setrlimit()&getrlimit()设置和获取Linux下进程可用资源

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本文介绍Linux系统中getrlimit和setrlimit两个API的功能及使用方法，包括如何获取和设置进程资源限制，如CPU时间、内存大小等，并通过示例代码展示具体的实现过程。

本文根据MSDA和百度空间的文章整理，向作者致谢。

参考文章链接

http://hi.baidu.com/phps/blog/item/7e3ba44410cf9580b3b7dc81.html
http://blog.youkuaiyun.com/msda/article/details/4463688

getrlimit && setrlimit

一个进程都有自己的一组资源限制，在Linux系统中我们可以通过 getrlimit()/setrlimit()这2个API来取得和设置资源:

#include <sys/resource.h>
int getrlimit(int resource, struct rlimit *rlim);
int setrlimit(int resource, const struct rlimit *rlim);

Both return: 0 if OK,nonzero on error

getrlimit用来取得setrlimit用来设置.

第一个参数表示要控制的资源,比如控制CPU、内存、文件描述符个数等等的控制,第二个参数是一个rlimit的结构体地址（指针），它的结构如下定义：
定义放在头文件/usr/include/bits/resource.h中
struct rlimit
   {
   /* The current (soft) limit. */
   rlim_t rlim_cur;
   /* The hard limit. */
   rlim_t rlim_max;
   };
结构体中 rlim_cur是要取得或设置的资源软限制的值，rlim_max是硬限制。

这两个值的设置有一个小的约束：
1）任何进程可以将软限制改为小于或等于硬限制
2）任何进程都可以将硬限制降低，但普通用户降低了就无法提高，该值必须等于或大于软限制
3）只有超级用户可以提高硬限制

注意：
1.两个函数不属于POSIX.1，但SVR4和4.3+BSD提供它们。注意,并非所有资源限制都受到SVR4和4.3+BSD的支持。==>what's SVR4????
SVR4在上面的结构中使用基本系统数据类型rlim。其它系统则将这两个成员定义为整型或长整型。

2.进程的资源限制通常是在系统初启时由0#进程建立的，然后由后续进程继承。在SVR4中，系统默认值可以查看文件/etc/conf/cfd/mtune ==> need to verification
在4.3+BSD中，系统默认值分散在多个头文件中。

3.资源限制影响到调用进程并由其子进程继承。这就意味着为了影响一个用户的所有后续进程，需将资源限制设置构造在shell之中。确实，Bourne Shell和Kornshell具有内部ulimit命令，CShell具有内部limit命令。(umask和chdir也必须是shell内部的)。较早的BourmeShell，例如由贝克莱提供的一种，不支持ulimit命令。较新的KornShell的ulimit命令具有-H和-s选择项，以分别检查和修改硬和软的限制，但它们尚末编写入文档。

这两个函数的resource参数取下列值之一。
一个无限的限制由常量RLIM_INFINITY指定（The value RLIM_INFINITY denotes no limit on a resource ）

RLIMIT_AS
The maximum size of the process鈙 virtual memory (address space) in bytes. This limit affects calls to brk(2), mmap(2) and mremap(2), which fail with the error ENOMEM upon exceeding this limit. Also automatic stack expansion will fail (and generate a SIGSEGV that kills the process when no alternate stack has been made available). Since the value is a long, on machines with a 32-bit long either this limit is at most 2 GiB, or this resource is unlimited.

