每一個進程都有自己的一組資源限制，在(*)inux系統中我們可以通過
#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 這二個參數都需要一個要控制的資源 比如控制CPU、內存、文件描述符個數等等的控制，作爲第一個參數傳入，第二個參數是一個rlimit的結構體地址（指針），他的結構如下定義：
定義放在頭文件/usr/include/bits/resource.h中
struct rlimit
{
/* The current (soft) limit. */
rlim_t rlim_cur;
/* The hard limit. */
rlim_t rlim_max;
};
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） 任何進程可以將軟限制改爲小於或等於硬限制
2） 任何進程都可以將硬限制降低，但普通用戶降低了就無法提高，該值必須等於或大於軟限制
3） 只有超級用戶可以提高硬限制
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 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鈙 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.
可以創建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.
每個進程能夠打開的最多文件數。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.
最大駐內存集字節長度(RSS)如果物理存儲器供不應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！ 可以在SHELL中預設這些值ulimit命令設置
小試牛刀（代碼只是演示，並沒有作錯誤處理）
#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);
/** 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;
}
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:!gcc test.c
:!./a.out
RLIMIT_NOFILE cur:1024
RLIMIT_NOFILE max:1024
RLIMIT_NOFILE cur:100
RLIMIT_NOFILE max:200