parse_printf_format / register_printf_function / printf_size / printf_size_info (3) - glibc

Split from the chapter glibc: stdio and Formatted I/O into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <printf.h>

size_t parse_printf_format(const char *restrict fmt, size_t n,
                           int *restrict argtypes);
int register_printf_function(int spec, printf_function func,
                             printf_arginfo_function arginfo);
int printf_size(FILE *restrict fp, const struct printf_info *info,
                const void *const *restrict args);
int printf_size_info(const struct printf_info *restrict info,
                     size_t n, int *restrict argtypes);

All four are declared in /usr/include/printf.h with no internal feature-test gate (lines 99-110, 137-138, 176-183); printf.h itself is the gate. register_printf_function carries __attribute_deprecated__ (line 108).

Semantics

parse_printf_format parses fmt and fills up to n entries of argtypes with the type codes (PA_INT, PA_STRING, …, or’d with PA_FLAG_LONG etc.) each conversion needs, returning the number of arguments required (manual, Parsing a Template String). register_printf_function defines conversion %spec by pairing an output handler with an arginfo function; flags like # and modifiers like l cannot be registered, and a null func removes the handler (manual, Registering New Conversions). printf_size is glibc’s predefined handler printing floats with k/m/g… suffixes – powers of 1024 for lowercase specifiers, 1000 for uppercase – together with its mandatory printf_size_info arginfo function (manual, Predefined Printf Handlers).

Gotchas and portability

• glibc-only: musl 1.2.5 has no printf.h at all and none of these symbols (verified absence in musl include/ and src/).
• Prefer register_printf_specifier (glibc 2.10): the old function cannot carry the size information user-defined types need.
• Registering at runtime mutates global printf state guarded by a non-recursive lock – do not register from signal handlers or while other threads print (manual safety notes).
• musl absence means format-extension code needs a fallback path.

References

• glibc manual: stdio.texi, nodes Customizing Printf, Registering New Conversions, Predefined Printf Handlers
• headers: /usr/include/printf.h
• symbols: nm -D --with-symbol-versions libc.so.6