CODEWITHSUNDEEP
cpebble-watchpebble-sdk
Kalyan02
Kalyan02

Reputation: 1434

snprintf bug on pebble

There seems to be some weirdness with snprintf implementation in Pebble Sdk 2.x

I modified the example program to show a click counter. The following code works, if I use str[1000] size

selectCount ++;
char str[1000], buf[100];
snprintf(str, 1000, "Click %d foo", selectCount);
snprintf(buf, 100,  "s(%d) one two l(%d)", sizeof(str), strlen(str) );

if(selectCount % 2)
  text_layer_set_text(text_layer, buf );
else
  text_layer_set_text(text_layer, str );

But if I reduce the buffer size of str to 400, the application crashes

selectCount ++;
char str[400], buf[100];
snprintf(str, 400, "Click %d foo", selectCount);
snprintf(buf, 100,  "s(%d) one two l(%d)", sizeof(str), strlen(str) );

if(selectCount % 2)
  text_layer_set_text(text_layer, buf );
else
  text_layer_set_text(text_layer, str );

But again, if I reduce the size of str to 20, it doesn't crash. Instead it truncates buf at 7 characters; and doesn't display str at all in a text_layer.

The values in the non-crashing cases look okay when I perform 

APP_LOG(APP_LOG_LEVEL_DEBUG, str);

Any help on this is appreciated. Thanks in advance.

Upvotes: 2

Views: 374

Answers (1)

toasted_flakes
toasted_flakes

Reputation: 2509

You're using a stack buffer, but text_layer_set_text doesn't copy its content. When the drawing occurs, the memory doesn't exist anymore.

From the documentation:

The string is not copied, so its buffer most likely cannot be stack allocated, but is recommended to be a buffer that is long-lived, at least as long as the TextLayer is part of a visible Layer hierarchy.

Upvotes: 1

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