That depends of Your C-Compiler.
In most Compilers will be used PORTx to access the output of a Port, or to activate the internal Pull Up's.
Where 'x' is the Port A,B,C.. and so on Example: PORTA.
There are 2 additional registers for each Port.
The first one is the DDRx register that selects if a Port is an Input or an Output.
The second is the PINx Register that shows the state of the Input Port of each Port.
Example:
DDRA=0x0F; //Select the lower nibble as Output and the higher Nibble as Input
PORTA=0x81; //Set the More significant bit Output as Pullup - It is a Input and Set the Low significant Bit to High - It's an Output.
A=PINA; //Readout the State of the Complete PORTA and store it into the variable A
Most Compilers support single bit access like "PORTA.0=1" - The PORTA - 0 = Low significant bit will be set to 1.
Be a little careful with the Port C. A few Ports of that will be used for the JTAG Interface and that is activated on a new Controller.
You must disable the JTAGen Fuse in the Fuse Setting to get full access to all Pins of Port C.
But the Controller Programming and Debugging via a JTAG Interface is not further possible until Change the State of the JTAGen Fuse.
But most people use ISP Programming and that doesn't depend of the JTAGEN.