C file functions crash unless root

knobby67 · 06-24-2011, 05:15 AM

Hi All,
I've encountered a problem when running a simple bit of code, I'm using c to read write data to a binary file. It works fine when I run as root. However when I run as a user it produces the file, however when I try to open / write after it's produced it caused a seg fault. Is there any reason file functions ( fopen etc ) would work for root and not a user and is there a way to fix this?

Thanks

Ivshti · 06-24-2011, 05:39 AM

Yes, you are not permitted to modify this file unless you are using root. If you want to make a file modifiable by a certain user, use the command chown.
For example,

Code:

chown user:group /tmp/file

If you don't know which group to use, type the username again, because typically there is a group with the same name for every user.
For example, if your file is in the path "/tmp/file", and your username is "knobby67", you use this:

Code:

chroot knobby67:knobby67 /tmp/file

You can prevent segfaults (and instead do error reporting) by checking the return value from the fopen function. If it equals NULL, you can print the error with the following code:

Code:

perror("the error is");

Which will print the last error with a prefix of "the error is". In order to use perror, you must include the stdlib.h header.

dwhitney67 · 06-24-2011, 06:15 AM

Quote:

Originally Posted by knobby67

Hi All,
I've encountered a problem when running a simple bit of code, I'm using c to read write data to a binary file. It works fine when I run as root. However when I run as a user it produces the file, however when I try to open / write after it's produced it caused a seg fault. Is there any reason file functions ( fopen etc ) would work for root and not a user and is there a way to fix this?

Thanks

How did you "produce" the file? Using fopen()? Does your code check to verify that you successfully created the file? If at a later point you attempt to open the file (presumably in another area of your code), did you check to see if the file was successfully opened before assuming it was ok to write to it?

MTK358 · 06-24-2011, 08:47 AM

Do you have permissions to read/write the file? Does your code check whether the file was successfully opened before using it?

knobby67 · 06-24-2011, 08:48 AM

Quote:

Originally Posted by dwhitney67

How did you "produce" the file? Using fopen()? Does your code check to verify that you successfully created the file? If at a later point you attempt to open the file (presumably in another area of your code), did you check to see if the file was successfully opened before assuming it was ok to write to it?

yes the file is produced, I can step through code and see it "appear" in the folder. No I don't check when I reopen as the code works fine when I'm root, so if I start with sudo, it's fine. It's only as a user that it seg faults.

Quote:

Originally Posted by Ivshti

use the command chown

Unfortunately I'm producing the file myself inside my C code , so using chown is not really possible, well mybe I could system( but that seems a bit long winded.

dwhitney67 · 06-24-2011, 10:03 AM

Quote:

Originally Posted by knobby67

yes the file is produced, I can step through code and see it "appear" in the folder. No I don't check when I reopen as the code works fine when I'm root, so if I start with sudo, it's fine. It's only as a user that it seg faults.

Please show the fopen() call that you use in your code. I am most interested in seeing a) the filename, and b) the mode used to open the file.

sundialsvcs · 06-24-2011, 06:45 PM

Most likely, the file is not accessible to you. Therefore, the fopen() call will return NULL.

If you subsequently use that value, without checking that it is null, a protection exception ("seg fault") will occur, because NULL corresponds to an address of $00000000 ... which is always inaccessible (indeed, for this very reason).

Programs must never "assume that all is well." That's one reason why you should never develop software on high-powered accounts. The software must graciously detect the condition and meaningfully report it to the user.

It is quite handy to use languages that will "throw an exception" when something goes wrong, but "C" is an older language that does not, ordinarily, do that.

lzgonline · 06-25-2011, 09:34 PM

It seems some problems with the program while creating the file in user mode,check if you have the permission to create file in the program directory in user mode,if not,just give the permission or excute program under the user home directory.

paulsm4 · 06-25-2011, 11:27 PM

Hi -

Quote:

Please show the fopen() call that you use in your code. I am most interested in seeing a) the filename, and b) the mode used to open the file.

And *MOST* interested in seeing if you check the error status

1. "Permissions" can cause you to fail to open a file.
2. Failing to open a file *cannot* directly cause a segmentation violation.
... However ...
3. Trying to use a file that you failed to open most definitely CAN and WILL cause an access violation

Q: Do you check for "NULL" in your fopen()?

knobby67 · 06-26-2011, 06:22 AM

Hi he's how I write to the file ( eg save data )

Code:

void StoreLastGameValues( LASTGAMERECALL *lastgame )
{
int count=0;
FILE *fp;
	if((fp=fopen("lastgamescorelist", "rb"))==NULL)
	{
		printf("\n DEBUG cannot open last game file\n");		/*no file so close and create*/
		fp=fopen("lastgamescorelist", "w+b");				/*create the file*/
		/********INIT THE ARRAY*****/
		for(count=0;count<11;count++) 
		{			
			LastGameList[count].time=time(NULL);			/*init the array*/
			for( int i = 0; i < 5; i++ ) LastGameList[ count ].position[ i ] = 0;
			fwrite(&LastGameList[ count ],sizeof( LASTGAMERECALL ),1,fp);
		}
		fclose(fp);
		return;
	}

