OPEN_MAX在Linux系统中定义在哪里？ [英] Where is OPEN_MAX defined for Linux systems?

问题描述

OPEN_MAX 是定义单个程序允许的最大打开文件数的常量。

开始Linux编程4 ^th 版，页101：

$ b

这个限制通常由常量OPEN_MAX定义limits.h，因系统而异，...

在我的系统中，文件限制。 h 在目录 /usr/lib/gcc/x86_64-linux-gnu/4.6/include-fixed 中没有这个常量。我是否看到错误的 limits.h 或者自2008年以来 OPEN_MAX 的位置发生了变化？

解决方案

值得一提的是，第四版的开始Linux编程于2007年发布;它的一部分可能有点过时。 （这不是对本书的批评，我没有读过。）

看来 OPEN_MAX 是不推荐使用，至少在Linux系统上。原因似乎是，可以同时打开的最大文件数量不是固定的，因此扩展为整数字面值的宏不是获取该信息的好方法。

还有另一个应该类似的宏 FOPEN_MAX ;我想不出为什么 OPEN_MAX 和 FOPEN_MAX ，如果它们都定义了，应该有不同的值。但是C语言标准要求 FOPEN_MAX ，所以系统没有选择不定义它。 C标准规定： FOPEN_MAX

展开为一个整数常量表达式， （
）实现保证的最小文件数可以同时打开

（如果minimum这个词很混乱，一个程序可以至少打开多个文件的保证。）

如果你想要当前可以打开的最大文件数量，请查看 sysconf（） 功能;在我的系统上， sysconf（_SC_OPEN_MAX）返回1024.（ sysconf（）手册页引用符号 OPEN_MAX 。这不是一个计数，而是一个由 sysconf（）识别的值，并且它没有在我的系统中定义。

我搜索了 OPEN_MAX （单词匹配，所以不包括 FOPEN_MAX ），并发现以下内容（这些显然只是简短的摘录）：
$ b

/ usr / include / X11 /Xos.h ：

 ＃ifdef __GNU__ 
 #define PATH_MAX 4096 
＃define MAXPATHLEN 4096 
 #define OPEN_MAX 256 / *我们定义一个合理的限制。 * / 
＃endif 
  

/ usr / include / i386- linux-gnu / bits / local_lim.h ：

  / *内核头文件污染了命名空间NR_OPEN符号
并定义了LINK_MAX，尽管文件系统具有不同的最大值。对于OPEN_MAX，
类似的事情是正确的：限制可以在
运行时改变，因此宏不能被定义。如有必要，在包含标题后删除此
。 * / 
 #ifndef NR_OPEN 
＃define __undef_NR_OPEN 
 #endif 
 #ifndef LINK_MAX 
＃define __undef_LINK_MAX 
 #endif 
 #ifndef OPEN_MAX 
＃define __undef_OPEN_MAX 
 #endif 
 #ifndef ARG_MAX 
＃define __undef_ARG_MAX 
 #endif 
  

/usr/include/i386-linux-gnu/bits/xopen_lim.h ：

  / *我们不提供
 
的固定值ARG_MAX`exec'函数参数的最大长度
包括环境数据。 
 
 ATEXIT_MAX可以用`atexit'注册的
的最大数量。 
 
 CHILD_MAX每个真实
用户ID的最大并发进程数。 
 
 OPEN_MAX任何时候一个进程可以打开
的最大文件数量。 
 
 PAGESIZE 
 PAGE_SIZE页面字节的大小。 
 
 PASS_MAX密码中有效字节的最大数量。 
 
我们只为
 
提供一个固定限制IOV_MAX一个进程有
可用于`readv'或writev'的`iovec'结构的最大数量。 
 
，如果这确实由底层系统修复的话。 
 * / 
  

OPEN_MAX is the constant that defines the maximum number of open files allowed for a single program.

According to Beginning Linux Programming 4^th Edition, Page 101 :

The limit, usually defined by the constant OPEN_MAX in limits.h, varies from system to system, ...

In my system, the file limits.h in directory /usr/lib/gcc/x86_64-linux-gnu/4.6/include-fixed does not have this constant. Am i looking at the wrong limits.h or has the location of OPEN_MAX changed since 2008 ?

解决方案

For what it's worth, the 4th edition of Beginning Linux Programming was published in 2007; parts of it may be a bit out of date. (That's not a criticism of the book, which I haven't read.)

It appears that OPEN_MAX is deprecated, at least on Linux systems. The reason appears to be that the maximum number of file that can be opened simultaneously is not fixed, so a macro that expands to an integer literal is not a good way to get that information.

There's another macro FOPEN_MAX that should be similar; I can't think of a reason why OPEN_MAX and FOPEN_MAX, if they're both defined, should have different values. But FOPEN_MAX is mandated by the C language standard, so system's don't have the option of not defining it. The C standard says that FOPEN_MAX

expands to an integer constant expression that is the minimum number of files that the implementation guarantees can be open simultaneously

(If the word "minimum" is confusing, it's a guarantee that a program can open at least that many files at once.)

If you want the current maximum number of files that can be opened, take a look at the sysconf() function; on my system, sysconf(_SC_OPEN_MAX) returns 1024. (The sysconf() man page refers to a symbol OPEN_MAX. This is not a count, but a value recognized by sysconf(). And it's not defined on my system.)

I've searched for OPEN_MAX (word match, so excluding FOPEN_MAX) on my Ubuntu system, and found the following (these are obviously just brief excerpts):

/usr/include/X11/Xos.h:

# ifdef __GNU__
#  define PATH_MAX 4096
#  define MAXPATHLEN 4096
#  define OPEN_MAX 256 /* We define a reasonable limit.  */
# endif

/usr/include/i386-linux-gnu/bits/local_lim.h:

/* The kernel header pollutes the namespace with the NR_OPEN symbol
   and defines LINK_MAX although filesystems have different maxima.  A
   similar thing is true for OPEN_MAX: the limit can be changed at
   runtime and therefore the macro must not be defined.  Remove this
   after including the header if necessary.  */  
#ifndef NR_OPEN
# define __undef_NR_OPEN
#endif
#ifndef LINK_MAX
# define __undef_LINK_MAX
#endif
#ifndef OPEN_MAX
# define __undef_OPEN_MAX
#endif
#ifndef ARG_MAX
# define __undef_ARG_MAX
#endif

/usr/include/i386-linux-gnu/bits/xopen_lim.h:

/* We do not provide fixed values for 

   ARG_MAX      Maximum length of argument to the `exec' function
                including environment data.

   ATEXIT_MAX   Maximum number of functions that may be registered
                with `atexit'.

   CHILD_MAX    Maximum number of simultaneous processes per real
                user ID. 

   OPEN_MAX     Maximum number of files that one process can have open
                at anyone time.

   PAGESIZE
   PAGE_SIZE    Size of bytes of a page.

   PASS_MAX     Maximum number of significant bytes in a password.

   We only provide a fixed limit for

   IOV_MAX      Maximum number of `iovec' structures that one process has
                available for use with `readv' or writev'.

   if this is indeed fixed by the underlying system.
*/

这篇关于OPEN_MAX在Linux系统中定义在哪里？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！