I was studying various compiler option in GCC and observing changes when I made changes in the standard to be used.
$ gcc Q1.c -Wall -save-temps -o Q1
$ vi Q1.s
I see one of the opcodes as
call __isoc99_scanf
and now when I compile with the C89 standards
$gcc Q1.c -Wall -save-temps -std=c89 -o Q1
$ vi Q1.s
I see the opcode as
call scanf
What is the difference between these two forms of scanf? Any link where I can see their source would be highly appreciated.
Answer
The reason is strict following of c99 disallow some existing GNU extension conversion specifiers.
In glibc 2.17, in libio/stdio.h there is this comment:
/* For strict ISO C99 or POSIX compliance disallow %as, %aS and %a[
GNU extension which conflicts with valid %a followed by 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);