	/*******READ IN DATA********/
	for( count=0; count<10; count++ ) 
	{
		int i = fread(&LastGameList[ count ],sizeof( LASTGAMERECALL ),1,fp);
		if( i == 1 ){ };  //needed for new version of gcc	
	}
	fclose(fp);

        /*mode to top of table*/
	for( count=10 ;count>=0; count-- ) 
	{
		LastGameList[ count+1 ]= LastGameList[ count ];	/*move up one*/
				
	}

	
	LastGameList[ 0 ] = *lastgame;

	/******WRITE DATA FILE***/
	fp=fopen("lastgamescorelist", "wb");
	for( count = 0; count < 11; count++ ) 
	{
		fwrite(&LastGameList[ count ],sizeof( LASTGAMERECALL ),1,fp);	
	}
	fclose(fp);
}

dwhitney67 · 06-26-2011, 07:45 AM

Thanks for the additional information; however it appears that there is still some gray areas.... how is LASTGAMERECALL defined, and how is LastGameList declared?

As for your function, it seems that initially you are checking to see if the file is readable, and hence implying that it exists. If it does not, then you open the file for writing so that you can initialize it with data.

If the file does happen to exist, then you read in all of its data, perform a shuffling of the data, and then re-write that data to the file.

You could have chosen to use stat() to determine if the file exists, and also devise separate functions that read data from the file, write data to the file, initialize the data, and lastly manipulate the data itself. Instead you opted to perform all of these steps (sans stat() call) in one function.

Anyhow, when you have the luxury of time, please answer the questions I posted above.

P.S. The 'b' option used with fopen() is ignored by most POSIX compliant systems, including Linux. It was deprecated long ago.

Wim Sturkenboom · 06-26-2011, 09:43 AM

You do NOT always check if fp is valid (not NULL) after opening the file; e.g. at the end of the shown code under 'write data file'.

You should always check if opening a file succeeded and use e.g. perror() to get the errors in a human readable message.

knobby67 · 06-26-2011, 10:26 AM

Quote:

Originally Posted by dwhitney67

T

Anyhow, when you have the luxury of time, please answer the questions I posted above.

P.S. The 'b' option used with fopen() is ignored by most POSIX compliant systems, including Linux. It was deprecated long ago.

Hi,
sorry if I didn't include what is needed, I think you mean how I first open the the file? This is done as follows

Code:

void InitLastGame( void )
{
int count = 0;
FILE *fp;
	if((fp=fopen("lastgamescorelist", "rb"))==NULL)
	{
		printf("\n DEBUG cannot open last game file\n");		/*no file so close and create*/
		fp=fopen("lastgamescorelist", "w+b");				/*create the file*/
		/********INIT THE ARRAY*****/
		for( count=0; count<11; count++) 
		{			
			LastGameList[ count ].time=time(NULL);		/*init the array*/
			LastGameList[ count ].playerposition[ 0 ] = 0;
			LastGameList[ count ].playerposition[ 1 ] = 0;
			LastGameList[ count ].playerposition[ 2 ] = 0;
			LastGameList[ count ].playerposition[ 3 ] = 0;
			LastGameList[ count ].playerposition[ 4 ] = 0;



			fwrite(&LastGameList[ count ],sizeof(LASTGAMERECALL ),1,fp);
		}
		fclose(fp);
		return;
	}

	/*******READ IN DATA********/
	for(count=0; count < 10 ; count++) 
	{
		int i = fread( &LastGameList[ count ],sizeof(LASTGAMERECALL),1,fp);
		if( i == 1 ){ };	
	}
	fclose(fp);
}

last game is just a structure holding player data

Code:

typedef struct lastgamerecall
{
time_t time;
int playerposition[ 5 ];
long score;

}LASTGAMERECALL;

dwhitney67 · 06-26-2011, 10:52 AM

It is this variable, LastGameList, that I am interested in seeing how it is declared. I can tell it is a global variable (btw, bad programming practice).

In some loops, you access LastGameList using indexes 0 through 10, in others 0 through 9. How many array slots have you declared for LastGameList.

Also, I've seen the following statement in your code; out of curiosity, why is it present?

Code:

if( i == 1 ){ };

theNbomr · 06-26-2011, 11:37 AM

I am surprised no one has suggested trying to open the file with a known-good tool. Since the OP has created a binary-formatted data file, I suggest od:

Code:

od -tx1 lastgamescorelist

This should clarify whether there is a file permissions error, or error in the OP's code.

--- rod.