extracting a specific part out of a string (errors in my code?)

snpnx · 01-15-2009, 06:18 AM

Hello guys,

I have the following task to do:

given a string (char * buf) in a specific way ("putfile $[path of file]") I want to extract the filename out of the filepath with a function called extract_name.

Here's the code:

Code:

char* extract_name(char* buf){
	int length = strlen(buf)-1;
	int count  = -1;
	int i = 0;
	char * fname;
	char c;
	for(i = length; i>=0; i--){	//go backwards through string to gain knowledge of filename length
		c = buf[length];
		if((c == '/') || (c == '\\') || (c == '$')){ //either of these chars show me I have reached the
													 //beginning of my filename, so I leave the loop
			break;
		}
		count++;
		length--;
	}
	fname = (char*)calloc(count+1, sizeof(char));	//allocate free space
	fname[count+1] = '\0';							//put string termination symbol
	length = strlen(buf)-1;							//reset length
	
	for(i = count; i>=0; i--){						//go backwards through string
		fname[i] = buf[length];						//and set the characters accordingly
		i--;
		length--;
	}
	return fname;									//return pointer to name
}

Now, my problem is, that I get no filename back. It's just empty.

Maybe you need to see this part of code too?

Code:

        char *name;
        name = extract_name(message);
	path  = (char*)calloc(10 + strlen(act_user->username) + strlen(name), sizeof(char)); 
									
									
	sprintf(path, "./uploads/%s/%s",act_user->username,name); //create  path-string

Anyone can tell me where exactly my error is? Or should I do it in a different way alltogether?

zacs7 · 01-15-2009, 06:29 AM

Yarr ugly!

Code:

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

#define MAX_PATH 256

char * extract_name(char * buf)
{
   char * path = NULL;

   path = malloc(MAX_PATH);
   if(!path)
      return NULL;

   sscanf(buf, "%*s %256s", path);

   return path; 
}

int main(void)
{
   char * a;

   a = extract_name("putfile /foo/bar");
   if(a)
   {
      printf("%s\n", a);
      free(a);
   }
   
   return 0;
}

I haven't done the work for you either. It's an ugly solution, you can do it your way and write a far nicer one. Also don't cast calloc().

1. You don't need to traverse backwards. You know the length of the filename is the length of the buffer - the start bit.
2. You can use strncpy() you know

.

In reality I'd probably strip all the trailing and leading spaces from the buffer, then again from the path after I parsed it. Probably with the above method if I was feeling lazy

MK27 · 01-15-2009, 06:37 AM

You should be aware of strrchr:

Code:

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

char *extract_name (char *fpathname) {
	char *ptr=strrchr(fpathname,'/');
	ptr++;
	return ptr;
}


int main() {
	char example[]="/home/user/somedir/afile.o",
		*shortname=extract_name(example);
	printf("%s\n",shortname);
}

BE AWARE THAT THIS RETURNS ONLY A POINTER INTO THE ORIGINAL STRING. However, it does work and demonstrate strrchr, which you need to look up.

zacs7 · 01-15-2009, 06:40 AM

Yes, but "./foo/bar" isn't the same as "/foo/bar" nor is "../foo/bar":-)

MK27 · 01-15-2009, 06:47 AM

I really don't think you should do this:

Quote:

Originally Posted by snpnx

if((c == '/') || (c == '\\') || (c == '$')){ //either of these chars show me I have

If portability is a factor use an ifdef statement:

Code:

#ifdef WIN32
#define SC 92     /* a "\"   */
#else 
#define SC 47     /* a  "/" */
#endif
[...]
char *ptr=strrchr(fpathname,SC);

ps. an honest question -- what OS allows "$" in filenames or paths?

MK27 · 01-15-2009, 06:48 AM

Quote:

Originally Posted by zacs7

Yes, but "./foo/bar" isn't the same as "/foo/bar" nor is "../foo/bar":-)

Well, that's true. Also, blue is not the same as green. Is this relevent?

snpnx · 01-15-2009, 06:57 AM

@zacs7: why no use of calloc?

@MK27: an honest question -- what OS allows "$" in filenames or paths?

Not a question of the OS, it was just needed since I did go backwards through the String and thought if the user puts in the filename without any leading './' I know it's right after the '$'. But I'll use strrchr, which I didn't know and let the user know that he needs to give me a '/' with the filename.

@all
Thanks for your help, it should be enough for me to figure it out. If I have questions again, I'll come back

zacs7 · 01-15-2009, 06:58 AM

> ps. an honest question -- what OS allows "$" in filenames or paths?
The ext2/3 filesystems. So Linux :-)

zacs7 · 01-15-2009, 06:59 AM

I said don't cast calloc(). See the FAQ.

MK27 · 01-15-2009, 07:13 AM

Quote:

Originally Posted by snpnx

Not a question of the OS, it was just needed since I did go backwards through the String and thought if the user puts in the filename without any leading './' I know it's right after the '$'. But I'll use strrchr, which I didn't know and let the user know that he needs to give me a '/' with the filename.

So you mean the end of the command prompt. That WILL NOT be in your string. Also, if the string does not contain any slashes, it is not a path and sticking one on the front won't change that. This is like saying I can make an apple out of an orange by putting a hat on it.

