What does ## mean in C?

Question

I was looking at the source code of the berkeley lab checkpoint/restart and found this pre-process macro definition:

  #define io_wrap(_op,_ctx,_file,_buf,_count) \
        cr_##_op((_ctx)->req->errbuf,(_file),(_buf),(_count))

Any idea what the hell this thing means?

I found this in blcr-0.8.5/vmadump4/vmadump.h

Thank you!

It means concatenate `cr_` with the value of `_op()` without adding the whitespace which would be added if they were separated by a space character. — Joachim Isaksson, Feb 19 '14 at 20:35

score 4 · Accepted Answer · edited May 23 '17 at 12:28

4

edited May 23 '17 at 12:28

Community

answered Feb 19 '14 at 20:35

Filipe Gonçalves

score 2 · Answer 2 · answered Feb 19 '14 at 20:35

2

## is for concatenation in the C preprocessor.

This result _op((_ctx)->req->errbuf,(_file),(_buf),(_count)) is concatenated with cr_

answered Feb 19 '14 at 20:35

Engineer2021

2 Answers2