Pointer Data Getting Cut Off

**carrotcake1029** · 01-01-2009

I've never seemed to be very good at pointers and decided to practice by writing a function to get the file extension of a given filename. It seems that it is working correctly for most filepaths except in the case of extremely long ones (not that this happens much but I still would like my function to support it).

It cuts off at the 'k' in the third alphabet.

Code:

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

void getext(const char *path, char *ext);

int main (void)
{
	char *path = "myfile.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz";
	char *ext = malloc(FILENAME_MAX * sizeof(char));
	
    getext(path, ext);
    printf("%s\n", ext);

    return 0;
}

void getext(const char *path, char *ext)
{
	char *pch;
	
	pch = strrchr(path, '.');
	
	memcpy(ext, pch + 1, *path - *pch);
}

By the way, FILENAME_MAX = 2048
Any pointers/critiques welcome! Happy New Year!

**tabstop** · 01-01-2009

What do you expect *path - *pch to be?

**matsp** · 01-01-2009

Perhaps you want to subtract the pointers in your memcpy line, rather than subtracting the CONTENTS at the pointer. [Also, make sure you copy the right amount - I haven't checked your calculations, but you want to copy 1 byte beyond the actual extension length, so that you get the terminating zero as well].

--
Mats

**stevesmithx** · 01-01-2009

Why not simply use strlen() instead.
I thought file extensions would be normally three or four characters long(in Windows of course)

**matsp** · 01-01-2009

Originally Posted by stevesmithx

Why not simply use strlen() instead.
I thought file extensions would be normally three or four characters long(in Windows of course)

In traditional FAT, the extension is 3 characters. But modern file-systems (including Unix ones that have been around for 20+ years) that allow long filenames are able to have as long an extension as the max filename size allows. This obviously includes the extended (long) FAT filenames.

To the original poster, what happens in your code with this:

Code:

char *fname = "foo/bar.baz/blah";
char ext[MAX_PATH]; 
getext(fname, ext);

[be aware also that slashes in Windows can be either forward or backward ones (at the C library level - the OS itself expects backslashes, '\\', whilst Unix/Linux only allows forward slashes for directory separators].

--
Mats

**stevesmithx** · 01-01-2009

Thanks for clarifying, Mats.

**carrotcake1029** · 01-01-2009

The only problem I am having now is that when no extension is found, I want to return a null pointer, but for some stupid reason I am not seeing, I don't know how to make it a null pointer can anyone help me out. The line I am trying to set it to null is highlighted in red.

Code:

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

void getext(const char *path, char *ext);

int main (void)
{
	char *path = "foo/bar.baz/blah";
	char *ext = malloc(FILENAME_MAX * sizeof(char));
	
    getext(path, ext);
    if (ext == NULL)
    	printf("No extension found\n");
    else
    	printf("Extension: %s\n", ext);

    return 0;
}

void getext(const char *path, char *ext)
{
	char *pch;
	
	pch = strrchr(path, '/');
	if (pch != NULL)
		pch = strrchr(pch, '.');
	else
		pch = strrchr(path, '.');

	if (pch == NULL)
		*ext = NULL;
	else
		memcpy(ext, pch + 1, strlen(pch));
}

**tabstop** · 01-01-2009

*ext is not a pointer. ext, on the other hand, is.

**carrotcake1029** · 01-01-2009

You're right, but it still triggers the else block in my main() if statement.

It just looks like

Extension:

Like its null, lol

**root4** · 01-01-2009

and you can drop the 'sizeof(char)' which is 1 by definition.

**root4** · 01-01-2009

for the null pointer problem, either work on [...temporarily removed, cf next post...] or more simply make your function returns 1 if it succeeds or 0 if it fails.

PS: *ext=NULL sets 0 as the first item of the string, ext=0 set the pointer to 0 but this is only a local variable so it has no effect once you're out of the function

**Elysia** · 01-01-2009

Originally Posted by carrotcake1029

You're right, but it still triggers the else block in my main() if statement.

It just looks like

Like its null, lol

Of course it is. The contents of the pointer is copied into the new local pointer in the function, and its contents gets destroyed when the function ends.
That is why we use pointers in the first place - to modify data in a local variable in another function.
But if you want to modify a pointer to set its value to NULL, how do we do now? Oh dear.
Think about it a while. Since a pointer is merely a variable, it must be able to be done the same way as regular variables, no?

**stevesmithx** · 01-01-2009

I think the problem is caused by faulty logic.
you expect pch to be null but its not.

**carrotcake1029** · 01-01-2009

Well, I am kind of depending on strrchr's return value

Return Value
A pointer to the last occurrence of character in str.
If the value is not found, the function returns a null pointer.

**matsp** · 01-01-2009

Originally Posted by stevesmithx

I think the problem is caused by faulty logic.
you expect pch to be null but its not.

Nope, str(r)chr() returns NULL if the searched for item is not found.

However, as Elysia says, you can not modify what a pointer points to, unless you pass a pointer to pointer. [Or you could return a pointer].

--
Mats

Thread: Pointer Data Getting Cut Off

Thread Tools

Search Thread

Display

Pointer Data Getting Cut Off

Similar Threads

Abnormal Program Termination when executed from C:/Program Files

Following CTools

Question regarding Memory Leak

HUGE fps jump

C diamonds and perls :°)