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How is it possible to declare a variable-length array as a global variable ?
when variable length array is declared in a function before the length is scanned, it compiles but does not run. it gives segmentation fault. when the same declaration statement is shifted below the scanning statement, it runs fine.
in case we want a variable length array globally available to all functions, how can we do that? problem here is the that the length of the array can only be scanned through some function only.
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The arrays can be declared and initialized globally as well as locally(i.e., in the particular scope of the program) in the program.
Dimensions used when declaring arrays in C must be positive integral constants or constant expressions. In C99, dimensions must still be positive integers, but variables can be used, so long as the variable has a positive value at the time the array is declared.
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 run time (instead of at compile time). In C, the VLA is said to have a variably modified type that depends on a value (see Dependent type).
As in case of scalar variables, we can also use external or global arrays in a program, i. e., the arrays which are defined outside any function. These arrays have global scope. Thus, they can be used anywhere in the program.
A variable length array (i.e. an array sized with a runtime value) can't be a global variable, because the expression you are using for the size must obviously be computed at compile time. It can only live on the stack. Presumably what you are getting is a static array with a size that depends on where in the code you are defining it (because you are redefining something it depends on).
Why can't you just use a global pointer and realloc() to size it as needed?
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You can't do that. Here's what the draft of the standard says:
6.7.6.2 Array declarators
2 If an identifier is declared as having a variably modified type, it shall be an ordinary identifier (as defined in 6.2.3), have no linkage, and have either block scope or function prototype scope. If an identifier is declared to be an object with static or thread storage duration, it shall not have a variable length array type.
Also,
10 EXAMPLE 4 All declarations of variably modified (VM) types have to be at either block scope or function prototype scope. Array objects declared with the _Thread_local, static, or extern storage-class specifier cannot have a variable length array (VLA) type. However, an object declared with the static storage-class specifier can have a VM type (that is, a pointer to a VLA type). Finally, all identifiers declared with a VM type have to be ordinary identifiers and cannot, therefore, be members of structures or unions.
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