Linux sysconf函数 -- get configurable system variables

• 函数原型

  long sysconf(int name);
  

• 头文件

  #include <unistd.h>
  

• 描述

  get configurable system variables.

  This interface is defined by IEEE Std 1003.1-1988 (``POSIX.1’’). A far more complete interface is available using sysctl(3).

  The sysconf() function provides a method for applications to determine the current value of a configurable system limit or option variable. The name argument specifies the system variable to be queried. Symbolic constants for each name value are found in the include file <unistd.h>. Shell programmers who need access to these parameters should use the getconf(1) utility.

  主要是来获取系统变量的。

• 参数

  	_SC_ARG_MAX
               The maximum bytes of argument to execve(2).
  
       _SC_CHILD_MAX
               The maximum number of simultaneous processes per user id.
  
       _SC_CLK_TCK
               The frequency of the statistics clock in ticks per second.
  
       _SC_IOV_MAX
               The maximum number of elements in the I/O vector used by readv(2), writev(2), recvmsg(2), and
               sendmsg(2).
  
       _SC_NGROUPS_MAX
               The maximum number of supplemental groups.
  
       _SC_NPROCESSORS_CONF
               The number of processors configured.
  
       _SC_NPROCESSORS_ONLN
               The number of processors currently online.
  
       _SC_OPEN_MAX
               The maximum number of open files per user id.
  
       _SC_PAGESIZE
               The size of a system page in bytes.
  
       _SC_STREAM_MAX
               The minimum maximum number of streams that a process may have open at any one time.
  
       _SC_TZNAME_MAX
               The minimum maximum number of types supported for the name of a timezone.
  
       _SC_JOB_CONTROL
               Return 1 if job control is available on this system, otherwise -1.
  
       _SC_SAVED_IDS
               Returns 1 if saved set-group and saved set-user ID is available, otherwise -1.
  
       _SC_VERSION
               The version of IEEE Std 1003.1 (POSIX.1) with which the system attempts to comply.
  
       _SC_BC_BASE_MAX
               The maximum ibase/obase values in the bc(1) utility.
  
       _SC_BC_DIM_MAX
               The maximum array size in the bc(1) utility.
  
       _SC_BC_SCALE_MAX
               The maximum scale value in the bc(1) utility.
  
       _SC_BC_STRING_MAX
               The maximum string length in the bc(1) utility.
  
       _SC_COLL_WEIGHTS_MAX
               The maximum number of weights that can be assigned to any entry of the LC_COLLATE order keyword in the
               locale definition file.
  
       _SC_EXPR_NEST_MAX
               The maximum number of expressions that can be nested within parenthesis by the expr(1) utility.
  
       _SC_LINE_MAX
               The maximum length in bytes of a text-processing utility's input line.
  
  	_SC_RE_DUP_MAX
  	             The maximum number of repeated occurrences of a regular expression permitted when using interval nota-
  	             tion.
  
       _SC_2_VERSION
               The version of IEEE Std 1003.2 (POSIX.2) with which the system attempts to comply.
  
       _SC_2_C_BIND
               Return 1 if the system s C-language development facilities support the C-Language Bindings Option, oth-
               erwise -1.
  
       _SC_2_C_DEV
               Return 1 if the system supports the C-Language Development Utilities Option, otherwise -1.
  
       _SC_2_CHAR_TERM
               Return 1 if the system supports at least one terminal type capable of all operations described in IEEE
               Std 1003.2 (POSIX.2), otherwise -1.
  
       _SC_2_FORT_DEV
               Return 1 if the system supports the FORTRAN Development Utilities Option, otherwise -1.
  
       _SC_2_FORT_RUN
               Return 1 if the system supports the FORTRAN Runtime Utilities Option, otherwise -1.
  
       _SC_2_LOCALEDEF
               Return 1 if the system supports the creation of locales, otherwise -1.
  
       _SC_2_SW_DEV
               Return 1 if the system supports the Software Development Utilities Option, otherwise -1.
  
       _SC_2_UPE
               Return 1 if the system supports the User Portability Utilities Option, otherwise -1.
  

• 返回值

  If the call to sysconf() is not successful, -1 is returned and errno is set appropriately. Otherwise, if the variable is associated with functionality that is not supported, -1 is returned and errno is not modified. Otherwise, the current variable value is returned.

• 举个例子

  #include <stdio.h>
  #include <unistd.h>
   
  #define ONE_MB (1024 * 1024)
   
  int main()
  {
      printf("The number of processors configured is :%ld\n",
          sysconf(_SC_NPROCESSORS_CONF));
      printf("The number of processors currently online (available) is :%ld\n",
          sysconf(_SC_NPROCESSORS_ONLN));
      printf ("The pagesize: %ld\n", sysconf(_SC_PAGESIZE));  
      printf ("The number of pages: %ld\n", sysconf(_SC_PHYS_PAGES));  
      printf ("The memory size: %lld MB\n", 
          (long long)sysconf(_SC_PAGESIZE) * (long long)sysconf(_SC_PHYS_PAGES) / ONE_MB );  
      printf ("The number of files max opened:: %ld\n", sysconf(_SC_OPEN_MAX));  
      printf("The number of ticks per second: %ld\n", sysconf(_SC_CLK_TCK));  
      printf ("The max length of host name: %ld\n", sysconf(_SC_HOST_NAME_MAX));  
      printf ("The max number of simultaneous processes per user: %ld\n", sysconf(_SC_CHILD_MAX)); 
      return 0;
  }
  

  输出结果为：

  The number of processors configured is :4
  The number of processors currently online (available) is :4
  The pagesize: 4096
  The number of pages: 2097152
  The memory size: 8192 MB
  The number of files max opened:: 256
  The number of ticks per second: 100
  The max length of host name: 255
  The max number of simultaneous processes per user: 709