c - __isoc99_scanf and scanf -

i studying various compiler option in gcc , observing changes when made changes in standard used.

$ gcc q1.c -wall -save-temps -o q1 $ vi q1.s 

i see 1 of opcodes as

 call    __isoc99_scanf 

and when compile c89 standards

$gcc q1.c -wall -save-temps -std=c89 -o q1 $ vi q1.s 

i see opcode as

call    scanf 

what difference between these 2 forms of scanf? link can see source highly appreciated.

the reason strict following of c99 disallow existing gnu extension conversion specifiers.

in glibc 2.17, in libio/stdio.h there comment:

/* strict iso c99 or posix compliance disallow %as, %as , %a[    gnu extension conflicts valid %a followed letter    s, s or [.  */ extern int __redirect (fscanf, (file *__restrict __stream,     const char *__restrict __format, ...),     __isoc99_fscanf) __wur; extern int __redirect (scanf, (const char *__restrict __format, ...),     __isoc99_scanf) __wur; extern int __redirect_nth (sscanf, (const char *__restrict __s,       const char *__restrict __format, ...),       __isoc99_sscanf);