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Double sscanf confilicts, overwrites over variables

Posted on 2008-10-25
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Last Modified: 2013-12-14
Hi

When I run this code it overwrites second parameter over first ones:

unsigned char test1[4];
unsigned char test2[4];
memset(test1, 0, 4);
sscanf(param2, "%d.%d.%d.%d", &(test1[0]), &(test1[1]), &(test1[2]), &(test1[3]));
memset(test2, 0, 4);
sscanf(param1, "%d.%d.%d.%d", &(test2[0]), &(test2[1]), &(test2[2]), &(test2[3]));


When I debug, when I reach sscanf sscanf(param2...) it parses test1 properly and I have proper data in test1

When I reach and finish execution of scanf(param1...) it overwrites data and makes test1 data 0

Please advice.
Thanks from now
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Question by:CSecurity
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15 Comments
 
LVL 53

Expert Comment

by:Infinity08
ID: 22803755
%d reads an integer value. You only provide room for one char. The size of an int is typically bigger than that of a char (often 4 times as big).

Either you want to read characters, and then you should use %c instead of %d, or you want to read integer values, and then you should provide enough room for an int.
0
 
LVL 17

Author Comment

by:CSecurity
ID: 22803939
it's IP address, %c not works but %d works... and it fits... Just when I call it second time just 2 lines below it corrupts previous data
0
 
LVL 53

Expert Comment

by:Infinity08
ID: 22804042
Did you understand what I meant in my previous post ?

%d will write 4 bytes (typically), not just 1 byte as you want it to. You'll need to either read them into 4 ints rather than 4 chars, or you'll have to read and process the 4 values one by one.
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LVL 17

Author Comment

by:CSecurity
ID: 22804059
>> read and process the 4 values one by one.

How?

reading them in 4 chars is not a case
0
 
LVL 17

Author Comment

by:CSecurity
ID: 22804063
>> read and process the 4 values one by one.

How?

reading them in 4 ints is not a case
0
 
LVL 53

Expert Comment

by:Infinity08
ID: 22804082
You read an int, and cast it to an unsigned char.
0
 
LVL 17

Author Comment

by:CSecurity
ID: 22804089
0
 
LVL 53

Expert Comment

by:Infinity08
ID: 22804102
>> It's your code

Heh, got me ;)

But, why not use inet_pton, which was my first suggestion ?
0
 
LVL 17

Author Comment

by:CSecurity
ID: 22804110
>> Heh, got me ;)
:-)

>> But, why not use inet_pton, which was my first suggestion ?
I want to use something like sscanf or alternatives to that, I don't want to include a lib, I need just a function or a piece of code
0
 
LVL 17

Author Comment

by:CSecurity
ID: 22804114
Also I should mention again that it works first time, when I re-call it just 2 lines below, it overwrites bytes above... But for first time, it works properly
0
 
LVL 53

Accepted Solution

by:
Infinity08 earned 500 total points
ID: 22804120
Try something like this instead :
const char *ip_addr = "192.168.0.1";
unsigned char ip[4] = { 0 };
unsigned int tmp[4] = { 0 };
if (4 == sscanf(ip_addr, "%u.%u.%u.%u", &(tmp[0]), &(tmp[1]), &(tmp[2]), &(tmp[3]))) {
  int i = 0;
  for (i = 0; i < 4; ++i) {
    ip[i] = (unsigned char) tmp[i];
  }
}
else {
  // error parsing the IP address !!
}

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LVL 53

Expert Comment

by:Infinity08
ID: 22804132
>> But for first time, it works properly

That's because of endianness. Your platform is apparently little endian ... But it's dangerous code, since you're writing past the end of the buffer (3 bytes past it to be exact), thus overwriting memory that doesn' t belong to the buffer.

If you'll only run this code on a little endian system with 32bit integers, you can still use my original code (from the previous question), as long as you increase the buffer size to 7 bytes instead of 4.
0
 
LVL 17

Author Closing Comment

by:CSecurity
ID: 31509956
Thank you so much!
0
 
LVL 53

Expert Comment

by:Infinity08
ID: 22804134
I've got to go now, but I'll be back in a few hours if there are further questions/problems.
0
 
LVL 17

Author Comment

by:CSecurity
ID: 22804136
Code works properly, thank you so much
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