

Issa dat si'ple.

Richard

 

[Irrwahn Grausewitz]

I open a binary file and want to write 0x00040700 to this file.
how can I set write buffer?

Do you want to
[1] write bytes in exactly the order you gave above (portable
result), or
[2] write an unsigned long value to the file (non-portable
result)?

---------------------------------------------------
typedef unsigned char UCHAR;
int iFD=open(szFileName,O_CREAT|O_BINARY|O_TRUNC|O_WRONLY,S_IREAD|S_IWRITE);
UCHAR buffer[5]; //???????????
write(iFD,buffer,5);
---------------------------------------------------

Neither open nor write are standard C functions.
What you want is something like:

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

int main( void )
{
unsigned char buf[] = { 0x00, 0x04, 0x07, 0x00 };
unsigned long ul = 0x00040700UL;
FILE *fp;

if ( ( fp = fopen( "foo", "wb" ) ) != NULL )
{
fwrite( buf, sizeof buf, 1, fp ); /* [1] */
fwrite( &ul, sizeof ul, 1, fp ); /* [2] */
fclose( fp );
return EXIT_SUCCESS;
}
return EXIT_FAILURE;
}

HTH
Regards

 

[Martin Dickopp]

Dear all,

I open a binary file and want to write 0x00040700 to this file.
how can I set write buffer?

Use shifts and mask operations (bitwise AND) to extract the individual
bytes. The details depend on the byte order in which you want to write
to the file.

No such function in standard C. Use `fopen'.

UCHAR buffer[5]; //???????????
write(iFD,buffer,5);

No such function in standard C. Use `fwrite'.

---------------------------------------------------

This program should give you same hints:


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

int main (void)
{
const unsigned long value = 0x00040700;
unsigned char buffer [4];
FILE *f;

/* This as known as "big-endian" byte order. */
buffer [0] = value >> 24;
buffer [1] = (value >> 16) & 0xFF;
buffer [2] = (value >> 8) & 0xFF;
buffer [3] = value & 0xFF;

f = fopen ("testfile", "wb");
if (f != NULL)
{
if (fwrite (buffer, sizeof *buffer, sizeof buffer, f) < sizeof buffer
|| fclose (f) == EOF)
{
fputs ("Error writing to testfile.\n", stderr);
return EXIT_FAILURE;
}
}
else
{
fputs ("Cannot open testfile for writing.\n", stderr);
return EXIT_FAILURE;
}

return 0;
}


Martin

 

[Martin Dickopp]

if (fwrite(buf, sizeof *buf, sizeof buf/sizeof *buf, outfile) !=
sizeof buf/sizeof *buf) {
puts("Handle a write error here.");
}
fclose(outfile);
/* For critical applications, you should even check the return value
of fclose(), but I rarely do this. */

Why do you check the return value of `fwrite' then? Most operating
systems don't write immediately to the underlying device when `fwrite'
is called, but have some buffering mechanism. Therefore, an error is
far more likely to show up in `fclose' than in `fwrite' on such systems.

Martin

 

[Martin Dickopp]

if (fwrite (buffer, sizeof *buffer, sizeof buffer, f) < sizeof buffer

That's inconsistent. Make that:

if (fwrite (buffer, 1, sizeof buffer, f) < sizeof buffer

Martin

 

[cylin]

Great. Thank all.

I use low-level I/O functions because I want to the speed faster than
stardard I/O functions.
Can't low-level I/O functions do this case?

Regards,
cylin.

 

[Martin Dickopp]

I use low-level I/O functions because I want to the speed faster than
stardard I/O functions.
Can't low-level I/O functions do this case?

They can, but they're off-topic in comp.lang.c, which is only about
standard C.

<OT>
They are also harder to use correctly. Note, e.g., that it is not
necessarily an indication of error if the POSIX function `write' writes
less bytes than requested.
</OT>

Martin

 

[Joona I Palaste]

Great. Thank all.

I use low-level I/O functions because I want to the speed faster than
stardard I/O functions.
Can't low-level I/O functions do this case?

First of all, your "low-level I/O functions" might not even be available
on all platforms. Second of all, there is no guarantee they will be any
faster than standard I/O functions. They could even be slower.

 

[CBFalconer]

I open a binary file and want to write 0x00040700 to this file.
how can I set write buffer?

Use shifts and mask operations (bitwise AND) to extract the
individual bytes. The details depend on the byte order in which
you want to write to the file.
.... snip ...

