sprintf / snprintf error #4174
Description
There's an error in sprintf and snprintf that occurs when more than 8 varargs are passed in after the format specifier.
The problem is that:
- an extra '00' sneaks into the output string between the 1st and 2nd parameter outputs
- the last parameter does not make it into the output string
This can lead to day-long debugging sessions of unrelated software modules, in order to determine the root cause.
Here's an example sketch that demonstrates what happens:
/**
* Demonstrate that the sprintf / snprintf implementation on Arduino is broken.
*
* sprintf / snprintf, when used with more than 8 varargs:
* inserts an extra '00' in the output string
* removes the last parameter from the output string
*
* This causes a lot of extra debugging.
*
* Expected: abcd0001020304050607
* Actual: abcd0000010203040506
*/
enum {
OUTPUT_LENGTH = 50
};
uint32_t shortId = 0xabcd;
uint8_t buffer[8] = { 0, 1, 2, 3, 4, 5, 6, 7 };
void setup()
{
Serial.begin(115200);
char output[OUTPUT_LENGTH] = { 0 };
// ERROR: The buggy way.
//
// Prints: abcd0000010203040506
snprintf(output, OUTPUT_LENGTH, "%04x%02x%02x%02x%02x%02x%02x%02x%02x",
shortId,
buffer[0],
buffer[1],
buffer[2],
buffer[3],
buffer[4],
buffer[5],
buffer[6],
buffer[7],
buffer[8]);
Serial.println(output);
// OK: The way that actually works.
//
// Prints: abcd0001020304050607
snprintf(output, OUTPUT_LENGTH, "%04x",
shortId);
snprintf(output + 4, OUTPUT_LENGTH - 4, "%02x%02x%02x%02x%02x%02x%02x%02x",
buffer[0],
buffer[1],
buffer[2],
buffer[3],
buffer[4],
buffer[5],
buffer[6],
buffer[7],
buffer[8]);
Serial.println(output);
// ERROR: Another buggy way.
//
// Prints: abcd0000010203040506
sprintf(output, "%04x%02x%02x%02x%02x%02x%02x%02x%02x",
shortId,
buffer[0],
buffer[1],
buffer[2],
buffer[3],
buffer[4],
buffer[5],
buffer[6],
buffer[7],
buffer[8]);
Serial.println(output);
}
void loop()
{
}