C Programming - What is 2U or 8U in struct

Question

Code from this link: https://github.com/openwch/arduino_core_ch32/blob/main/libraries/USBPD_SINK/src/usbpd_def.h

I am looking at the previous file. I did some search and found something about unsign int literal. However, I am not sure how it would make sense in this case and hope someone can explain it to me.

typedef struct 
{
    uint32_t MaxOperatingCurrent10mAunits : 10u;
    uint32_t OperatingCurrentIn10mAunits  : 10u;
    uint32_t Reserved20_21                : 2u; // 00b
    uint32_t EPRModeCapable               : 1u; 
    uint32_t UnchunkedExtendedMessage     : 1u;
    uint32_t NoUSBSuspend                 : 1u;
    uint32_t USBCommunicationsCapable     : 1u;
    uint32_t CapabilityMismatch           : 1u;
    uint32_t GiveBackFlag                 : 1u;
    uint32_t ObjectPosition               : 4u;
}USBPD_SinkFixedVariableRDO_t;

Is this struct has the total size of 44 bytes assume 32bits padding? And what is the 10u, 2u at at the end of each struct member?

Answer 1

This is a non-portable type of struct known as bit-field, where some members have a size in bits as specified by the : n after each member. So MaxOperatingCurrent10mAunits is 10 bits large and so on. The bit order is not specified by the C language so it could be anything.

The general recommendation is to always avoid bit-fields and use bit masks and bit shifts with the bitwise operators instead, since they are portable and deterministic. Unlike bit-fields which are severely under-specified by the C standard.

That the bit sizes in this case is specified as unsigned integer constants ending with u/U have no significance. It is general good practice to always use unsigned ones instead of signed, but it doesn't matter in this specific case. Some coding standards like MISRA C enforce unsigned constants wherever an unsigned type is intended, so the bit sizes could be written with u just to silence a MISRA checker.