Ask a syntax question in gcc-4.3.2 source code

Ask a syntax problem in the source code of the gcc-4.3.2-Linux general technology-Linux programming and kernel information, the following is detailed reading. Ask a syntax question in gcc-4.3.2 source code
I don't know what it means to get a line (00046) About _ iobuf in the source code of gcc-4.3.2.tar.bz2 today. The details are as follows:
\ Gcc-4.3.2 \ gcc \ testsuite \ gcc. c-torture \ compile \ 920428-2.c
00036: extern struct _ iobuf {
00037: int _ cnt;
00038: char * _ ptr;
00039: char * _ base;
00040: int _ bufsiz;
00041: short _ flag;
00042: char _ file;
00043:} _ iob [];
00044: typedef unsigned long size_t;
00045: typedef char * va_list;
00046: struct _ iobuf * (const char * filename, const char * type );

[As follows:]


Question 1: Is this a function? Why is there no function name?



In fact, this is also unintentional discovery. I originally wanted to see how the fopen function is implemented, but after looking for a long time, I saw the fopen Statement (as shown below)
\ Gcc \ gcc-4.3.2 \ fixdocumdes \ tests \ base \ stdio. h
00091: extern FILE * fopen (const char * _ filename, const char * _ type );
Question 2: Where is the fopen function implemented?

Thank you for your help.