Why does/did C allow implicit function and typeless variable declarations?

Question

Why is it sensible for a language to allow implicit declarations of functions and typeless variables? I get that C is old, but allowing to omit declarations and default to int() (or int in case of variables) doesn't seem so sane to me, even back then.

So, why was it originally introduced? Was it ever really useful? Is it actually (still) used?

Note: I realise that modern compilers give you warnings (depending on which flags you pass them), and you can suppress this feature. That's not the question!

---

Example:

int main() {
  static bar = 7; // defaults to "int bar"
  return foo(bar); // defaults to a "int foo()"
}

int foo(int i) {
  return i;
}

Answer 1

See Dennis Ritchie's "The Development of the C Language": http://cm.bell-labs.com/who/dmr/chist.html

For instance,

In contrast to the pervasive syntax variation that occurred during the creation of B, the core semantic content of BCPL—its type structure and expression evaluation rules—remained intact. Both languages are typeless, or rather have a single data type, the 'word', or 'cell', a fixed-length bit pattern. Memory in these languages consists of a linear array of such cells, and the meaning of the contents of a cell depends on the operation applied. The + operator, for example, simply adds its operands using the machine's integer add instruction, and the other arithmetic operations are equally unconscious of the actual meaning of their operands. Because memory is a linear array, it is possible to interpret the value in a cell as an index in this array, and BCPL supplies an operator for this purpose. In the original language it was spelled rv, and later !, while B uses the unary *. Thus, if p is a cell containing the index of (or address of, or pointer to) another cell, *p refers to the contents of the pointed-to cell, either as a value in an expression or as the target of an assignment.

This typelessness persisted in C until the authors started porting it to machines with different word lengths:

The language changes during this period, especially around 1977, were largely focused on considerations of portability and type safety, in an effort to cope with the problems we foresaw and observed in moving a considerable body of code to the new Interdata platform. C at that time still manifested strong signs of its typeless origins. Pointers, for example, were barely distinguished from integral memory indices in early language manuals or extant code; the similarity of the arithmetic properties of character pointers and unsigned integers made it hard to resist the temptation to identify them. The unsigned types were added to make unsigned arithmetic available without confusing it with pointer manipulation. Similarly, the early language condoned assignments between integers and pointers, but this practice began to be discouraged; a notation for type conversions (called `casts' from the example of Algol 68) was invented to specify type conversions more explicitly. Beguiled by the example of PL/I, early C did not tie structure pointers firmly to the structures they pointed to, and permitted programmers to write pointer->member almost without regard to the type of pointer; such an expression was taken uncritically as a reference to a region of memory designated by the pointer, while the member name specified only an offset and a type.

Programming languages evolve as programming practices change. In modern C and the modern programming environment, where many programmers have never written assembly language, the notion that ints and pointers are interchangeable may seem nearly unfathomable and unjustifiable.

Answer 2

It's the usual story — hysterical raisins (aka 'historical reasons').

In the beginning, the big computers that C ran on (DEC PDP-11) had 64 KiB for data and code (later 64 KiB for each). There was a limit to how complex you could make the compiler and still have it run. Indeed, there was scepticism that you could write an O/S using a high-level language such as C, rather than needing to use assembler. So, there were size constraints. Also, we are talking a long time ago, in the early to mid 1970s. Computing in general was not as mature a discipline as it is now (and compilers specifically were much less well understood). Also, the languages from which C was derived (B and BCPL) were typeless. All these were factors.

The language has evolved since then (thank goodness). As has been extensively noted in comments and down-voted answers, in strict C99, implicit int for variables and implicit function declarations have both been made obsolete. However, most compilers still recognize the old syntax and permit its use, with more or less warnings, to retain backwards compatibility, so that old source code continues to compile and run as it always did. C89 largely standardized the language as it was, warts (gets()) and all. This was necessary to make the C89 standard acceptable.

There is still old code around using the old notations — I spend quite a lot of time working on an ancient code base (circa 1982 for the oldest parts) which still hasn't been fully converted to prototypes everywhere (and that annoys me intensely, but there's only so much one person can do on a code base with multiple millions of lines of code). Very little of it still has 'implicit int' for variables; there are too many places where functions are not declared before use, and a few places where the return type of a function is still implicitly int. If you don't have to work with such messes, be grateful to those who have gone before you.