ASSERT(3)              FreeBSD Library Functions Manual              ASSERT(3)

NAME

     assert, static_assert - expression verification macro

SYNOPSIS

     #include <assert.h>

     assert(expression);

     static_assert(expression);

     static_assert(expression, message);

DESCRIPTION

     The assert() macro tests the given scalar expression, and if it is false,
     a diagnostic message is written to stderr and the function abort(3) is
     called, effectively terminating the calling process.

     If expression is true, the assert() macro does nothing.

     In all compilation modes, 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
     assert() in the style of static_assert(), which would otherwise silently
     evaluate as always true via the comma operator.

     The assert() macro may be removed at compile time by defining NDEBUG as a
     macro (e.g., by using the cc(1) option -DNDEBUG).  Unlike most other
     include files, <assert.h> may be included multiple times.  Each time
     whether or not NDEBUG is defined determines the behavior of assert from
     that point forward until the end of the unit or another inclusion of
     <assert.h>.

     The assert() macro should only be used for ensuring the developer's
     expectations hold true.  It is not appropriate for regular run-time error
     detection.

     In pre-C23 compilation modes static_assert() is implemented as a macro
     and expands to _Static_assert(), and, contrarily to 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 _Static_assert()
     containing a string literal message was introduced in C11 standard, and
     the other form with no string literal conforms to C23 standard.

     In C23 and later, static_assert() is a language keyword, and
     _Static_assert() is provided as an obsolescent alternative spelling that
     should not be used for new code and development.

EXAMPLES

     The assertion:
           assert(1 == 0);
     generates a diagnostic message similar to the following:
           Assertion failed: (1 == 0), function main, file main.c, line 100.

     The following assert tries to assert there was no partial read:
           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 NDEBUG is defined, changing the semantics of the program.

     The following example asserts that the iov_len member of the compound
     literal, reflecting the value of len, is non-zero.  The compound literal
     contains a comma that is not protected by parentheses, which the variadic
     assert() macro handles transparently:
           assert((struct iovec){ buf, len }.iov_len);

     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:
           static_assert(sizeof(struct S) == 16, "size mismatch");
     If none is provided, it only points at the constraint.

SEE ALSO

     abort2(2), abort(3)

STANDARDS

     The assert() macro conforms to ISO/IEC 9899:2024 ("ISO C23").

     The static_assert() macro conforms to ISO/IEC 9899:2011 ("ISO C11").  In
     ISO/IEC 9899:2024 ("ISO C23"), it is a language keyword; whether the
     macro is defined or not depends on compilation mode.

HISTORY

     The assert macro first appeared in Version 7 AT&T UNIX.  Starting with
     FreeBSD 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 ISO/IEC
     9899:2024 ("ISO C23"), but made available in all compilation modes.

FreeBSD 15.1-STABLE-HBSD         May 17, 2026                        ASSERT(3)