The format specifier %a for printf() in C

The %a formatting specifier is new in C99. It prints the floating-point number in hexadecimal form. This is not something you would use to present numbers to users, but it's very handy for under-the-hood/technical use cases.

As an example, this code:

printf("pi=%a\n", 3.14);

prints:

pi=0x1.91eb86p+1

The excellent article linked in the comments explains that this should be read "1.91EB86₁₆ * 2¹" (that is, the p is for power-of-two the floating-point number is raised to). In this case, "1.91EB86₁₆" is "1.5700000524520874₁₀". Multiply this by the "2¹", and you get "3.140000104904175₁₀".

Note that this also has the useful property of preserving all bits of precision, and presenting them in a robust way. For instance you could use this to serialize floating point numbers as text, and not have to worry about repeating/infinite decimals.

Also note that strtod() can convert floating point numbers in hexadecimal form back to actual numbers. Not 100% sure about sscanf() and friends, the documentation was not very clear and I've never used that.

As far as an example of why you would want to use the hex representation, you may want to use %a to precisely represent a floating point value being sent to another machine for processing.

We're using this currently for unit testing an embedded controller by sending data from a simulated plant model that is emulating sensors and actuators to the embedded processor via a UART where the embedded processor does it's control processing and returns feedback (again, float represented as %a) back to the plant model to close the loop.