C语言系统资源控制（getrlimit && setrlimit)

每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
#include
<sys/resource.h>
int getrlimit(int resource, struct rlimit
*rlim);
int setrlimit(int resource, const struct rlimit
*rlim);
这2个API来取得和设置资源。
getrlimit用来取得，setrlimit用来设置。这二个参数都需要一个要控制的资源，比如控制CPU、内存、文件描述符个数等等的控制，作为第一个参数传入，第二个参数是一个rlimit的结构体地址（指针），他的结构如下定义：
定义放在头文件/usr/include/bits/resource.h中
struct
rlimit

{

rlim_t
rlim_cur;

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
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
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.
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
need

to 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.
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

小试牛刀（代码只是演示，并没有作错误处理）
#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