assert.3

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.Dd May 17, 2026
.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 scalar
.Ar expression ,
and if it is false,
a diagnostic message is written to
.Dv stderr
and the function
.Xr abort 3
is called, effectively terminating the calling process.
.Pp
If
.Ar expression
is true,
the
.Fn assert
macro does nothing.
.Pp
The
.Fn assert
macro accepts a variable number of arguments.
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 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.
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
In pre-C23 compilation modes
.Fn static_assert
is implemented as a macro and 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 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);"
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 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"");"
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-2023 .
.Pp
The
.Fn static_assert
conforms to
.St -isoC-2011 ,
wherein defined as a macro, and
.St -isoC-2023 ,
as a language keyword, depending on the compilation mode.
.Sh HISTORY
The
.Nm
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.