sysconf、pathconf和fpathconf函数

1. 限制

Unix系统实现定义了很多幻数和常量，这些在不同程度上依从POSIX，也遵从POSIX.1标准。这就有助于软件的可移植性。

以下两种类型的限制是必须的：

1) 编译时限制(例如，短整型的最大值是什么？)

2) 运行时限制(例如，文件名可以有多少个字符？)

编译时限制可在头文件中定义，程序在编译时可以包含头文件。但是，运行时限制则要求进程调用一个函数以获取此种限制值。

为了解决此类问题：

1) 编译时限制(例如，短整型的最大值是什么？)

2) 不与文件或目录相关联的运行时限制(sysconf函数)

3) 与文件或目录相关联的运行时限制(pathconf和fpathconf函数)

2. 函数原型

#include
long sysconf(int name);
long pathconf(const char *pathname, int name);
long fpathname(int filedes, int name);
所有函数返回值：若成功则返回相应值；若出错则返回-1.

返回值：

（1）如果 name 不是一个合适的常量，则所有三个函数都会返回-1，并将 errno 设置为EINVAL。

（2）有些 name可以返回变量的值（返回值>=0），或者返回-1，这表示该值是不确定的，此时不改变 errno的值。

因此，程序要检查错误，应该在调用 sysconf() 之前将 errno 设置为 0 ，然后如果返回-1，则检验到错误。

表1 sysconf的限制及name参数，用于标识系统限制。以__SC__开始的常量用作标识运行时限制的sysconf参数。

Name of limit	Description	name argument
ARG_MAX	maximum length, in bytes, of arguments to the exec functions	_SC_ARG_MAX
ATEXIT_MAX	maximum number of functions that can be registered with the atexit function	_SC_ATEXIT_MAX
CHILD_MAX	maximum number of processes per real user ID	_SC_CHILD_MAX
clock ticks/second	number of clock ticks per second	_SC_CLK_TCK
COLL_WEIGHTS_MAX	maximum number of weights that can be assigned to an entry of the LC_COLLATE order keyword in the locale definition file	_SC_COLL_WEIGHTS_MAX
HOST_NAME_MAX	maximum length of a host name as returned by gethostname	_SC_HOST_NAME_MAX
IOV_MAX	maximum number of iovec structures that can be used with readv or writev	_SC_IOV_MAX
LINE_MAX	maximum length of a utility's input line	_SC_LINE_MAX
LOGIN_NAME_MAX	maximum length of a login name	_SC_LOGIN_NAME_MAX
NGROUPS_MAX	maximum number of simultaneous supplementary process group IDs per process	_SC_NGROUPS_MAX
OPEN_MAX	maximum number of open files per process	_SC_OPEN_MAX
PAGESIZE	system memory page size, in bytes	_SC_PAGESIZE
PAGE_SIZE	system memory page size, in bytes	_SC_PAGE_SIZE
RE_DUP_MAX	number of repeated occurrences of a basic regular expression permitted by the regexec and regcomp functions when using the interval notation /{m,n/}	_SC_RE_DUP_MAX
STREAM_MAX	maximum number of standard I/O streams per process at any given time; if defined, it must have the same value as FOPEN_MAX	_SC_STREAM_MAX
SYMLOOP_MAX	number of symbolic links that can be traversed during pathname resolution	_SC_SYMLOOP_MAX
TTY_NAME_MAX	length of a terminal device name, including the terminating null	_SC_TTY_NAME_MAX
TZNAME_MAX	maximum number of bytes for the name of a time zone	_SC_TZNAME_MAX

表2 sysconf的限制及name参数，用于标识系统限制。以__PC__开始的常量用作标识运行时限制的pathconf参数和fpathconf参数。

Name of limit	Description	name argument
FILESIZEBITS	minimum number of bits needed to represent, as a signed integer value, the maximum size of a regular file allowed in the specified directory	_PC_FILESIZEBITS
LINK_MAX	maximum value of a file's link count	_PC_LINK_MAX
MAX_CANON	maximum number of bytes on a terminal's canonical input queue	_PC_MAX_CANON
MAX_INPUT	number of bytes for which space is available on terminal's input queue	_PC_MAX_INPUT
NAME_MAX	maximum number of bytes in a filename (does not include a null at end)	_PC_NAME_MAX
PATH_MAX	maximum number of bytes in a relative pathname, including the terminating null	_PC_PATH_MAX
PIPE_BUF	maximum number of bytes that can be written atomically to a pipe	_PC_PIPE_BUF
SYMLINK_MAX	number of bytes in a symbolic link	_PC_SYMLINK_MAX