Replace an image in C using binary mode

I think you need to read the contents you want to write first (that is read newpic into an array (characters should be ok ) before trying to write it ot background pic).

scrub that....
The size you enter should be the size of the file (which I'm not sure whether sizeof will return correctly).
You can use fseek to SEEK_END to set a pointer to the end of the file, and then use ftell to work out the size.

scrub that....
The size you enter should be the size of the file (which I'm not sure whether sizeof will return correctly).
You can use fseek to SEEK_END to set a pointer to the end of the file, and then use ftell to work out the size.

ok forgive me for perhaps sounding a bit newbie here, but in stdio.h it says the syntax is this:

fseek (FILE*, long, int);

and

ftell (FILE*);

I'm not entirely sure what to put in 'long' in fseek and whether either of these are supposed to display any information on the screen?

Neither of these display information.

Use SEEK_END with fseek. That will position the file cursor to the end of the file. Next, call ftell() to return the byte displacement from the begining of the file. That will be the file size you need to read into memory using a single fread().

Neither of these display information.

Use SEEK_END with fseek. That will position the file cursor to the end of the file. Next, call ftell() to return the byte displacement from the begining of the file. That will be the file size you need to read into memory using a single fread().

ok slowly getting there. So far I have:

#include "stdio.h"
#include "stdlib.h"

int main() {

int x;
FILE *newpic;
newpic=fopen("c:\\Dave\\C-programming\\newpic.jpg", "rb");
fseek (newpic, SEEK_END , x);
fread(x, ftell(x), ftell(x)/ftell(x), newpic);
FILE *background;
background=fopen("c:\\Dave\\C-programming\\stbedes.jpg", "wb");
fwrite(x, ftell(x), ftell(x)/ftell(x), background);
}

My compiler is coming up with errors for the lines with fread and fwrite.
What am I doing wrong?

This compiles but I did not test it:

Thanks but unfortunately it didn't work. I'm not entirely sure what malloc does and I presume int* is some kind of pointer.

I opened up stbedes.jpg in notepad to see if it actually had anything in it but it did not have more than a single symbol so perhaps the program only copied 1 character into the file? If so, would fputc be of any use?

Thanks but unfortunately it didn't work. I'm not entirely sure what malloc does and I presume int* is some kind of pointer.

I opened up stbedes.jpg in notepad to see if it actually had anything in it but it did not have more than a single symbol so perhaps the program only copied 1 character into the file? If so, would fputc be of any use?

malloc alocates memory so that fread can read in a specifed amount of data(specifed by secound fread argument) from the block which x points.

Savage

This compiles but I did not test it:

Expand|Select|Wrap|Line Numbers

1. FILE *newpic;
2. int* x;
3. newpic=fopen("c:\\Dave\\C-programming\\newpic.jpg", "rb");
4. //
5. //WeaknessForCats
6. //You need to seek from the origin (byte 0) of the file)
7. //                                        V
8. fseek (newpic, SEEK_END ,0);
9. //
10. //Now determine file size:
11. //ftell() will return the position of the end of the file.
12. //That's where fseek has positioned the cursor.
13. int length = ftell(newpic);
14. //
15. //Now create an array of the correct size:
16. //
17. x = (int*)malloc(length * sizeof(int));
18. fread(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), newpic);
19. FILE *background;
20. background=fopen("c:\\Dave\\C-programming\\stbedes.jpg", "wb");
21. fwrite(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), background);
22. //
23. //Dont forget to delete the array
24. //
25. free(x);
26.  

anybody know why this doesn't work?

I said I didn't test it. The bug is here:
x is an int*. So sizeof(x) is the sizeof the pointer (4 bytes) divided by
sizeof(x[0]) which is the sizeof an int so now you have 4/4 or 1.

Use the length of the file.

This trick:

sizeof(x)/sizeof(x[0])

to get the number of elements in an array DOES NOT WORK in all cases. In fact, it works in only one case: The array is a stack variable in the function where you use this trick. In all other cases, the name of the array is the address of element 0 and an address is 4 bytes so you always get 4 for the sizeof the array.

C and C++ require that you keep the number of elements in a variable and you pass the variable around with the array name.

In fact, in C++, you shold be using a vector rather than an array. Issues like like are handled by the vector code. The number of elements in a vector is alwways vector<>::size().

Ok thanks very much. I have put:

fread(x, sizeof(x[0]), length, newpic);

instead of:

fread(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), newpic);

Hasn't worked though lol. Did I do something wrong?

Ok thanks very much. I have put:

fread(x, sizeof(x[0]), length, newpic);

instead of:

fread(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), newpic);

Hasn't worked though lol. Did I do something wrong?

What do u mean it hasn't worked?

Is it still copying just one character or is it something else?

Savage

What do u mean it hasn't worked?

Is it still copying just one character or is it something else?

Savage

Still seems to be copying one character when I open stbedes.jpg in notedpad. And when I try to view stbedes.jpg as a picture it has nothing to show. Not sure why that is. This is my source code:

#include "stdio.h"
#include "stdlib.h"

int main() {

FILE *newpic;
int* x;
newpic=fopen("c:\\Dave\\C-programming\\newpic.jpg", "rb");
fseek (newpic, SEEK_END ,0);.
int length = ftell(newpic);
x = (int*)malloc(length * sizeof(int));
fread(x, sizeof(x[0]), length, newpic);
FILE *background;
background=fopen("c:\\Dave\\C-programming\\stbedes.jpg", "wb");
fwrite(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), background);
free(x);
}

use length in your write as well as your read:
eg

use length in your write as well as your read:
eg

Expand|Select|Wrap|Line Numbers

1. fwrite(x, sizeof(x[0]), length, background);
2.  

