Editing Printf (section)

=== Custom data type formatting ===

A common way to handle formatting with a custom data type is to format the custom data type value into a [[string (computer science)|string]], then use the {{code|%s|c}} specifier to include the serialized value in a larger message.

Some printf-like functions allow extensions to the [[escape-character]]-based [[mini-language]], thus allowing the programmer to use a specific formatting function for non-builtin types. One is the (now [[deprecated]]) [[glibc]]'s [https://www.gnu.org/software/libc/manual/html_node/Customizing-Printf.html {{code|register_printf_function()}}]. However, it is rarely used due to the fact that it conflicts with [[static program analysis|static format string checking]]. Another is [http://www.and.org/vstr/#cust-fmt Vstr custom formatters], which allows adding multi-character format names.

Some applications (like the [[Apache HTTP Server]]) include their own printf-like function, and embed extensions into it. However these all tend to have the same problems that {{code|register_printf_function()}} has.

The [[Linux kernel]] <code>[[printk]]</code> function supports a number of ways to display kernel structures using the generic {{code|%p|c}} specification, by {{em|appending}} additional format characters.<ref>{{cite web | url= https://docs.kernel.org/core-api/printk-formats.html | title= How to get printk format specifiers right | work = The Linux Kernel documentation | first1= Randy | last1= Dunlap | first2= Andrew | last2= Murray | publisher= [[Linux Foundation]] | date= n.d. | access-date= 2025-02-12 | archive-date= 2025-02-06 | archive-url= https://web.archive.org/web/20250206200419/https://docs.kernel.org/core-api/printk-formats.html | url-status= live }}</ref> For example, {{code|%pI4|c}} prints an [[IPv4 address]] in dotted-decimal form. This allows static format string checking (of the {{code|%p|c}} portion) at the expense of full compatibility with normal printf.