I saw a couple posts asking about thread safety for sprintf online. Is it safe to use sprintf from stdio.h within a FreeRTOS task or is that a bad idea? If it is not a good idea, what is the alternative because I really need this functionality.
The short answer is “it depends on the chip, compiler, and library you are using”.
The longer answer is that in some situations in which FreeRTOS is used it is ok to call sprintf() from multiple tasks, unlike printf() which will always generate a conflict when accessing whichever console is used. However, if you are using a standard GCC sprintf(), then be aware that it will use a huge amount of stack, and might pull in all the floating point libraries which will have the effect of doubling the binary image size.
Many of the demo applications in FreeRTOS include a file called prtinf-stdarg.c which contains a small and limited but very stack light sprintf() implementation. Be aware that not all the printf-stdarg.c files implement snprintf() as they just ignore the buffer size argument. The FreeRTOS+ download contains some more featured implementations too.
The [v]s[n]printf family is very useful but it its definitions and usage are a bit controversial ( see e.g. http://linux.die.net/man/3/snprintf )
Some implementations are stack hungry, up to > 1 KB. Some implementations are not interrupt-proof. You will have to try it out to know.
I attached an implementation that is interrupt-proof and uses a modest amount of stack, about 80 bytes.
snprintf : the official versions may return an amount that it bigger that the size supplied. If that happens, the result may not nul terminated (GCC and MSC that I just tried do write a nul at the end).
The attached version will always write a terminating nul and it will return at most
size - 1 bytes.
char buffer[ 6 ]; int rc = snprintf( buffer, sizeof buffer, "%s", "Hello world" );
official snprintf returns “Hello” with value 11
attached snprintf returns “Hello” with value 5
After I pressed “Post” I saw that Richard had responded earlier
The source that I attached indeed contains the full-featured version of
[v]s[n]printf() with an extensive printf syntax and that also checks for the buffer size. You find it in the The FreeRTOS+ downloads.
Note that the usage of the user-provided function:
void vOutputChar( const char cChar, const TickType_t xTicksToWait )
is a bit historical. The idea was to that if the buffer is NULL, the function
vOutputChar() will be called for each character:
sprintf( NULL, "This output goes to the user function\n" );