Index: head/share/man/man3/assert.3
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--- head/share/man/man3/assert.3	(revision 327465)
+++ head/share/man/man3/assert.3	(revision 327466)
@@ -1,90 +1,98 @@
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-.Dd January 26, 1999
+.Dd January 1, 2018
 .Dt ASSERT 3
 .Os
 .Sh NAME
 .Nm assert
 .Nd expression verification macro
 .Sh SYNOPSIS
 .In assert.h
 .Fn assert expression
 .Sh DESCRIPTION
 The
 .Fn assert
 macro tests the given
 .Ar expression
 and if it is false,
 the calling process is terminated.
 A
 diagnostic message is written to
 .Dv stderr
 and the function
 .Xr abort 3
 is called, effectively terminating the program.
 .Pp
 If
 .Ar expression
 is true,
 the
 .Fn assert
 macro does nothing.
 .Pp
 The
 .Fn assert
 macro
 may be removed at compile time by defining
 .Dv NDEBUG
 as a macro
 (e.g., by using the
 .Xr cc 1
 option
 .Fl D Ns Dv NDEBUG ) .
+Unlike most other include files,
+.In assert.h
+may be included multiple times.
+Each time whether or not
+.Dv NDEBUG
+is defined determines the behavior of assert from that point forward
+in the compilation unit until the end of file or another include of
+.In assert.h .
 .Sh EXAMPLES
 The assertion:
 .Pp
 .Dl "assert(1 == 0);"
 .Pp
 generates a diagnostic message similar to the following:
 .Pp
 .Dl "Assertion failed: (1 == 0), function main, file assertion.c, line 100."
 .Sh SEE ALSO
 .Xr abort 3
 .Sh STANDARDS
 The
 .Fn assert
 macro conforms to
 .St -isoC-99 .
 .Sh HISTORY
 An
 .Nm
 macro appeared in
 .At v7 .