I have some struct that I need to print frequently. For now, I am using a classical print wrapper around this struct :
void printf_mystruct(struct* my_struct)
{
if (my_struct==NULL) return;
printf("[value1:%d value2:%d]", struct->value1, struct->value2);
}
This function is handy, but is also really limited. I cannot prepen or append some text without making a new wrapper. I know that I can use va_arg family to be able to prepend or apprend some text, but I feel like I would be re-implementing the wheel.
I am wondering if it's possible to write a customizing function to printf. I would like to be able to write something like this :
register2printf("%mys", &printf_mystruct);
...
if (incorrect)
printf("[%l] Struct is incorrect : %mys\n", log_level, my_struct);
Is this possible ? How can I do this ?
NB: I am under Ubuntu Linux 10.04 and I use gcc.
Sorry, but some answers are incorrect on Linux with Glibc
On Linux with a GNU Glibc, you can customize printf: you would call
register_printf_function
to e.g. define the meaning of %Y
in your printf
format strings.
However, this behavior is Glibc specific, and might even become obsolete... I'm not sure I would recommend this approach!
If coding in C++, the C++ stream library has manipulators which you could extend, and you can also overload for your types the operator <<
etc.
You could consider writing a GCC plugin helping that (and improving the typechecking of some extended printf
). It won't be easy (probably a few weeks or months of work), and it would be GCC version specific (not the same plugin code for GCC 7 and GCC 8). you might add some specific #pragma
to inform your plugin about extra control string specifiers like your %Y
and the type expected for them. Your plugin should change the handling of format
attribute (perhaps in gcc/tree.c
)