c - Why does printf require a mandatory parameter? -

the definition of printf function in c language is:

int printf(const char * _format, ...); 

the same scanf , lot of similar functions managed variable number of arguments.

why there _format mandatory parameter?

the format string mandatory because way c's variable argument macros work depends on @ least 1 argument being present, , using find others.

specifically, read other variable arguments, use va_start (then va_arg repeatedly, once each variable argument want read). when call va_start, need pass format string (or, more generally, last non-varying parameter function).

for example, acts printf, prints both stdout , file of choice:

void tee(file *f, char const *fmt, ...) {      va_list ap;     va_start(ap, fmt);     vprintf(fmt, ap);     va_end(ap);     va_start(ap, fmt);     vfprintf(f, fmt, ap);     va_end(ap); } 

this uses vprintf , vfprintf, doesn't (directly) use va_arg itself, va_start , va_end, that's enough show how fmt involved in using va_start.

at 1 time, wasn't needed. when c shiny , new, have function equivalent to: int f(...);.

during first c standardization effort, however, eliminated in favor of macros noted above (va_start, va_arg, va_end) require @ least 1 named parameter. older macros placed number of requirements on calling convention:

1. parameters passed same way, regardless of type or number.
2. it's easy find first argument passed.

with conventional c calling convention (all arguments passed on stack, arguments pushed right left) true. looked @ top of stack, moved backward past return address, , there first argument.

with other calling conventions, things weren't simple though. example, pushing arguments left right means first argument (the format string, in case of printf) buried arbitrary distance down stack, arbitrary number of other arguments after it.

the way came deal pass previous (named) argument va_start (and va_start macro use address of argument). if push right left, give address whatever distance needed down stack, va_arg can walk up stack retrieve other variable arguments.

this apparently seen acceptable compromise, since functions take variable arguments take @ least 1 named parameter anyway.