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.Dd April 20, 2021
.Dt ASSERT 3
.Os
.Sh NAME
.Nm assert ,
.Nm static_assert
.Nd expression verification macro
.Sh SYNOPSIS
.In assert.h
.Fn assert expression
.Fn static_assert expression
.Fn static_assert expression message
.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
until the end of the unit or another include of
.In assert.h .
.Pp
The
.Fn assert
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
.Fn static_assert
macro expands to
.Fn _Static_assert ,
and, contrarily to
.Fn assert ,
makes assertions at compile-time.
Once the constraint is violated, the compiler produces a diagnostic
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.
.Sh EXAMPLES
The assertion:
.Dl "assert(1 == 0);"
generates a diagnostic message similar to the following:
.Dl "Assertion failed: (1 == 0), function main, file main.c, line 100."
.Pp
The following assert tries to assert there was no partial read:
.Dl "assert(read(fd, buf, nbytes) == nbytes);"
However, there are two problems.
First, it checks for normal conditions, rather than conditions that
indicate a bug.
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.
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"");"
If none is provided, it only points at the constraint.
.Sh SEE ALSO
.Xr abort2 2 ,
.Xr abort 3
.Sh STANDARDS
The
.Fn assert
macro conforms to
.St -isoC-99 .
.Pp
The
.Fn static_assert
macro conforms to
.St -isoC-2011 .
.Sh HISTORY
An
.Nm
macro appeared in
.At v7 .