Posted on 2015-12-08 22:35
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1, 三个标准1.1, ISO C标准由ISO/IEC维护开发最新版本是C11，共有29个标准头文件。1.2, POSIX是一系列由IEEE制定的标准POSIX包括ISO C标准库函数。POSIX标准的1988版本是IEEE 1003.1-1988，经过修改后作为IEEE Std.1003.1-1990提交ISO，成为国际标准ISO/IEC 9945-1:1990,该标准通常称为POSIX.1。当前最新版本是POSIX.1-2008，由IEEE和Open Group共同开发。1.3, SUS是POSIX的超集，其系统接口全集称为XSIThe core specifications of the SUS are developed and maintained by the Austin Group, which is a joint working group of IEEE, ISO JTC 1 SC22 and The Open Group.只有遵循XSI的实现才能称为UNIX系统。当前的最新版本是SUSv4。1.4, 找到一些网址C11http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=57853POSIX.1-2008http://pubs.opengroup.org/onlinepubs/9699919799/https://standards.ieee.org/findstds/standard/1003.1-2008.htmlSUSv4https://www2.opengroup.org/ogsys/jsp/publications/PublicationDetails.jsp?publicationid=12310https://en.wikipedia.org/wiki/Single_UNIX_Specification#cite_note-112, 限制2.1 两种限制编译时限制和运行时限制。编译时限制通过头文件获取；不与文件或目录相关的运行时限制通过sysconf函数获取；与文件或目录相关的运行时限制通过pathconf和fpathconf函数获取。2.2 ISO C限制都是编译时限制，主要定义在<limits.h>里面。http://en.cppreference.com/w/c/types/limits2.3 POSIX限制和XSI限制书中列出的都是实现中必须支持的各种最小值，特定系统实际支持的限制值需要通过头文件或者三个函数函数获取。三个函数的name参数是限制名前面加_SC_或者_PC_前缀得到。http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html2.4 书中代码/*
* If name is invalid, -1 is returned, and errno is set to EINVAL.
* Otherwise, the value returned is the value of the system resource and errno is not changed.
* In the case of options, a positive value is returned if a queried option is available, and -1 if it is not.
* In the case of limits, -1 means that there is no definite limit.
*/
#include "apue.h"
#include <errno.h>
#include <limits.h>
#ifdef OPEN_MAX
static long openmax = OPEN_MAX;
#else
static long openmax = 0;
#endif
/*
* If OPEN_MAX is indeterminate, we're not
* guaranteed that this is adequate
*/
#define OPEN_MAX_GUESS 256
long
open_max(void)
{
if (openmax == 0) { /* first time through */
errno = 0;
if ((openmax = sysconf(_SC_OPEN_MAX)) < 0) {
if (errno == 0)
openmax = OPEN_MAX_GUESS; /* it's indeterminate */
else
err_sys("sysconf error for _SC_OPEN_MAX";)
}
}
return(openmax);
}
3, 选项3.1, 选项确定方式编译时选项定义在<unistd.h>中；与文件或目录无关的选项用sysconf确定；与文件或目录有关的选项用pathconf或者fpathconf确定；http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/unistd.h.html3.2, 选项确定流程如果符号常量未定义，对_POSIX前缀的选项，将_POSIX前缀替换为_SC或_PC前缀，对_XOPEN前缀的选项，在_XOPEN前面加上_SC或_PC前缀，然后调用sysconf, pathconf或fpathconf函数。如果符号常量已经定义，则有三种可能：值为-1，不支持相应的选项；值大于0，支持相应的选项；值为0，需调用函数确定选项是否支持。注：某些系统可能出现定义了符号常量，但没有定义值的情况。3.4, 代码示例先占坑。4, 功能测试宏Feature test macros allow the programmer to control the definitions that are exposed by system header files when a program is compiled.NOTE: In order to be effective, a feature test macro must be defined before including any header files. This can be done either in the compilation command (cc -DMACRO=value) or by defining the macro within the source code before including any headers.see man page feature_test_macros(7).