Why is glibc's sscanf vastly slower than fscanf on Linux?

Question

I am using GCC 4.8 and glibc 2.19 on an x86_64 Linux.

While playing with different input methods for a different question, I compared fscanf and s

Answer 1

It looks like glibc's sscanf() scans the source string for the length before doing anything else.

sscanf() (in stdio-common/sscanf.c) is essentially a wrapper around a call to _IO_vsscanf() (in libio/iovsscanf.c). And one of the first things that _IO_vsscanf() does is initialize its own _IO_strfile structure by calling _IO_str_init_static_internal() (in libio/strops.c) which calculates the length of the string if it's not provided.