C issue 1029: Spelling of __LINE_

Authors: Jay Ghiron
Date: 2026-03-05
Submitted against: C23
Status: Open
Cross-references: 0464, 1016

Issue 1016 was resolved as not allowing digit separators in __LINE__. However, there are other issues with the permissible spellings of __LINE__. Specifically relating to #line __LINE__ as mentioned in DR 464. First, if __LINE__ expands to a binary constant or a hexadecimal constant then #line __LINE__ is invalid. Second, if __LINE__ expands to include an integer suffix then #line __LINE__ is also invalid. Third, if __LINE__ expands to an octal constant then #line __LINE__ will possibly result in a different value. For example:

//line 1
//line 2
//line 3
//line 4
//line 5
//line 6
//line 7
#line __LINE__
__LINE__

As indicated in DR 464, this could expand to eight or nine. However if an octal constant were used, this could instead expand to ten or eleven. Since #line 010 is interpreted as setting the line number to ten, not eight.

Question 1

Question 2

Is it intended that #line __LINE__ cannot have more than two possible results due to octal constants possibly being used?

Question 3

Outside of #line __LINE__, should the spelling of __LINE__ be strictly defined so that stringization and concatenation have well defined results? For example:

#define CAT(X,Y,...)__VA_OPT__(X)##__VA_OPT__(Y)
#define DEFINE int CAT(v,__LINE__,,);CAT(v,__LINE__,,)=42
int main(){
DEFINE;
}

There are many programs like this that rely on the spelling of __LINE__ being consistent within a line. See also C++ CWG 3142.

At the August 2026 (Ottawa) meeting of WG14, it was agreed that __LINE__ should be decimal or possibly 0 so that #line __LINE__ works and there is a unique possible spelling for a given line number. Joseph Myers took an action item to propose wording for this, preferably similar to the wording used in C++.

The following proposed wording is based on C2y draft N3886; for C23 (as amended by issue 1016), "integer literal" and "decimal literal" should be read as "integer constant" and "decimal constant".

Issue 1029: Spelling of __LINE__

Question 1

Question 2

Question 3

Issue 1029: Spelling of `LINE`