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sizeof cannot be used in C preprocessor expressions, such as #if, because it is an element of the programming language, not of the preprocessor syntax, which has no data types. The following example in C++ uses the operator sizeof with variadic templates.
The sizeof operator on such a struct gives the size of the structure as if the flexible array member were empty. This may include padding added to accommodate the flexible member; the compiler is also free to re-use such padding as part of the array itself.
size_t is an unsigned integer type used to represent the size of any object (including arrays) in the particular implementation. The operator sizeof yields a value of the type size_t. The maximum size of size_t is provided via SIZE_MAX, a macro constant which is defined in the <stdint.h> header (cstdint header in C++).
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It should be noted that, since sizeof is a compile-time operation, it is impossible to use sizeof to determine the size of an array if it cannot be evaluated at compile time. That's incorrect since C99 introduces variable-length arrays. In that case, the size of the array can only be evaluated at run-time, and sizeof() will be evaluated at runtime.
This is an accepted version of this page This is the latest accepted revision, reviewed on 17 February 2025. General-purpose programming language "C programming language" redirects here. For the book, see The C Programming Language. Not to be confused with C++ or C#. C Logotype used on the cover of the first edition of The C Programming Language Paradigm Multi-paradigm: imperative (procedural ...
A snippet of C code which prints "Hello, World!". The syntax of the C programming language is the set of rules governing writing of software in C. It is designed to allow for programs that are extremely terse, have a close relationship with the resulting object code, and yet provide relatively high-level data abstraction.
Without such precautions, most programs may compile only on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to the reliance on compiler- or platform-specific attributes such as the exact size of certain data types and byte endianness.