This is the platform dependent conversion specifier for scanning and printing values of type #gint64. See also #G_GINT16_FORMAT.
Some platforms do not support scanning and printing 64-bit integers, even though the types are supported. On such platforms %G_GINT64_FORMAT is not defined. Note that scanf() may not support 64-bit integers, even if %G_GINT64_FORMAT is defined. Due to its weak error handling, scanf() is not recommended for parsing anyway; consider usinginstead.