NAME

getrlimit, setrlimit — control maximum system resource consumption

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/resource.h>

int
getrlimit(int resource, struct rlimit *rlp);

int
setrlimit(int resource, const struct rlimit *rlp);

DESCRIPTION

Limits on the consumption of system resources by the current process and each process it creates may be obtained with the getrlimit() call, and set with the setrlimit() call. Resources of an arbitrary process can be obtained/changed using sysctl(3).

The resource parameter is one of the following:

RLIMIT_AS

The maximum amount (in bytes) of virtual memory the process is allowed to map.

RLIMIT_CORE

The largest size (in bytes) core file that may be created.

RLIMIT_CPU

The maximum amount of CPU time (in seconds) to be used by each process.

RLIMIT_DATA

The maximum size (in bytes) of the data segment for a process; this defines how far a program may extend its break with the sbrk(2) system call.

RLIMIT_FSIZE

The largest size (in bytes) file that may be created.

RLIMIT_MEMLOCK

The maximum size (in bytes) which a process may lock into memory using the mlock(2) function.

RLIMIT_NOFILE

The maximum number of open files for this process.

RLIMIT_NPROC

The maximum number of simultaneous processes for this user id.

RLIMIT_NTHR

The maximum number of simultaneous threads (Lightweight Processes) for this user id. Kernel threads and the first thread of each process are not counted against this limit.

RLIMIT_RSS

The maximum size (in bytes) to which a process's resident set size may grow. This imposes a limit on the amount of physical memory to be given to a process; if memory is tight, the system will prefer to take memory from processes that are exceeding their declared resident set size.

RLIMIT_SBSIZE

The maximum size (in bytes) of the socket buffers set by the setsockopt(2) SO_RCVBUF and SO_SNDBUF options.

RLIMIT_STACK

The maximum size (in bytes) of the stack segment for a process; this defines how far a program's stack segment may be extended. Stack extension is performed automatically by the system.

A resource limit is specified as a soft limit and a hard limit. When a soft limit is exceeded a process may receive a signal (for example, if the CPU time or file size is exceeded), but it will be allowed to continue execution until it reaches the hard limit (or modifies its resource limit). The rlimit structure is used to specify the hard and soft limits on a resource,

struct rlimit { 
	rlim_t	rlim_cur;	/* current (soft) limit */ 
	rlim_t	rlim_max;	/* hard limit */ 
};

Only the super-user may raise the maximum limits. Other users may only alter rlim_cur within the range from 0 to rlim_max or (irreversibly) lower rlim_max.

An “infinite” value for a limit is defined as RLIM_INFINITY.

Because this information is stored in the per-process information, this system call must be executed directly by the shell if it is to affect all future processes created by the shell. Thus, shells provide built-in commands to change the limits (limit for csh(1), or ulimit for sh(1)).

The system refuses to extend the data or stack space when the limits would be exceeded in the normal way: a brk(2) call fails if the data space limit is reached. When the stack limit is reached, the process receives a segmentation fault (SIGSEGV); if this signal is not caught by a handler using the signal stack, this signal will kill the process.

A file I/O operation that would create a file larger that the process' soft limit will cause the write to fail and a signal SIGXFSZ to be generated; this normally terminates the process, but may be caught. When the soft CPU time limit is exceeded, a signal SIGXCPU is sent to the offending process.

RETURN VALUES

A 0 return value indicates that the call succeeded, changing or returning the resource limit. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

ERRORS

The getrlimit() and setrlimit() will fail if:

[EFAULT]

The address specified for rlp is invalid.

[EINVAL]

Specified resource was invalid; or, in the setrlimit() call, the specified rlim_cur exceeds the specified rlim_max.

[EPERM]

The limit specified to setrlimit() would have raised the maximum limit value, and the caller is not the super-user.

The setrlimit() function may fail if:

[EINVAL]

The limit specified to setrlimit() cannot be lowered, because current usage is already higher than the limit.

SEE ALSO

csh(1), sh(1), mlock(2), setsockopt(2), sigaction(2), sigaltstack(2), libquota(3), sysctl(3)

HISTORY

The getrlimit() function call appeared in 4.2BSD.

BUGS

The resource limit RLIMIT_RSS is not implemented in uvm(9) which means that process memory size limits are not enforced.