Compatibility of vsnprintf and the macros from inttypes

Question

I'm rewriting some code to be compatible with 32-bit and 64-bit architectures, and I'm having trouble with a vsnprintf call. It doesn't appear that vsnprintf handles the fixed size integer types from inttypes.h properly on either architecture.

Here is the relevant code:

void formatString(char *buffer, int size, char *format, ...)
{
    va_list va;

    /* Format the data */
    va_start( va, format );
    vsnprintf( (char *)buffer, size, format, va );
    va_end( va );
}

int main(int argc, char *argv[])
{
    char buffer[2048];

    printf("The format string: %s\n", stringsLookup(0));
    formatString(&buffer[0], sizeof(buffer), stringsLookup(0), 1, 2);
    printf("The output string: %s\n", buffer);

    return 0;
}

The output is as follows:

The format string: action=DoSomething&Val1=%"PRIx32"&Val2=%x
The output string: action=DoSomething&Val1=%"PRIx32"&Val2=1

You can see that the %"PRIx32" portion of the format string was not replaced with the value '1' as expected. Is this a known issue? Is there a work around?

I will mention that if I hard code the strings in the source, the preprocessor appears to convert "%PRIu32" to the appropriate macro for the architecture and the call to vsnprintf works. Unfortunately I need to be able to load the strings.

Update

Some additional background: When I moved from a 32-bit system to a 64-bit system, I had to fix the size of certain variables. I declared them as uint32_t. I also changed the places where they were printed to clean up compiler warnings. The previous code used printf("%lx") . I used printf("%"PRIx32) . I need to do something similar with the call to vsnprintf.

As I've mentioned, if I hard code the string in the source code, the preprocessor appears to convert "%"PRIx32 to "%lx" or "%x" appropriately. Unfortunately, I'm running into trouble when I have to load the strings from a file. The preprocessor can't help me there.

Answer 1

PRIx32 is a macro whose name should not appear textually even in the format string. You are almost certainly using it wrong, unless it expands to a string that contains "PRIx32" (it almost certainly doesn't).

A typical use is printf("Number: %" PRIx32 "...", arg); .

In the typical idiom above, "Number: %" PRIx32 "..." is expanded to, say, "Number: %" "lX" "..." , which by a peculiarity of C syntax is equivalent to "Number: %lX..."

If you need to create the format string dynamically, use strcat or other string-manipulation functions. Do not write the equivalent of "Number: %\\"PRIx32\\"..." .

Just remember that PRIx32 expands to a string literal, and don't write "%PRIx32" , that does not make sense.

EDIT:

If you are loading the format string from a file, information that I suggested you provide in a comment 45 minutes ago, then you have to do your own substitution when the file is loaded from. Invent a syntax similar to the % syntax of printf, and write your own function to recognize it and substitute it with what is right on the architecture the program is running on.

Note that from a security point of view, if you load format strings from a file, whoever controls the file controls what the program does .

Also note that printf("Number: %llx\\n", (unsigned long long) e); almost always work. It can only disappoint you if your compiler has an integer type wider than unsigned long long and you use it.