Format (printf, scanf) warning do not appear. -Wformat not respected?

Showing results for 
Search instead for 
Did you mean: 

Format (printf, scanf) warning do not appear. -Wformat not respected?

Contributor IV


I'm using demo code from MCUXpresso SDK 2.9.0 to compile a "bare-metal" program for an ARM Cortex-M33 based microcontroller (specifically, the LPC55S28 but this should be irrelevant to the question).

I added this example code to the main() function (and the relevant include):

double f = 1.0;
printf("%i\n", f);

Compiling this code should give a warning as the format '%i' expects argument of type 'int', but variable f has type 'double'. The warning is based on the compiler option "-Wformat" that is enabled by option "-Wall" (which is enabled by default for this project).

However, no format warning appears.

I wonder how this can be fixed? I find the warning messages very helpful. 

I checked the compiler call for compiling this source file and when I remove the option "-specs=redlib.specs" from it, the warning message appears.

Is the "-specs" option required, what does it do and why does it inhibit "-Wformat"?



0 Kudos
5 Replies

Senior Contributor IV

Contributor IV

Hi @converse , thanks for suggesting to switch to newlib. 

To try your suggestion, I switched the project to newlib using "Quicksettings".

Using newlib, the warning message appears as expected. 

However, now the compiler emits a warning regarding the following lines:

uint32_t i = 0;
printf("%u\n", i);

The warning reads 

warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'uint32_t' {aka 'long unsigned int'} [-Wformat=]

Obviously, with switching to newlib a different stdint.h header file has been included. Using newlib, uint32_t is now defined as long unsigned int

In comparison, Redlib defines uint32_t as unsigned int

To fix this, I'll have to adapt all format strings accordingly.

Anything else important that the change to newlib brings along?

0 Kudos

Senior Contributor IV

Suggest you read this:

Basically, redlib is a cut-down C90 library to try to minimise the size of the library (with a few C99 features). Newlib (and it's smaller variant newlib-nano) are based on C99 (and later) so are compliant with more modern C standards.

If you are trying to follow modern standard, you should use newlib.

Contributor IV

Dear @converse , of course I've read the page you linked to. While I'm not using a lot of libc functions, I'd like to use and follow a modern C standard. I always use the <stdint.h> and <stdbool.h> headers which are C99 but they are also provided by Redlib.

I'm confused why suddenly gcc warns about the incompatibility of format "%u" and uint32_t type argument.

When I compile the code using, e.g., gcc 9.3.0 (Ubuntu via WSL) I don't get this warning.

0 Kudos

Senior Contributor IV

Its because there is no guarantee that format specifiers will match the definition across platforms. You will need to use %ul in LPCXpresso and %u on Ubuntu.

The only way to guarantee this is to use the format specifier macros - which looks ugly and unreadable, but avoids errors...

#include <inttypes.h>

uint32_t value;

printf("%"PRIu32"\n", value) ;


and look for "format macro constants"

0 Kudos