don't understand declaration "unsigned int ip_v:4"

[jian]

Hi,

I am looking at netinet/ip.h in linux code, and the declaration:
unsigned int ip_hl:4 make me confused.

I can understand "unsigned int ip_hl", but, what does ":4" here mean?

Very grateful to any answer.

Jian

 

[Andreas Kahari]

I am looking at netinet/ip.h in linux code, and the declaration:
unsigned int ip_hl:4 make me confused.

It's probably a bit field in a struct.

 

[James Hu]

I am looking at netinet/ip.h in linux code, and the declaration:
unsigned int ip_hl:4 make me confused.

I can understand "unsigned int ip_hl", but, what does ":4" here mean?

This kind of syntax is only allowed for a union or struct field, and
declares a bit-field. In this case, it is declaring ip_hl to hold
values that fit in 4 bits.

-- James