C語言系統資源控制（getrlimit && setrlimit)

每個進程都有本身的一組資源限制，在Linux系統中咱們能夠經過

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

這2個API來取得和設置資源。
getrlimit用來取得，setrlimit用來設置。這二個參數都須要一個要控制的資源，好比控制CPU、內存、文件描述符個數等等的控制，做爲第一個參數傳入，第二個參數是一個rlimit的結構體地址（指針），他的結構以下定義：
定義放在頭文件/usr/include/bits/resource.h中

struct rlimit
{
    rlim_t rlim_cur;
    rlim_t rlim_max;
}；

結構體中，rlim_cur是要取得或設置的資源軟限制的值，rlim_max是硬限制。
這兩個值的設置有一個小的約束：
1） 任何進程能夠將軟限制改成小於或等於硬限制
2） 任何進程均可以將硬限制下降，但普通用戶下降了就沒法提升，該值必須等於或大於軟限制
3） 只有超級用戶能夠提升硬限制
一個無限的限制由常量RLIM_INFINITY指定（The       value       RLIM_INFINITY       denotes no limit on a resource ）

RLIMIT_AS
                   The       maximum       size       of       the       process     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       gen-
                   erate       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
                   Maximum size of core file. When 0 no core dump files       are       cre-
                   ated.       When nonzero, larger dumps are truncated to this size.
設定最大的core文件，當值爲0時將禁止core文件非0時將設定產生的最大core文件大小爲設定的值
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 need
                   to catch this signal should perform an orderly termination upon
                   first receipt of SIGXCPU.)
CPU時間的最大量值（秒），當超過此軟限制時向該進程發送SIGXCPU信號
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.
數據段的最大字節長度
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.
能夠建立的文件的最大字節長度，當超過此軟限制時向進程發送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.
每一個進程可以打開的最多文件數。更改此限制將影響到sysconf函數在參數_SC_CHILD_MAX中的返回值
RLIMIT_OFILE is the BSD name for RLIMIT_NOFILE.
這裏BSD系統中RLIMIT_NOFILE的別名
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.
每一個實際用戶ID所擁有的最大子進程數，更改此限制將影響到sysconf函數在參數_SC_CHILD_MAX中返回的值
RLIMIT_RSS
                   Specifies the limit (in pages) of the       process       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.
最大駐內存集字節長度(RSS)若是物理存儲器供不該求則內核將從進程處取回超過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)).
棧的最大長度
RLIMIT——VMEM 可映照地址空間的最大字節長茺，這影響到mmap函數
這些限制影響到調用進程並由子進程繼承！能夠在SHELL中預設這些值ulimit命令設置

小試牛刀（代碼只是演示，並無做錯誤處理）

#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)；
     r.rlim_cur=100;
     r.rlim_max=200;
     if (setrlimit(RLIMIT_NOFILE,&r)<0){
         fprintf(stderr,"setrlimit error\n")；
         exit(1);
     }
     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；
}
:!gcc test.c                                                                                                                                                    
:!./a.out
RLIMIT_NOFILE cur:1024
RLIMIT_NOFILE max:1024
RLIMIT_NOFILE cur:100
RLIMIT_NOFILE max:200