Is there something wrong with my usage of sprintf?

Question

I'm was using this function in Arduino to pad out a string to 40 characters (the width of my LCD), so that in the process of writing it it clears any characters left behind from the last write.

Problem is when I use it, it stops my RF24 module from being able to send...(everything else still runs, and it recieves) weird I know...

 void printLine(char* line){ sprintf(line, "%-40s", line); lcd.writeString(line); }

It was pointed out that should not write into line from line, i corrected this, but same problem. This function also has the same problem:

void emptyLine(){ sprintf(line, "%-40s", ""); lcd.writeString(line); }

And here is declaration of 'line' (from top of code)

char line[lcdCols];

possible duplicate of Is sprintf(buffer, "%s […]", buffer, […]) safe? — alk
– alk, Commented Jun 8, 2014 at 9:07
With your new code, there's simply not enough information to go on. Where is blank defined? What is lcd.writeString()? — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 8, 2014 at 9:31
sorry about blank, it was from trial of using 2 bufers, after initial answer, i have corrected it to original code. lcd.writeString comes from: code.google.com/p/arduino-t6963c/source/browse/T6963_Lib/… — Hayden Thring
– Hayden Thring, Commented Jun 8, 2014 at 9:34
Ok, so now it's apparent that you forgot to leave room for the null terminator... — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 8, 2014 at 9:36
I changed 40 to 39 and its working again :) thanks, I forgot about that too. — Hayden Thring
– Hayden Thring, Commented Jun 8, 2014 at 9:57

Oliver Charlesworth · Accepted Answer · 2014-06-08 08:58:22Z

You're trying to write into the same character buffer that you're reading from.

From the C99 standard:

sprintf [...] If copying takes place between objects that overlap, the behavior is undefined.

In practice, this is probably just causing an infinite loop.

No this is not it, I tried with a new buffer and same problem. Editing Question.

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