OPEN_MAX在哪里为Linux系统定义？

看来OPEN_MAX已被弃用,至少在Linux系统上是这样.原因似乎是可以同时打开的最大文件数不固定,因此扩展为整数字的宏不是获取该信息的好方法.

另一个宏FOPEN_MAX应该是相似的;我想不出OPEN_MAX和FOPEN_MAX如果定义的话应该有不同的值的原因.但是FOPEN_MAX是C语言标准的强制要求,因此系统没有选择不定义它. C标准说FOPEN_MAX

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

(如果“最小”这个词令人困惑,则可以保证程序可以同时打开至少那么多文件.)

如果您想要当前可以打开的最大文件数,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.这不是计数,而是sysconf()识别的值.并且它没有在我的系统上定义.)

我在我的Ubuntu系统上搜索了OPEN_MAX(字匹配,因此不包括FOPEN_MAX),并发现以下内容(这些显然只是简短的摘录)：

/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.
*/