Issue 1028: jmp_buf as an incomplete array type

Authors: Jay Ghiron
Date: 2026-03-03
Submitted against: C23
Status: Review

No wording exists currently to say that jmp_buf must be a complete object type:

The type declared is

jmp_buf

which is an array type suitable for holding the information needed to restore a calling environment.

(C23 7.13.1 "Non-local jumps" paragraph 3.)

If jmp_buf were an incomplete object type, this would break almost all uses of it. Interestingly, the example of longjmp in C23 7.13.3.1 paragraph 5 would be fine because it defines an object of type jmp_buf by using a tentative definition.

Suggested correction

Modify C23 7.13.1 paragraph 3:

jmp_buf

which is a complete object type that is an array type suitable for holding the information needed to restore a calling environment.

Note: This wording is suggested instead of "complete array type" to clarify that jmp_buf must have a fundamental alignment requirement.

Comment from Issues list maintainer on 2026-08-21:

At the August 2026 (Ottawa) meeting of WG14, a modified version of the suggested correction was accepted (noting that issue 1058 should be responded to in a way such that "complete array type" is sufficient to guarantee a fundamental alignment requirement), subject to review at the next meeting.

Proposed correction

Modify C23 7.13.1 paragraph 3:

jmp_buf

which is an a complete array type suitable for holding the information needed to restore a calling environment.