Thread: fseek() Question

Hi Guys,

fseek() return 0 on succses and nonZero when failed.

For example the following code returns -1

Code:

FILE *fpRead;
int iResult;

fpRead=fopen("textfile.txt","rt");
if (fpRead==NULL)
{
    puts("File not found!");
    exit(1);
}

iResult=fseek(fpRead,-10,SEEK_SET);

This make sense because I am trying to move backward from the beginning of the file.

My question is, Why is the following code don't return the same error and show succses??

Code:

FILE *fpRead;
int iResult;

fpRead=fopen("textfile.txt","rt");
if (fpRead==NULL)
{
    puts("File not found!");
    exit(1);
}

iResult=fseek(fpRead,100000L,SEEK_END);

My text file is only 340 bytes and still this code progress 100,000 bytes, How can it be?

Thank you !!!

What's the "rt" mode?

Anyway... SEEK_END doesn't seek backward, it just sets the relative position. You are seeking FORWARD in the file, which is perfectly fine to do.

Originally Posted by Elysia

rt = read, text

Code:

Mode  	Meaning
"r" 	Open a text file for reading
"w" 	Create a text file for writing
"a" 	Append to a text file
"rb" 	Open a binary file for reading
"wb" 	Create a binary file for writing
"ab" 	Append to a binary file
"r+" 	Open a text file for read/write
"w+" 	Create a text file for read/write
"a+" 	Open a text file for read/write
"rb+" 	Open a binary file for read/write
"wb+" 	Create a binary file for read/write
"ab+" 	Open a binary file for read/write

Didn't know there was another extra string mode.

Originally Posted by Elysia

From msdn's docs.

That explains it.

The c, n, t, S, R, T and D mode options are Microsoft extensions for fopen and _fdopen and should not be used where ANSI portability is desired.

Originally Posted by Elysia

rt = read, text
Anyway, fseek seeks in the file relative to what you specify. Since SEEK_SET is fixed at the beginning of the file, and you try to seek a negative number, you're going to seek... before the file? Doesn't make sense, does it?
The second example with SEEK_END is also poor, because it tries to seek beyond the file, but I'm guessing fseek just sets it to the end of the file.

You can seek beyond the current end of file, this is a perfectly well-defined operation. This creates a 1 megabyte file consisting of zero bytes:

Code:

FILE *fp = fopen(filename, "w");
fseek(fp, 1024*1024-1, SEEK_SET);
fputc(0, fp);
fclose(fp);

Originally Posted by Aia

Didn't know there was another extra string mode.

There isn't, it's some non-standard Microsoft thing.

>My question is, Why is the following code don't return the same error and show succses??
>You can seek beyond the current end of file, this is a perfectly well-defined operation. This creates a 1 megabyte file consisting of zero bytes:
So As brewbuck said that the FILE pointer can be seeked beyond file size which is completly valid .And hence you could get any error message.

ssharish

Originally Posted by Elysia

The second example with SEEK_END is also poor, because it tries to seek beyond the file, but I'm guessing fseek just sets it to the end of the file.

That's the thing!, fseek DOESN'T point to the end of the file.
After running the following code:

Code:

fpRead=fopen(cFilename,"r");
	if (fpRead==NULL)
	{
		puts("File not found!");
		exit(1);
	}

	if (fseek(fpRead,100000L,SEEK_END))
	{
		puts("Error skipping file");
		exit(1);
	}


	printf("%ld\n",ftell(fpRead));

The result is: 100612, even that my file length is 612 bytes.
So were am I actully pointing?

Thanks again