Patchwork [v2,2/5] Remove getc and putc macros from the public stdio.h.

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Submitter Zack Weinberg
Date Feb. 5, 2018, 11:34 p.m.
Message ID <48dd1f0e9d32e9a96f6c712e828efc3d5b1bb189.1517872307.git.zackw@panix.com>
Download mbox | patch
Permalink /patch/25828/
State New
Headers show

Comments

Zack Weinberg - Feb. 5, 2018, 11:34 p.m.
The getc and putc macros in the public stdio.h expand to call _IO_getc
and _IO_putc respectively.  As _IO_getc, fgetc, and getc are all aliases
for the same function, and _IO_putc, fputc, and putc are also all aliases
for the same function, the macros are pointless.  The C standard does
not require getc and putc to be macros, so let's just not have macros.
All four symbols are exported from libc.so at the same, ancient symbol
version, so there should be no risks for binary compatibility.  Similarly,
the getchar and putchar inlines in bits/stdio.h forward to getc and putc
instead of their _IO_ aliases.

As a change from longstanding historical practice, this does seem
like it might break _something_, so there is a note in NEWS, which
is also a convenient place to advise people that if they thought getc
and putc had reduced per-character overhead they should consider using
getc_unlocked and putc_unlocked instead.  (These are also not macros,
but when optimizing, they are inlines.)

This patch changes installed stripped libraries, since several of the
ancillary libraries and programs use getc and/or putc, so they
formerly referenced _IO_getc / _IO_putc.

	* libio/stdio.h: Don't define getc or putc as macros.
	* libio/bits/stdio.h (getchar, putchar): Use getc and putc,
	not _IO_getc and _IO_putc.
---
 NEWS               | 7 +++++++
 libio/bits/stdio.h | 4 ++--
 libio/stdio.h      | 8 --------
 3 files changed, 9 insertions(+), 10 deletions(-)
Joseph Myers - Feb. 6, 2018, 12:01 a.m.
On Mon, 5 Feb 2018, Zack Weinberg wrote:

> diff --git a/NEWS b/NEWS
> index 3ac57eca4ee..eceab2b2be2 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -15,6 +15,13 @@ Deprecated and removed features, and other changes affecting compatibility:
>  
>    [Add deprecations, removals and changes affecting compatibility here]
>  
> + * The stdio.h functions 'getc' and 'putc' are no longer defined as macros.
> +   This was never required by the C standard, and the macros just expanded
> +   to call alternative names for the same functions.  If you hoped getc and
> +   putc would provide performance improvements over fgetc and fputc, instead
> +   investigate using (f)getc_unlocked and (f)putc_unlocked, and, if
> +   necessary, flockfile and funlockfile.

The "[Add deprecations, removals and changes affecting compatibility 
here]" placeholder should be removed by this patch, as the first one to 
put something in this section for 2.28.

The patch is OK with that change.

There is a theoretical use for implementation-namespace aliases of even 
C90 standard functions such as getc and putc - properly, if a function is 
used in a *macro* defined in a standard header, it should be used via such 
an alias to avoid possible shadowing of the standard function name by a 
block-scope variable in a scope where the macro is used (standard function 
names aren't generally reserved with block scope).  But that doesn't 
affect this patch at all, since there are no such macros making use of 
either getc or putc (and in any case, the _IO_getc and _IO_putc function 
exports still exist should they prove to be of use in future, and any 
aliases for this issue could be defined in headers with asm redirection 
without needing library exports at all).
Zack Weinberg - Feb. 6, 2018, 1:03 a.m.
On Mon, Feb 5, 2018 at 7:01 PM, Joseph Myers <joseph@codesourcery.com> wrote:
> On Mon, 5 Feb 2018, Zack Weinberg wrote:
>> diff --git a/NEWS b/NEWS
>> index 3ac57eca4ee..eceab2b2be2 100644
>> --- a/NEWS
>> +++ b/NEWS
>> @@ -15,6 +15,13 @@ Deprecated and removed features, and other changes affecting compatibility:
>>
>>    [Add deprecations, removals and changes affecting compatibility here]
>>
>> + * The stdio.h functions 'getc' and 'putc' are no longer defined as macros.
>> +   This was never required by the C standard, and the macros just expanded
>> +   to call alternative names for the same functions.  If you hoped getc and
>> +   putc would provide performance improvements over fgetc and fputc, instead
>> +   investigate using (f)getc_unlocked and (f)putc_unlocked, and, if
>> +   necessary, flockfile and funlockfile.
>
> The "[Add deprecations, removals and changes affecting compatibility
> here]" placeholder should be removed by this patch, as the first one to
> put something in this section for 2.28.

Doh! That was a merge botch on my end.

> The patch is OK with that change.

I have made that change and pushed patches 1 and 2 of this series.

Also, the full patchset is now available as git branch
'zack/no-libio-h' if anyone wants to experiment with it.

zw

Patch

diff --git a/NEWS b/NEWS
index 3ac57eca4ee..eceab2b2be2 100644
--- a/NEWS
+++ b/NEWS
@@ -15,6 +15,13 @@  Deprecated and removed features, and other changes affecting compatibility:
 
   [Add deprecations, removals and changes affecting compatibility here]
 
+ * The stdio.h functions 'getc' and 'putc' are no longer defined as macros.
+   This was never required by the C standard, and the macros just expanded
+   to call alternative names for the same functions.  If you hoped getc and
+   putc would provide performance improvements over fgetc and fputc, instead
+   investigate using (f)getc_unlocked and (f)putc_unlocked, and, if
+   necessary, flockfile and funlockfile.
+
 Changes to build and runtime requirements:
 
   [Add changes to build and runtime requirements here]
diff --git a/libio/bits/stdio.h b/libio/bits/stdio.h
index d287083de16..eb42b22153c 100644
--- a/libio/bits/stdio.h
+++ b/libio/bits/stdio.h
@@ -43,7 +43,7 @@  vprintf (const char *__restrict __fmt, _G_va_list __arg)
 __STDIO_INLINE int
 getchar (void)
 {
-  return _IO_getc (stdin);
+  return getc (stdin);
 }
 
 
@@ -78,7 +78,7 @@  getchar_unlocked (void)
 __STDIO_INLINE int
 putchar (int __c)
 {
-  return _IO_putc (__c, stdout);
+  return putc (__c, stdout);
 }
 
 
diff --git a/libio/stdio.h b/libio/stdio.h
index 95bc902a82c..33de3813bbd 100644
--- a/libio/stdio.h
+++ b/libio/stdio.h
@@ -483,10 +483,6 @@  extern int getc (FILE *__stream);
    marked with __THROW.  */
 extern int getchar (void);
 
-/* The C standard explicitly says this is a macro, so we always do the
-   optimization for it.  */
-#define getc(_fp) _IO_getc (_fp)
-
 #ifdef __USE_POSIX199506
 /* These are defined in POSIX.1:1996.
 
@@ -523,10 +519,6 @@  extern int putc (int __c, FILE *__stream);
    marked with __THROW.  */
 extern int putchar (int __c);
 
-/* The C standard explicitly says this can be a macro,
-   so we always do the optimization for it.  */
-#define putc(_ch, _fp) _IO_putc (_ch, _fp)
-
 #ifdef __USE_MISC
 /* Faster version when locking is not necessary.