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In computer programming, a variable-length array (VLA), also called variable-sized or runtime-sized, is an array data structure whose length is determined at runtime, instead of at compile time. [1] In the language C , the VLA is said to have a variably modified data type that depends on a value (see Dependent type ).
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++).
C99 standardised variable-length arrays (VLAs) within block scope. Such array variables are allocated based on the value of an integer value at runtime upon entry to a block, and are deallocated at the end of the block. [3] As of C11 this feature is no longer required to be implemented by the compiler.
In C and C++ arrays do not support the size function, so programmers often have to declare separate variable to hold the size, and pass it to procedures as a separate parameter. Elements of a newly created array may have undefined values (as in C), or may be defined to have a specific "default" value such as 0 or a null pointer (as in Java).
Array types in C are traditionally of a fixed, static size specified at compile time. The more recent C99 standard also allows a form of variable-length arrays. However, it is also possible to allocate a block of memory (of arbitrary size) at run-time, using the standard library's malloc function, and treat it as an array.
C99 also allows variable length arrays that have the length specified at runtime, [2] although the feature is considered an optional implementation in later versions of the C standard. In such cases, the sizeof operator is evaluated in part at runtime to determine the storage occupied by the array.
ANSI C, ISO C, and Standard ... compound literals, variable-length arrays, flexible array ... platform-specific attributes such as the exact size of certain data ...
C struct data types may end with a flexible array member [1] with no specified size: struct vectord { short len ; // there must be at least one other data member double arr []; // the flexible array member must be last // The compiler may reserve extra padding space here, like it can between struct members };