Yup,u heard TheMan

;)

Savage

use length in your write as well as your read:
eg

Expand|Select|Wrap|Line Numbers

1. fwrite(x, sizeof(x[0]), length, background);
2.  

I have done this now and the image has still not been transferred. I opened the image in notepad to see if any more had been transferred than before. There are now 3 characters, before there was one.

I have done this now and the image has still not been transferred. I opened the image in notepad to see if any more had been transferred than before. There are now 3 characters, before there was one.

Maybe malloc doesn't allocate enough memory.

Try allocating memory for different data type.

Maybe malloc doesn't allocate enough memory.

Try allocating memory for different data type.

Nope no change when allocating memory for char or float.

Have you tried printing out the intermediate variables.....
Is the length you get from fseek correct?
What is the size of x?
Can you narrow down whether the read or write is causing the error?

Have you tried printing out the intermediate variables.....
Is the length you get from fseek correct?
What is the size of x?
Can you narrow down whether the read or write is causing the error?

The problem is in length.try printing it out.

Savage

The problem is in length.try printing it out.

Savage

Sorry for double post but I found the frustration maker(tm):

it was fseek:

right now ur fseek is called like:fseek(newpic,SEEK_END,0) ,but it should be

fseek(newpic,0,SEEK_END)

Savage

Sorry for double post but I found the frustration maker(tm):

it was fseek:

right now ur fseek is called like:fseek(newpic,SEEK_END,0) ,but it should be

fseek(newpic,0,SEEK_END)

Savage

Right well the content of the file is now: Ä 0 h"0 (when opened in notepad)

It is not displaying any image.

Printing length and x does not work because the compiler is telling me that I'm trying to do an invalid conversion from int to char.

Right well the content of the file is now: Ä 0 h"0 (when opened in notepad)

It is not displaying any image.

Printing length and x does not work because the compiler is telling me that I'm trying to do an invalid conversion from int to char.

How did u tryed to print length?

It should be the same as the size of the file that u are coping

Savage

How did u tryed to print length?

It should be the same as the size of the file that u are coping

Savage

I just did printf( length ).

I just did printf( length ).

Like:

printf("%d",length);

??

Savage

Like:

printf("%d",length);

??

Savage

Lol I can't believe I missed that, sorry for being stupid. It printed: 101054

I have also just opened stbedes.jpg in notepad again. It appears that it has 4 characters but it seems to have a lot of spaces, and I would guess there are 101050 spaces because that would make up the rest of length and there are so many spaces that it is quite believable. Why is it coming up with 4 characters and 101050 spaces though?

Incidently (though I don't think this is the cause of your problem),
you are allocating WAY more memory than you need.....
you don't need to multiply (length* sizeof(int)) = the length is already the total number of bytes

Incidently (though I don't think this is the cause of your problem),
you are allocating WAY more memory than you need.....
you don't need to multiply (length* sizeof(int)) = the length is already the total number of bytes

Yes that's true.

However when u remove sizeof(int) final file will have size of 0 bytes,or atleast that's what happens to me.


Savage

you could use sizeof(char)....

you could use sizeof(char)....

Allready tryed that.

Same thing happens.

Savage

Before reading the file, you need to rewind it. The fseek command put you at the end of it, so no data is being read into your buffer. Use:

rewind(file_pointer)

where file_pointer refers to the file pointer of your input file, I believe it was "newpic".

thanks,
eric

Before reading the file, you need to rewind it. The fseek command put you at the end of it, so no data is being read into your buffer. Use:

rewind(file_pointer)

where file_pointer refers to the file pointer of your input file, I believe it was "newpic".

thanks,
eric

LOL,we all missed that.

Sharp eye eric..

Savage

hate to say it but it's still not working :S

hate to say it but it's still not working :S

Can you shouw us your code?

And please can you tell us how it's not working?

Does it sitll copie a few characters or is it something else?

Savage

fread (and I think fwrote also), return a value, this value represents the size read (or written).....
YOu might like to retrieve these values and print them to see whether one of these functions is causing the unexpected behaviour

fread (and I think fwrote also), return a value, this value represents the size read (or written).....
YOu might like to retrieve these values and print them to see whether one of these functions is causing the unexpected behaviour

Sorry Deman, but you are wrong, fread and fwrite returns number of elements readed/writen..

Still, OP can multiply with sizeof one element and that way discover unexpected behaviour, if there is one..

Savage

fread and fwrite returns number of elements readed/writen..

Are you suggesting that number of elements does not constitute size (You'll notice units were never mentioned)?

Are you suggesting that number of elements does not constitute size (You'll notice units were never mentioned)?

It depandes on what is the size of the element user requested when he called fread or fwrite.If it is 1byte then you are correct,but if it is more bytes per element he will need to multiply returned value with size of single element readed/writen

Savage

Granted, to get the size in bytes, number of elements needs to be mulktiplied by number of bytes in an element. But number of elements is still size (3 metres is still 3 metres even if it is 300 centimetres as well).

While printing the size in bytes may also be useful, I think it may also be helpful to print the size in number of elements, after all, you should probably consider data in its' rawest form if you want to find such peculiarities.

I appreciate that the size returned by these functions is not the size in bytes, however, I never suggested that it was, and feel my suggestion was worthwhile nonetheless.....

Granted, to get the size in bytes, number of elements needs to be mulktiplied by number of bytes in an element. But number of elements is still size (3 metres is still 3 metres even if it is 300 centimetres as well).

While printing the size in bytes may also be useful, I think it may also be helpful to print the size in number of elements, after all, you should probably consider data in its' rawest form if you want to find such peculiarities.

I appreciate that the size returned by these functions is not the size in bytes, however, I never suggested that it was, and feel my suggestion was worthwhile nonetheless.....

Sorry.

I thought that you are trying to tell that those functions returns size in bytes.

Savage