RLIMIT_CORE ==》bytes or blocks??A:对于此函数来说，单位应该是Byte，但是当调用ulimit查看或者设置core_file的大小时，单位为blocks.
Maximum size of core file. When 0 no core dump files are created. When nonzero, larger dumps are truncated to this size.
设定最大的core文件，当值为0时将禁止core文件非0时将设定产生的最大core文件大小为设定的值

RLIMIT_CPU
CPU time limit in seconds. When the process reaches the soft limit, it is sent a SIGXCPU signal. The default action for this signal is to terminate the process. However, the signal can be caught, and the handler can return control to the main program. If the process continues to consume CPU time, it will be sent SIGXCPU once per second until the hard limit is reached, at which time it is sent SIGKILL.(This latter point describes Linux 2.2 and 2.4 behaviour. Implementations vary in how they treat processes which continue to consume CPU time after reaching the soft limit. Portable applications that needto catch this signal should perform an orderly termination upon first receipt of SIGXCPU.)
CPU时间的最大量值（秒），当超过此软限制时向该进程发送SIGXCPU信号

RLIMIT_DATA
   The maximum size of the process data segment (initialized
   data, uninitialized data, and heap). This limit affects calls
   to brk() and sbrk(), which fail with the error ENOMEM upon
   encountering the soft limit of this resource.
数据段的最大字节长度
RLIMIT_FSIZE
   The maximum size of files that the process may create.
   Attempts to extend a file beyond this limit result in delivery
   of a SIGXFSZ signal. By default, this signal terminates a pro-
   cess, but a process can catch this signal instead, in which
   case the relevant system call (e.g., write(), truncate()) fails
   with the error EFBIG.
可以创建的文件的最大字节长度，当超过此软限制时向进程发送SIGXFSZ
RLIMIT_MEMLOCK
   The maximum number of bytes of virtual memory that may be
   locked into RAM using mlock() and mlockall().
RLIMIT_NOFILE
   Specifies a value one greater than the maximum file descriptor
   number that can be opened by this process. Attempts (open(),
   pipe(), dup(), etc.) to exceed this limit yield the error
   EMFILE.
每个进程能够打开的最多文件数。更改此限制将影响到sysconf函数在参数_SC_CHILD_MAX中的返回值
RLIMIT_OFILE is the BSD name for RLIMIT_NOFILE.
这里BSD系统中RLIMIT_NOFILE的别名
RLIMIT_NPROC
   The maximum number of processes that can be created for the
   real user ID of the calling process. Upon encountering this
   limit, fork() fails with the error EAGAIN.
每个实际用户ID所拥有的最大子进程数，更改此限制将影响到sysconf函数在参数_SC_CHILD_MAX中返回的值
RLIMIT_RSS
   Specifies the limit (in pages) of the process鈙 resident set
   (the number of virtual pages resident in RAM). This limit only
   has effect in Linux 2.4 onwatrds, and there only affects calls
   to madvise() specifying MADVISE_WILLNEED.
最大驻内存集字节长度(RSS)如果物理存储器供不应求则内核将从进程处取回超过RSS的部份
RLIMIT_STACK
   The maximum size of the process stack, in bytes. Upon reaching
   this limit, a SIGSEGV signal is generated. To handle this sig-
   nal, a process must employ an alternate signal stack (sigalt-
   stack(2)).
栈的最大长度
RLIMIT——VMEM 可映照地址空间的最大字节长茺，这影响到mmap函数
这些限制影响到调用进程并由子进程继承！可以在SHELL中预设这些值ulimit命令设置

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/resource.h>
int main(void)
{
   struct rlimit r;
   if(getrlimit(RLIMIT_NOFILE,&r)<0)
   {
   fprintf(stderr,"getrlimit error\n");
   exit(1);
   }
   printf("RLIMIT_NOFILE cur:%d\n",r.rlim_cur);
   printf("RLIMIT_NOFILE max:%d\n",r.rlim_max);

   /** set limit **/
   r.rlim_cur=100;
   r.rlim_max=200;
   if (setrlimit(RLIMIT_NOFILE,&r)<0)
   {
   fprintf(stderr,"setrlimit error\n")
   exit(1);
   }

   /** get value of set **/
   if(getrlimit(RLIMIT_NOFILE,&r)<0)
   {
   fprintf(stderr,"getrlimit error\n");
   exit(1);
   }
   printf("RLIMIT_NOFILE cur:%d\n",r.rlim_cur);
   printf("RLIMIT_NOFILE max:%d\n",r.rlim_max);
   return 0;
}