flex.skl calls fileno(), which is not ISO C99 function

[Explorer09]

I caught this warning when building flex in -std=c99 mode:

./stage1flex   -o stage1scan.c ./scan.l
  CC       flex-stage1scan.o
stage1scan.c: In function ‘yy_init_buffer’:
stage1scan.c:4757:48: warning: implicit declaration of function ‘fileno’; did you mean ‘mblen’? [-Wimplicit-function-declaration]
         b->yy_is_interactive = file ? (isatty( fileno(file) ) > 0) : 0;
                                                ^~~~~~
                                                mblen

In glibc, fileno() prototype is exposed only if _POSIX_C_SOURCE is defined to >= 1. Although flex will build with _GNU_SOURCE defined (AC_USE_SYSTEM_EXTENSIONS in configure script), the problem is that generated scanners may not build if the user's environment requires strict ISO C99, and I consider this a bug.

So this issue report is about two things:

1. The #define _POSIX_C_SOURCE 1 is missing in flex.skl when the scanner is specified neither always-interactive or never-interactive option.
2. We didn't document that we use POSIX functions to detect whether the input is "interactive", and so brings surprise to users that expect it builds in non-POSIX environments.

[westes]

What about rewriting to not use fileno() and being C99 compliant that way?

[westes]

I checked against my copy of the glibc reference manual and it doesn't note anything special about fileno() as well. Which is unfortunate given what you describe.

[Explorer09]

@westes
For your first question, it won't help much if we replace fileno() with something else since the routine already depends on isatty(), and so I'm pursuing only to have the feature macro defined.
For your second question, glibc manual doesn't explicitly mention about feature macro requirements, but Linux man pages does, as well as the code of "/usr/include/stdio.h" and "features.h":

/*stdio.h*/
#ifdef	__USE_POSIX
/* Return the system file descriptor for STREAM.  */
extern int fileno (FILE *__stream) __THROW __wur;
#endif /* Use POSIX.  */

/*features.h*/
#if defined _POSIX_SOURCE || _POSIX_C_SOURCE >= 1 || defined _XOPEN_SOURCE
# define __USE_POSIX	1
#endif

[westes]

On Monday, 4 September 2017, 8:44 am -0700, "Kang-Che Sung (宋岡哲)" ***@***.***> wrote: @westes For your first question, it won't help much if we replace fileno() with something else since the routine already depends on isatty(), and so I'm pursuing only to have the feature macro defined.

Fair enough. At some level, however, It seems reasonable to assume a few things -- C99 and POSIX are both hardly new, for instance. If it's just a matter of ensuring that feature macros get defined in ways people expect, that is reasonable. But isatty() has been around since SVR4 and 4.3BSD according to the man page, so I'm not worried about systems that "forget" to declare it.

For your second question, glibc manual doesn't explicitly mention about feature macro requirements, but Linux man pages does, as well as the code of "/usr/include/stdio.h" and "features.h":

I was not doubting your description. It's just unfortunate that the glibc reference manual I have on my system doesn't make any note of the feature macros needed to provide the function.

…

-- Will Estes Flex Project Maintainer westes575@gmail.com https://github.com/westes/flex