Lots of man pages.

man/man3/assert.3

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+.TH ASSERT 3
+.SH NAME
+assert \- check program invariants
+.SH SYNOPSIS
+.B #include <u.h>
+.br
+.B #include <libc.h>
+.PP
+.B
+#define assert if(cond);else _assert("cond")
+.PP
+.B
+void _assert(int cond)
+.SH DESCRIPTION
+.I Assert
+is a preprocessor macro that
+(via
+.IR _assert )
+prints a message and calls
+.I abort
+when
+.I cond
+is false.
+.SH SOURCE
+.B /sys/src/libc/port/_assert.c