c++ overflow problem Post: 302256213

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Top Forums Programming c++ overflow problem Post 302256213 by mshindo on Saturday 8th of November 2008 07:29:07 PM

11-08-2008

Registered User

c++ overflow problem

I have come accross a rather strange problem with two buffers that seem to be overriding each other.

Code:

bool killSession(const Session &session){
//Get user
	FILE* fp = fopen("conf/sessions.current", "rt");
	char line[1024]; char user[1024]; int x = 0; int leaseTime;
	printf("key: %s \n" , session.macAddress);

	while(fgets(line, 1024, fp) != NULL){
			x++;	
			int id; char addr[1024];
			sscanf(line, "%i %s %s %i", &id, addr, user, &leaseTime);
			addr[17] = '\0';
			
			printf("key: %s \naddr: %s \n", session.macAddress, addr);
			
			if(strcmp(addr, session.macAddress) != 0){
				printf("mac address found in file: %s \n", line);
				int gid =  getGroupId(user);
				char* accessType =  checkAccessType(gid);
				//modifyFirewall(accessType, session.macAddress, 0);
				break;
			}
		}
	//Finally remove session from session.current
	deleteLine("conf/sessions.current", x);
	fclose(fp);
return true;
}

gives the output:

Code:

key: 00:13:A9:A5:5D:EF 
key: 00:10 fakesessionthat michael 1999999999
 
addr: fakesessionthat 
mac address found in file: 0 fakesessionthat michael 1999999999

From that output it would seem that in the while loop that reads the session file session.macAddress is getting replaced by a part of the line variable. Have I actually managed to buffer overflow my own program.

Can someone please explain how ive gone wrong.

Thankyou

Cheers

mshindo

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fixsub

fixsub(3alleg4) 						  Allegro manual						   fixsub(3alleg4)

NAME

       fixsub - Safe function to subtract fixed point numbers clamping underflow. Allegro game programming library.

SYNOPSIS

       #include <allegro.h>

       fixed fixsub(fixed x, fixed y);

DESCRIPTION

       Although  fixed point numbers can be subtracted with the normal '-' integer operator, that doesn't provide any protection against overflow.
       If overflow is a problem, you should use this function instead. It is slower than using integer operators, but if  an  overflow	occurs	it
       will set `errno' and clamp the result, rather than just letting it wrap. Example:

	  fixed result;
	  /* This will put 4965 into `result'. */
	  result = fixsub(itofix(5000), itofix(35));
	  /* Sets `errno' and puts -32768 into `result'. */
	  result = fixsub(itofix(-31000), itofix(3000));
	  ASSERT(!errno); /* This will fail. */

RETURN VALUE

       Returns the clamped result of subtracting `y' from `x', setting `errno' to ERANGE if there was an overflow.

SEE ALSO

       fixadd(3alleg4), fixmul(3alleg4), fixdiv(3alleg4)

Allegro 							   version 4.4.2						   fixsub(3alleg4)