This program should give you same hints:

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

int main (void)
{
const unsigned long value = 0x00040700;
unsigned char buffer [4];
FILE *f;

/* This as known as "big-endian" byte order. */
buffer [0] = value >> 24;
buffer [1] = (value >> 16) & 0xFF;
buffer [2] = (value >> 8) & 0xFF;
buffer [3] = value & 0xFF;

f = fopen ("testfile", "wb");
if (f != NULL)
{
if (fwrite (buffer, sizeof *buffer, sizeof buffer, f) < sizeof buffer
|| fclose (f) == EOF)
{
fputs ("Error writing to testfile.\n", stderr);
return EXIT_FAILURE;
}
}
else
{
fputs ("Cannot open testfile for writing.\n", stderr);
return EXIT_FAILURE;
}

return 0;
}

There is nothing wrong with your code above, and it illustrates
most things admirably, I suggest that the use of embedded tests
will facilitate a clearer order of things. This is a style
question, not a flame, and just a suggestion. My version follows:

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

int main (void)
{
const unsigned long value = 0x00040700;
unsigned char buffer [4];
FILE *f;

/* This as known as "big-endian" byte order. */
buffer [0] = value >> 24;
buffer [1] = (value >> 16) & 0xFF;
buffer [2] = (value >> 8) & 0xFF;
buffer [3] = value & 0xFF;

if (!(f = fopen("testfile", "wb"))) {
fputs("Cannot open testfile for writing.\n", stderr);
}
else if ((fwrite(buffer, sizeof *buffer, sizeof buffer, f)
< sizeof buffer)
|| (EOF == fclose(f))) {
fputs("Error writing to testfile.\n", stderr);
}
else {
return 0;
}
return EXIT_FAILURE;
}

although the neophyte is still going to have trouble comprehending
the combined fwrite/fclose failure test. I have also added the
odd extraneous parentheses to make the meanings explicit.

This now follows the pattern "if phasefails exit else nextphase".

--
Some useful references:
<http://www.ungerhu.com/jxh/clc.welcome.txt>
<http://www.eskimo.com/~scs/C-faq/top.html>
<http://benpfaff.org/writings/clc/off-topic.html>
<http://anubis.dkuug.dk/jtc1/sc22/wg14/www/docs/n869/> (C99)

 

[Dan Pop]

fclose(outfile);
/* For critical applications, you should even check the return value
of fclose(), but I rarely do this. */

Bad idea: most of the things that can go wrong (after fopen succeeded)
happen at fclose time.

To optimise the error checking, when I generate an output file in a
compact succession of operations, I only check fclose and a fflush call
immediately preceding it (which is probably redundant).

OTOH, long running programs that generate output constantly (e.g. one
printf call per main loop iteration) should check each output call,
even if it's on stdout (that gets redirected to a file when the program
is run in non-interactive mode). No point in continuing the execution
if the program can no longer generate output.

Dan

 

[Dan Pop]

I use low-level I/O functions because I want to the speed faster than
stardard I/O functions.

Where did you get this silly idea from? The low-level I/O functions have
a relatively high cost and the stardard I/O functions are designed to
minimise the number of low-level I/O function calls, by doing some local
buffering.

As a result of this, it is trivially easy for the ignorant to slow down
his I/O by one order of magnitude, while an expert can only speed up his
I/O by a small percentage, by using the low-level functions instead of the
standard ones and removing one level of buffering, when and where it is
not necessary.

Can't low-level I/O functions do this case?

Why bother?

Dan

 

[kal]

Use shifts and mask operations (bitwise AND) to extract the individual

bytes. The details depend on the byte order in which you want to write
to the file.

IMHO one is better off using "htnl()." This will work fine so long as
all you are writing are 32 bit values.

 

[Christopher Benson-Manica]

IMHO one is better off using "htnl()." This will work fine so long as
all you are writing are 32 bit values.

I suspect you meant htonl().

 

[Martin Dickopp]

IMHO one is better off using "htnl()."

Wheather such a non-standard method of doing things is preferable when a
standard method exists is certainly debatable.

Martin

 

[Richard Bos]

Why do you check the return value of `fwrite' then? Most operating
systems don't write immediately to the underlying device when `fwrite'
is called, but have some buffering mechanism. Therefore, an error is
far more likely to show up in `fclose' than in `fwrite' on such systems.

Good question. Guess I'm just too used to interactive output.

Richard

 

[Richard Bos]

I suspect you meant htonl().

And _I_ suspect that function is not ISO.

Richard

 

[Christopher Benson-Manica]

And _I_ suspect that function is not ISO.

Going off-topic doesn't have to mean making things up

 

[kal]

I suspect you meant htonl().

Yes, it is "htonl()." Thank you very much. It was my mistake.

Are there any equivalent functions in the standard C library?

Is there even the concept of byte ordering in C?

 

[Dan Pop]

Yes, it is "htonl()." Thank you very much. It was my mistake.

Are there any equivalent functions in the standard C library?
Nope.

Is there even the concept of byte ordering in C?

Nope. In theory the bits could be scattered all over the place, rather
than being nicely grouped in bytes, i.e. the physical representation of
a short could be:

b0 b7 b5 sign b4 b3 b1 b10 | b2 b14 b8 b9 b6 b12 b13 b12
---------------------------+----------------------------
first byte second byte

as long as all the operations generate the specified results (and unsigned
short replaces the sign bit by bit 15). Of course, this is not going
to happen in practice, but the standard is perfectly happy with it.
Some very popular (at the time) old architectures had glitches at the
byte ordering level (neither big endian nor little endian for 32-bit
integers) but they are gone now.

Dan