Why do C compilers prepend underscores to external names?

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It was common practice for C compilers to prepend a leading underscore to all external scope program identifiers to avert clashes with contributions from runtime language support

If the runtime support is provided by the compiler, you would think it would make more sense to prepend an underscore to the few external identifiers in the runtime support instead!

When C compilers first appeared, the basic alternative to programming in C on those platforms was programming in assembly language, and it was (and occasionally still is) useful to link together object files written in assembler and C. So really (IMHO) the leading underscore added to external C identifiers was to avoid clashes with the identifiers in your own assembly code.

(See also GCC’s asm label extension; and note that this prepended underscore can be considered a simple form of name mangling. More complicated languages like C++ use more complicated name mangling, but this is where it started.)

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