vfprintf writes formatted output to a stream from a va_list. It lets a variadic wrapper delegate formatting instead of manually retrieving and classifying each argument. section 16.5.2
Forwarding a debug argument list
int printf_debug(const char *format, ...) { int ret = 0; if (iodebug) { va_list va; va_start(va, format); ret = vfprintf(iodebug, format, va); va_end(va); } return ret; } The wrapper initializes va with va_start, passes it and the format string to vfprintf, then finishes with va_end. The stream is selected separately from the formatting logic. section 16.5.2
Use the surrounding variadic interface carefully:
- Include stdarg.h and manage va_list with va_start and va_end. section 16.5.2
- Keep the format string and supplied arguments consistent; vfprintf removes the need for manual case analysis, but not the need for a valid argument convention. section 16.5.2
- Prefer a non-variadic interface for new designs when an equivalent fixed interface is practical. section 16.5.2
20 FILE* iodebug = 0;
21
22 /**
23 ** @brief Prints to the debug stream @c iodebug
24 **/
25 #ifdef __GNUC__
26 __attribute__((format(printf, 1, 2)))
27 #endif
28 int printf_debug(const char *format, ...) {
29 int ret = 0;
30 if (iodebug) {
31 va_list va;
32 va_start(va, format);
33 ret = vfprintf(iodebug, format, va);
34 va_end(va);
35 }
36 return ret;
37 }
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Inspect the complete printf_debug wrapper, including stream checking, va_list setup, vfprintf forwarding, and cleanup. section 16.5.2
