MESSAGE
| DATE | 2015-06-26 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [LIU Comp Sci] Fwd: strange function types in unistd.h
|
From owner-learn-outgoing-at-mrbrklyn.com Fri Jun 26 18:24:36 2015
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Date: Fri, 26 Jun 2015 18:24:09 -0400
From: Ruben Safir
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To: Hangout , learn-at-nylxs.com
Subject: [LIU Comp Sci] Fwd: strange function types in unistd.h
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Just something worth bringing to your attention and for learning about
advanced C programming in Unix and GCC
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From: ruben safir
Newsgroups: comp.unix.programmer
Subject: strange function types in unistd.h
Date: Wed, 24 Jun 2015 23:39:08 -0400
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Xref: panix comp.unix.programmer:234761
Can someone walk me through the exact meaning of this line
279 /* Values for the second argument to access.
280 These may be OR'd together. */
281 #define R_OK 4 /* Test for read permission. */
282 #define W_OK 2 /* Test for write permission. */
283 #define X_OK 1 /* Test for execute permission. */
284 #define F_OK 0 /* Test for existence. */
285
286 /* Test for access to NAME using the real UID and real GID. */
287 extern int access (const char *__name, int __type) __THROW
__nonnull ((1));
288
287 - throw and nonnul? I'm not familiar with this C syntax
Ruben
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From: Barry Schwarz
Newsgroups: comp.unix.programmer
Subject: Re: strange function types in unistd.h
Date: Wed, 24 Jun 2015 21:00:59 -0700
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Xref: panix comp.unix.programmer:234762
On Wed, 24 Jun 2015 23:39:08 -0400, ruben safir
wrote:
>Can someone walk me through the exact meaning of this line
>
>
> 279 /* Values for the second argument to access.
> 280 These may be OR'd together. */
> 281 #define R_OK 4 /* Test for read permission. */
> 282 #define W_OK 2 /* Test for write permission. */
> 283 #define X_OK 1 /* Test for execute permission. */
> 284 #define F_OK 0 /* Test for existence. */
> 285
> 286 /* Test for access to NAME using the real UID and real GID. */
> 287 extern int access (const char *__name, int __type) __THROW
>__nonnull ((1));
> 288
>
>287 - throw and nonnul? I'm not familiar with this C syntax
C is case sensitive and the underscores are part of the tokens. So it
is __THROW and __nonnull (with two ells).
The standard reserves tokens starting with two underscores for
extensions provided by the implementation. These fit that definition.
Just as a guess, it appears that __nonull directs the compiler to
generate some additional code when the function named access is
called. This code would verify that argument 1 is not NULL. __THROW
probably identifies the action if the verification fails.
--
Remove del for email
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From: ruben safir
Newsgroups: comp.unix.programmer
Subject: Re: strange function types in unistd.h
Date: Thu, 25 Jun 2015 01:04:58 -0400
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Xref: panix comp.unix.programmer:234763
On 06/25/2015 12:00 AM, Barry Schwarz wrote:
> The standard reserves tokens starting with two underscores for
> extensions provided by the implementation.
the implementation in this case is gcc?
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From: Xavier Roche
Newsgroups: comp.unix.programmer
Subject: Re: strange function types in unistd.h
Date: Thu, 25 Jun 2015 07:25:12 +0200
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Le 25/06/2015 07:04, ruben safir a ?crit :
> the implementation in this case is gcc?
Yes, mostly. __nonnull is just expanding to nonnull attributes
/usr/include/sys/cdefs.h:
#if __GNUC_PREREQ (3,3)
# define __nonnull(params) __attribute__ ((__nonnull__ params))
...
See
nonnull (arg-index, ...)
The nonnull attribute specifies that some function parameters should be non-null pointers. For instance, the declaration:
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull (1, 2)));
causes the compiler to check that, in calls to my_memcpy, arguments dest and src are non-null. If the compiler determines that a null pointer is passed in an argument slot marked as non-null, and the -Wnonnull option is enabled, a warning is issued. The compiler may also choose to make optimizations based on the knowledge that certain function arguments will not be null.
If no argument index list is given to the nonnull attribute, all pointer arguments are marked as non-null. To illustrate, the following declaration is equivalent to the previous example:
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull));
[ Note on '__attribute__ ((__nonnull__' :
You may also specify attributes with `__' preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use __noreturn__ instead of noreturn. ]
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