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I have been going through a bit of history of C, and I find that in earlier versions of C, like in C89 standard, it is mandatory to declare variables at the beginning of a block.
But I also find there are some relaxations from C99 standard specification, where variable can be declared anywhere before it is used.
My question is why the earlier versions made it mandatory? my emphasis is to know if there was any technical difficulties in designing the compiler at those days, that prevented them identifying declarations at any point.
Also, with a compiler design perspective I understand, with such a restriction in C89, it is easy to handle variable declarations and usage with the help of an intermediate file to store the mappings. But are there methods that can handle the case without using an intermediary file, say some memory based storage?.
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The first one declares a variable of type char with the identifier c . The second is declares a variable of type int with the identifier area . The third one declares a variable of type float with the identifier num .
A declaration is used to announce the existence of the entity to the compiler; this is important in those strongly typed languages that require functions, variables, and constants, and their types to be specified with a declaration before use, and is used in forward declaration.
Modern C compilers such as gcc and clang support the C99 and C11 standards, which allow you to declare a variable anywhere a statement could go. The variable's scope starts from the point of the declaration to the end of the block (next closing brace). You can also declare variables inside for loop initializers.
Initialization of Variables in Java Dynamic - Dynamic means that in Java, you can declare variables anywhere in the program, because when the statement is executed the memory is assigned to them.
If the compiler sees a consolidated list of all the local/automatic variables up front, it can immediately work out the total amount by which to move the stack pointer to reserve stack memory for them - just one operation on the stack pointer. If it handles the variables in dribs and drabs as they're encountered in the function, moving the stack pointer incrementally, then there ends up being more opcodes dedicated to stack setup and stack pointer updates. It's important that the stack pointer be up to date whenever a further function call's performed. Newer compilers do a tiny bit of extra work to patch back in the amount by which to move the stack pointer after all the function's been considered. (I'd hazard that the effort's so minimal that the early Standard was shaped more by the conceptual appeal of knowing what to do up front than the effort of being more flexible, but if you just want to get something working - why make extra efforts?)
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C99 Rationale didn't directly explain why it was not permited in C89, but did say it was added in C99 because it was permited in other languages and the it has been found useful.
Rationale for International Standard — Programming Languages — C
§6.2.4 Storage durations of objects
A new feature of C99: C89 requires all declarations in a block to occur before any statements. On the other hand, many languages similar to C (such as Algol 68 and C++) permit declarations and statements to be mixed in an arbitrary manner. This feature has been found to be useful and has been added to C99.
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