assert.3: Update as per C23

Signed-off-by:	Faraz Vahedi <kfv@kfv.io>
Reviewed by:	fuz
MFC after:	1 month
Pull Request:	https://github.com/freebsd/freebsd-src/pull/2203

share/man/man3/assert.3

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 20, 2021
+.Dd May 17, 2026
 .Dt ASSERT 3
 .Os
 .Sh NAME
@@ -40,15 +40,14 @@
 .Sh DESCRIPTION
 The
 .Fn assert
-macro tests the given
-.Ar expression
+macro tests the given scalar
+.Ar expression ,
 and if it is false,
-the calling process is terminated.
-A diagnostic message is written to
+a diagnostic message is written to
 .Dv stderr
 and the function
 .Xr abort 3
-is called, effectively terminating the program.
+is called, effectively terminating the calling process.
 .Pp
 If
 .Ar expression
@@ -57,6 +56,22 @@ the
 .Fn assert
 macro does nothing.
 .Pp
+In all compilation modes,
+.Fn assert
+is defined as a macro with an ellipsis parameter, consistent with the
+C23 standard.
+This allows expressions containing commas to be passed directly without
+requiring an extra pair of enclosing parentheses.
+Only a single scalar expression is evaluated.
+Supplying multiple arguments is prohibited, and hence are top-level comma
+operators.
+In particular, this guards against accidentally writing
+.Fn assert
+in the style of
+.Fn static_assert ,
+which would otherwise silently evaluate as always true via the comma
+operator.
+.Pp
 The
 .Fn assert
 macro
@@ -73,7 +88,7 @@ may be included multiple times.
 Each time whether or not
 .Dv NDEBUG
 is defined determines the behavior of assert from that point forward
-until the end of the unit or another include of
+until the end of the unit or another inclusion of
 .In assert.h .
 .Pp
 The
@@ -82,9 +97,9 @@ macro should only be used for ensuring the developer's expectations
 hold true.
 It is not appropriate for regular run-time error detection.
 .Pp
-The
+In pre-C23 compilation modes
 .Fn static_assert
-macro expands to
+is implemented as a macro and expands to
 .Fn _Static_assert ,
 and, contrarily to
 .Fn assert ,
@@ -94,8 +109,14 @@ message including the string literal message, if provided.
 The initial form of the
 .Fn _Static_assert
 containing a string literal message was introduced in C11 standard, and
-the other form with no string literal is to be implemented by C2x and
-some compilers may lack its adoption at present.
+the other form with no string literal conforms to C23 standard.
+.Pp
+In C23 and later,
+.Fn static_assert
+is a language keyword, and
+.Fn _Static_assert
+is provided as an obsolescent alternative spelling that should not be
+used for new code and development.
 .Sh EXAMPLES
 The assertion:
 .Dl "assert(1 == 0);"
@@ -111,7 +132,20 @@ Second, the code will disappear if
 .Dv NDEBUG
 is defined, changing the semantics of the program.
 .Pp
-The following asserts that the size of the S structure is 16.
+The following example asserts that the
+.Va iov_len
+member of the compound literal, reflecting the value of
+.Va len ,
+is non-zero.
+The compound literal contains a comma that is not protected by
+parentheses, which the variadic
+.Fn assert
+macro handles transparently:
+.Dl assert((struct iovec){ buf, len }.iov_len);
+.Pp
+The following asserts that the size of the
+.Vt S
+structure is 16.
 Otherwise, it produces a diagnostic message which points at the
 constraint and includes the provided string literal:
 .Dl "static_assert(sizeof(struct S) == 16, ""size mismatch"");"
@@ -123,14 +157,26 @@ If none is provided, it only points at the constraint.
 The
 .Fn assert
 macro conforms to
-.St -isoC-99 .
+.St -isoC-2023 .
 .Pp
 The
 .Fn static_assert
 macro conforms to
 .St -isoC-2011 .
+In
+.St -isoC-2023 ,
+it is a language keyword; whether the macro is defined or not
+depends on compilation mode.
 .Sh HISTORY
-An
+The
 .Nm
-macro appeared in
+macro first appeared in
 .At v7 .
+Starting with
+.Fx 15.2 ,
+it accepts a variadic argument list, allowing expressions
+containing commas, such as compound literals, to be passed
+without requiring extra enclosing parentheses.
+This conforms to
+.St -isoC-2023 ,
+but made available in all compilation modes.