The Development of the C Language

                                   The Development of the C Language
Ritchie

incorporated many of the innovative aspects of Multics, including an explicit notion of a process

as a locus of control, a treestructured

file system, a command interpreter as userlevel

program,

simple representation of text files, and generalized access to devices. They excluded others, such

as unified access to memory and to files. At the start, moreover, he and the rest of us deferred

another pioneering (though not original) element of Multics, namely writing almost exclusively in

a higherlevel

language. PL/I, the implementation language of Multics, was not much to our

tastes, but we were also using other languages, including BCPL, and we regretted losing the

advantages of writing programs in a language above the level of assembler, such as ease of writing

and clarity of understanding. At the time we did not put much weight on portability; interest

in this arose later.

Thompson was faced with a hardware environment cramped and spartan even for the time: rm, cat, cp commands) were completed. After this point, the operating system was

the DEC PDP7

on which he started in 1968 was a machine with 8K 18bit

words of memory and

no software useful to him. While wanting to use a higherlevel

language, he wrote the original

Unix system in PDP7

assembler. At the start, he did not even program on the PDP7

itself, but

instead used a set of macros for the GEMAP assembler on a GE635

machine. A postprocessor

generated a paper tape readable by the PDP7.

These tapes were carried from the GE machine to the PDP7

for testing until a primitive

Unix kernel, an editor, an assembler, a simple shell (command interpreter), and a few utilities

(like the Unix

selfsupporting:

programs could be written and tested without resort to paper tape, and development

continued on the PDP7

itself.

Thompson’s PDP7

assembler outdid even DEC’s in simplicity; it evaluated expressions with a fixed name thata.out, explains a bit of Unix etymology; it is the possibly needed a system programming language. After a rapidly scuttled

and emitted the corresponding bits. There were no libraries, no loader or link editor: the entire

source of a program was presented to the assembler, and the output file

emerged was directly executable. (This name,

output of the assembler. Even after the system gained a linker and a means of specifying another

name explicitly, it was retained as the default executable result of a compilation.)

Not long after Unix first ran on the PDP7,

in 1969, Doug McIlroy created the new system’s

first higherlevel

language: an implementation of McClure’s TMG [McClure 65]. TMG is a language

for writing compilers (more generally, TransMoGrifiers) in a topdown,

recursivedescent

style that combines contextfree

syntax notation with procedural elements. McIlroy and Bob

Morris had used TMG to write the early PL/I compiler for Multics.

Challenged by McIlroy’s feat in reproducing TMG, Thompson decided that Unix

it had not even been named yet

attempt at Fortran, he created instead a language of his own, which he called B. B can be

thought of as C without types; more accurately, it is BCPL squeezed into 8K bytes of memory

and filtered through Thompson’s brain. Its name most probably represents a contraction of

BCPL, though an alternate theory holds that it derives from Bon [Thompson 69], an unrelated

language created by Thompson during the Multics days. Bon in turn was named either after his

wife Bonnie, or (according to an encyclopedia quotation in its manual), after a religion whose rituals

involve the murmuring of magic formulas.

Origins: the languages

BCPL was designed by Martin Richards in the mid1960s

while he was visiting MIT, and

was used during the early 1970s for several interesting projects, among them the OS6 operating

system at Oxford [Stoy 72], and parts of the seminal Alto work at Xerox PARC [Thacker 79].

We became familiar with it because the MIT CTSS system [Corbato 62] on which Richards

worked was used for Multics development. The original BCPL compiler was transported both to

Multics and to the GE635

GECOS system by Rudd Canaday and others at Bell Labs [Canaday

69]; during the final throes of Multics’s life at Bell Labs and immediately after, it was the language

of choice among the group of people who would later become involved with Unix.

BCPL, B, and C all fit firmly in the traditional procedural family typified by Fortran and