Pick, pick pick...anyway good luck

matsp · 01-15-2009, 07:21 AM

Code:

	for(i = count; i>=0; i--){						//go backwards through string
		fname[i] = buf[length];						//and set the characters accordingly
		i--;
		length--;
	}

Code in red does what??

Code:

	fname = (char*)calloc(count+1, sizeof(char));	//allocate free space
	fname[count+1] = '\0';							//put string termination symbol

Probably should not access outside of the member you have allocated - it tends to cause bad things to happen. Remember, an n-element array has valid a valid index range of 0 .. (n-1).

--
Mats

snpnx · 01-15-2009, 09:56 AM

Quote:

Originally Posted by matsp

Code:

	for(i = count; i>=0; i--){						//go backwards through string
		fname[i] = buf[length];						//and set the characters accordingly
		i--;
		length--;
	}

The code in red was an error, I did it with a while loop before and forgot to take out the decrement.

Quote:

Code:

	fname = (char*)calloc(count+1, sizeof(char));	//allocate free space
	fname[count+1] = '\0';							//put string termination symbol

Probably should not access outside of the member you have allocated - it tends to cause bad things to happen. Remember, an n-element array has valid a valid index range of 0 .. (n-1).

--
Mats

Right, you should not access outside of the allocation, I dunno why I didn't see that O_o

MK27 · 01-15-2009, 10:04 AM

Quote:

Originally Posted by zacs7

> ps. an honest question -- what OS allows "$" in filenames or paths?
The ext2/3 filesystems. So Linux :-)

Okay, $ is allowed in a filename as long as it's not the first character (eg. the$.file) but in this:

Code:

prompt$ /home/user/file.txt

$ is not part of the filename or path.

01-15-2009, 06:29 AM	#2
zacs7 Woof, woof! Join Date: Mar 2007 Location: Australia Posts: 3,139	Yarr ugly! Code: #include <stdio.h> #include <stdlib.h> #define MAX_PATH 256 char * extract_name(char * buf) { char * path = NULL; path = malloc(MAX_PATH); if(!path) return NULL; sscanf(buf, "%s %256s", path); return path; } int main(void) { char a; a = extract_name("putfile /foo/bar"); if(a) { printf("%s\n", a); free(a); } return 0; } I haven't done the work for you either. It's an ugly solution, you can do it your way and write a far nicer one. Also don't cast calloc(). 1. You don't need to traverse backwards. You know the length of the filename is the length of the buffer - the start bit. 2. You can use strncpy() you know . In reality I'd probably strip all the trailing and leading spaces from the buffer, then again from the path after I parsed it. Probably with the above method if I was feeling lazy __________________ "I.T. gets the chicky-babes" - M. Kelly bakefile \| vim Last edited by zacs7; 01-15-2009 at 06:34 AM.
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01-15-2009, 06:37 AM	#3
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,946	You should be aware of strrchr: Code: #include <stdio.h> #include <string.h> char extract_name (char fpathname) { char ptr=strrchr(fpathname,'/'); ptr++; return ptr; } int main() { char example[]="/home/user/somedir/afile.o", shortname=extract_name(example); printf("%s\n",shortname); } BE AWARE THAT THIS RETURNS ONLY A POINTER INTO THE ORIGINAL STRING. However, it does work and demonstrate strrchr, which you need to look up. __________________ Accuracy and integrity mean nothing if you don't make it past the censors...PYTHAGORAS Last edited by MK27; 01-15-2009 at 06:44 AM.
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01-15-2009, 06:40 AM	#4
zacs7 Woof, woof! Join Date: Mar 2007 Location: Australia Posts: 3,139	Yes, but "./foo/bar" isn't the same as "/foo/bar" nor is "../foo/bar":-) __________________ "I.T. gets the chicky-babes" - M. Kelly bakefile \| vim
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01-15-2009, 06:57 AM	#7
snpnx Registered User Join Date: Jan 2009 Posts: 3	@zacs7: why no use of calloc? @MK27: an honest question -- what OS allows "$" in filenames or paths? Not a question of the OS, it was just needed since I did go backwards through the String and thought if the user puts in the filename without any leading './' I know it's right after the '$'. But I'll use strrchr, which I didn't know and let the user know that he needs to give me a '/' with the filename. @all Thanks for your help, it should be enough for me to figure it out. If I have questions again, I'll come back
snpnx is offline

01-15-2009, 06:58 AM	#8
zacs7 Woof, woof! Join Date: Mar 2007 Location: Australia Posts: 3,139	> ps. an honest question -- what OS allows "$" in filenames or paths? The ext2/3 filesystems. So Linux :-) __________________ "I.T. gets the chicky-babes" - M. Kelly bakefile \| vim
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01-15-2009, 06:59 AM	#9
zacs7 Woof, woof! Join Date: Mar 2007 Location: Australia Posts: 3,139	I said don't cast calloc(). See the FAQ. __________________ "I.T. gets the chicky-babes" - M. Kelly bakefile \| vim
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01-15-2009, 07:21 AM	#11
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	Code: for(i = count; i>=0; i--){ //go backwards through string fname[i] = buf[length]; //and set the characters accordingly i--; length--; } Code in red does what?? Code: fname = (char*)calloc(count+1, sizeof(char)); //allocate free space fname[count+1] = '\0'; //put string termination symbol Probably should not access outside of the member you have allocated - it tends to cause bad things to happen. Remember, an n-element array has valid a valid index range of 0 .. (n-1). -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
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