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
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.