main functionThis issue has been automatically converted from the original issue lists and some formatting may not have been preserved.
Authors: WG14, Martin Sebor
Date: 2015-09-10
Reference document: N1960
Submitted against: C11 / C17
Status: Closed
Converted from: n2396.htm
The text of the standard isn't entirely clear as to whether or not the function
main can be used by strictly conforming C programs in hosted environments. The
following passages quote the permissions and requirements that at the same time
suggest that main may not be used by such programs, and that there may be more
than the one call to the function made by the implementation at program startup.
§5.1.2.2.2 Program execution says:
In a hosted environment, a program may use all the functions, macros, type definitions, and objects described in the library clause (clause 7).
suggesting that, since main is not described in the library clause but rather
in §5.1.2.2.1, it may not be used by a program.
However, §5.1.2.2.3 Program termination, immediately following the section quoted above, then goes on to state (emphasis added):
If the return type of the
mainfunction is a type compatible withint, a return from the initial call to themainfunction is equivalent to calling theexitfunction with the value returned by themainfunction as its argument; ...
In addition, §7.21.3 Files contains the following sentence (emphasis also added):
If the
mainfunction returns to its original caller, all open files are closed...
Finally, since the C++ standard explicitly prohibits programs from calling or
otherwise using main, one might expect C to do the same. However the
references to main's initial call and its original caller in the latter two
paragraphs suggest otherwise.
The question was raised and discussed on the committee's mailing list starting
with message (SC22WG14.13780) valid uses of
main. In response, members of
the committee who participated in the preparation of the version of the C
standard that introduced the words clarified that the intent was and remains for
C to allow programs to use main. In particular, the intent of §5.1.2.2.2
Program execution is to grant permission to programs to use the facilities of
the standard library but not to preclude the uses of main or other symbols
defined by them.
However, since the function main is special and unlike any other symbol
defined by a program, and since C++ contains a rule to the contrary, we feel
that the intent isn't sufficiently clearly and unambiguously reflected in the
quoted passages or the rest of the standard, and that a clarification is called
for.
In light of the intent of the passages quoted above as made clear by the mailing list discussion, we offer two proposals to clarify the text of the standard.
Change §5.1.2.2.2 Program execution as follows:
In a hosted environment, a program may use the function
mainas well as all the functions, macros, type definitions, and objects described in the library clause (clause 7).
Add a footnote to the end of §5.1.2.2.2 Program execution, with the following text:
A program may also use the function
main.
Comment from WG14 on 2016-10-21:
Oct 2015 meeting
The committee does not agree that any further clarification is needed in the standard. We know of no actual confusion in a practical sense on this matter. As such, the committee agrees with and draws substantially from (SC22WG14.13787) RE: RE: RE: valid uses of main in the formulation of its Proposed Committee Response below.
As there is no "only" in 5.1.2.2.2 the interpretation should be that the statement is granting permission, not making a restriction. It is drawing a distinction between freestanding environments, where only a subset of the library can be used, and hosted environments, where all of the library can be used. Programs are always free, in either kind of environment, to use things in addition to the library, like their own functions and objects. Additionally, the reference in 5.1.2.2.3 to the initial call to
mainstrongly suggests that recursive calls are allowed.