Character array weirdness

Question

I'm using Energia to program one of TI's MCU. Since Energia is based off the Arduino IDE, I'm hoping someone can help me here.

I'm building a simple UDP packet sender and receiver app. The MCU starts up and connects to the local Wifi network and waits for a packet from another device on the same network.

This packet is collected in a char array char packetBuffer[16];

I want to split this array into two separate arrays each of 8 elements. These are char lampCmdRead[8]; and char pwdReadBuffer[8]; Now the network bit works great. I can send the packet through my desktop and receive it on the MCU. However when I split this array I see that the pwdReadBuffer shows the same contents as packetBuffer even though they are of different sizes. How is this possible?

Here is the code:

char packetBuffer[16];
char lampCmdRead[8];
char pwdReadBuffer[8];

// This is code from the 'void loop()' section
int packetSize = Udp.parsePacket();

if(packetSize) {
 int len = Udp.read(packetBuffer, 16);
 if (len > 0) packetBuffer[len] = 0;
 Serial.println();

 Serial.println("packetBuffer:");
 Serial.println(packetBuffer);
 Serial.println();

 for(ctr = 0; ctr < 8; ctr++) {
   pwdReadBuffer[ctr] = packetBuffer[ctr];
 }
 if (len > 8) pwdReadBuffer[len] = 0;

 for(ctr = 0; ctr < 8; ctr++) {
   lampCmdRead[ctr] = packetBuffer[ctr+8];
 }    
 if (len > 8) lampCmdRead[len] = 0;

 Serial.println("pwdReadBuffer:");
 Serial.println(pwdReadBuffer);
 Serial.println();

 Serial.println("lampCmdRead:");
 Serial.println(lampCmdRead);
 Serial.println();
}

And here is the output:

Received packet of size 16
From 192.168.1.101, port 55056

packetBuffer:
2qZ@X51Rredledon

pwdReadBuffer:
2qZ@X51Rredledon

lampCmdRead:
redledon

You're overflowing your buffers.

Majenko
– Majenko

2016-06-04 21:19:03 +00:00
Commented Jun 4, 2016 at 21:19 — Majenko
– Majenko, Commented Jun 4, 2016 at 21:19
Also you're not terminating your strings properly.

Majenko
– Majenko

2016-06-04 21:19:48 +00:00
Commented Jun 4, 2016 at 21:19 — Majenko
– Majenko, Commented Jun 4, 2016 at 21:19

Chris Stratton · Accepted Answer · 2016-06-04 21:20:11Z

if (len > 8) pwdReadBuffer[len] = 0;

This line is unsafe. You declared pwdReadBuffer to be only 8 bytes in size, but this line of code will either do nothing, or write outside of the target buffer, with unpredictable results.

You make a related mistake here:

if (len > 8) lampCmdRead[len] = 0;

First the test should be >= but again, such a value would be beyond the end of the target buffer.

Also consider that if you receive 8 or more characters, you will have no room for a terminating null in pwdReadBuffer, and so cannot pass it to anything that expects a null terminated string.

If you want to use null-terminate storage for a string of up to 8 bytes size, you need 9 bytes of storage. And of course you need to only use in-range indices to access the elements.

James Waldby - jwpat7 · Accepted Answer · 2016-06-04 21:20:27Z

This behavior (where contents of pwdReadBuffer print out the same as those of packetBuffer) is occurring because you haven't properly null-terminated your strings.

After you copy the first 8 characters of packetBuffer to pwdReadBuffer the statement if (len > 8) pwdReadBuffer[len] = 0; gets executed. It sets pwdReadBuffer[16] to 0, ie, references out of array bounds.

Apparently lampCmdRead is allocated directly after pwdReadBuffer, so when you copy the next 8 characters of packetBuffer to lampCmdRead, they are being copied, in effect, to pwdReadBuffer[8] ... pwdReadBuffer[15], so that the println prints a 16-character string, 2qZ@X51Rredledon.

For each of your three arrays, allocate an extra byte for a null terminator.

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