CODEWITHSUNDEEP
c
Bilbo92
Bilbo92

Reputation: 11

Reserved words vs standard identifier, how to see the difference

So I'm completely new to programming. I currently study computer science and have just read the first 200 pages of my programming book, but there's one thing I cannot seem to see the difference between and which havn't been clearly specified in the book and that's reserved words vs. standard identifiers - how can I see from code if it's one or the other.

I know the reserved words are some that cannot be changed, while the standard indentifiers can (though not recommended according to my book). The problem is while my book says reserved words are always in pure lowercase like,

(int, void, double, return)

it kinda seems to be the very same for standard indentifier like,

(printf, scanf)

so how do I know when it is what, or do I have to learn all the reserved words from the ANSI C, which is the current language we are trying to learn, (or whatever future language I might work with) to know when it is when?

Upvotes: 0

Views: 628

Answers (2)

ForceBru
ForceBru

Reputation: 44838

Reserved words are part of the language's syntax. C without int is not C, but something else. They are built into the language and are not and cannot be defined anywhere in terms of this particular language.

For example, if is a reserved keyword. You can't redefine it and even if you could, how would you do this in terms of the C language? You could do that in assembly, though.

The standard library functions you're talking about are ordinary functions that have been included into the standard library, nothing more. They are defined in terms of the language's syntax. Also, you can redefine these functions, although it's not advised to do so as this may lead to all sorts of bugs and unexpected behavior. Yet it's perfectly valid to write:

int puts(const char *msg) {
    printf("This has been monkey-patched!\n");
    return -1;
}

You'd get a warning that'd complain about the redefinition of a standard library function, but this code is valid anyway.

Now, imagine reimplementing return:

unknown_type return(unknown_type stuff) {
    // what to do here???
}

Upvotes: 0

Jonathan Leffler
Jonathan Leffler

Reputation: 754010

First off, you'll have to learn the rules for each language you learn as it is one of the areas that varies between languages. There's no universal rule about what's what.

Second, in C, you need to know the list of keywords; that seems to be what you're referring to as 'reserved words'. Those are important; they're immutable; they can't be abused because the compiler won't let you. You can't use int as a variable name; it is always a type.

Third, the C preprocessor can be abused to hijack anything; if you compile with #define double int in effect, you get what you deserve, but there's nothing much to stop you doing that.

Fourth, the only predefined variable name is __func__, the name of the current function.

Fifth, names such as printf() are defined by the standard library, but the standard library has to be implemented by someone using a C compiler; ask the maintainers of the GNU C library. For a discussion of many of the ideas behind the treaty between the standard and the compiler writers, and between the compiler writers and the programmers using a compiler, see the excellent book The Standard C Library by P J Plauger from 1992. Yes, it is old and the modern standard C library is somewhat bigger than the one from C90, but the background information is still valid and very helpful.

Upvotes: 2

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