lib/libsys/getrlimit.2

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.Dd July 25, 2024
.Dt GETRLIMIT 2
.Os
.Sh NAME
.Nm getrlimit ,
.Nm setrlimit
.Nd control maximum system resource consumption
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/time.h
.In sys/resource.h
.Ft int
.Fn getrlimit "int resource" "struct rlimit *rlp"
.Ft int
.Fn setrlimit "int resource" "const struct rlimit *rlp"
.Sh DESCRIPTION
Limits on the consumption of system resources by the current process
and each process it creates may be obtained with the
.Fn getrlimit
system call, and set with the
.Fn setrlimit
system call.
.Pp
The
.Fa resource
argument is one of the following:
.Bl -tag -width RLIMIT_FSIZEAA
.It Dv RLIMIT_AS
The maximum amount (in bytes) of virtual memory the process is
allowed to map.
.It Dv RLIMIT_CORE
The largest size (in bytes)
.Xr core 5
file that may be created.
.It Dv RLIMIT_CPU
The maximum amount of cpu time (in seconds) to be used by
each process.
.It Dv 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
.Xr sbrk 2
function.
.It Dv RLIMIT_FSIZE
The largest size (in bytes) file that may be created.
.It Dv RLIMIT_KQUEUES
The maximum number of kqueues this user id is allowed to create.
.It Dv RLIMIT_MEMLOCK
The maximum size (in bytes) which a process may lock into memory
using the
.Xr mlock 2
system call.
.It Dv RLIMIT_NOFILE
The maximum number of open files for this process.
.It Dv RLIMIT_NPROC
The maximum number of simultaneous processes for this user id.
.It Dv RLIMIT_NPTS
The maximum number of pseudo-terminals this user id is allowed to create.
.It Dv RLIMIT_PIPEBUF
The maximum total size of in-kernel buffers for bi-directional pipes/fifos
that this user id is allowed to consume.
The buffers for kernel FIFOs created on the first open of a filesystem
object created by
.Pq Xr mkfifo 2
are also charged to the user ID of the process opening it,
not the FIFO's filesystem owner.
Despite somewhat unexpected, this is in fact fair, since user of the fifo
is not necessary its creator.
.It Dv RLIMIT_RSS
When there is memory pressure and swap is available, prioritize eviction of
a process' resident pages beyond this amount (in bytes).
When memory is not under pressure, this rlimit is effectively ignored.
.Pp
Processes that exceed their set
.Dv RLIMIT_RSS
are not signalled or halted.
The limit is merely a hint to the VM daemon to prefer to deactivate pages from
processes that have exceeded their set
.Dv RLIMIT_RSS .
.It Dv RLIMIT_SBSIZE
The maximum size (in bytes) of socket buffer usage for this user.
This limits the amount of network memory, and hence the amount of
mbufs, that this user may hold at any time.
.It Dv 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.
.It Dv RLIMIT_SWAP
The maximum size (in bytes) of the swap space that may be reserved or
used by all of this user id's processes.
This limit is enforced only if bit 1 of the
.Va vm.overcommit
sysctl is set.
Please see
.Xr tuning 7
for a complete description of this sysctl.
.It Dv RLIMIT_UMTXP
The limit of the number of process-shared posix thread library objects
allocated by user id.
.It Dv RLIMIT_VMEM
An alias for
.Dv RLIMIT_AS .
.El
.Pp
A resource limit is specified as a soft limit and a hard limit.
When a soft limit is exceeded, a process might or might not receive a signal.
For example, signals are generated when the cpu time or file size is exceeded,
but not if the address space or RSS limit is exceeded.
A program that exceeds the soft limit is allowed to continue execution until it
reaches the hard limit, or modifies its own resource limit.
Even reaching the hard limit does not necessarily halt a process.
For example, if the RSS hard limit is exceeded, nothing happens.
.Pp
The
.Vt rlimit
structure is used to specify the hard and soft limits on a resource.
.Bd -literal -offset indent
struct rlimit {
	rlim_t	rlim_cur;	/* current (soft) limit */
	rlim_t	rlim_max;	/* maximum value for rlim_cur */
};
.Ed
.Pp
Only the super-user may raise the maximum limits.
Other users
may only alter
.Fa rlim_cur
within the range from 0 to
.Fa rlim_max
or (irreversibly) lower
.Fa rlim_max .
.Pp
An
.Dq infinite
value for a limit is defined as
.Dv RLIM_INFINITY .
.Pp
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;
.Ic limit
is thus a built-in command to
.Xr csh 1 .
.Pp
The system refuses to extend the data or stack space when the limits
would be exceeded in the normal way: a
.Xr brk 2
function fails if the data space limit is reached.
When the stack limit is reached, the process receives
a segmentation fault
.Pq Dv SIGSEGV ;
if this signal is not
caught by a handler using the signal stack, this signal
will kill the process.
.Pp
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
.Dv SIGXFSZ
to be
generated; this normally terminates the process, but may be caught.
When
the soft cpu time limit is exceeded, a
.Dv SIGXCPU
signal is sent to the
offending process.
.Pp
When most operations would allocate more virtual memory than allowed by the
soft limit of
.Dv RLIMIT_AS ,
the operation fails with
.Dv ENOMEM
and no signal is raised.
A notable exception is stack extension, described above.
If stack extension would allocate more virtual memory than allowed by the soft
limit of
.Dv RLIMIT_AS ,
a
.Dv SIGSEGV
signal will be delivered.
The caller is free to raise the soft address space limit up to the hard limit
and retry the allocation.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The
.Fn getrlimit
and
.Fn setrlimit
system calls
will fail if:
.Bl -tag -width Er
.It Bq Er EFAULT
The address specified for
.Fa rlp
is invalid.
.It Bq Er EPERM
The limit specified to
.Fn setrlimit
would have
raised the maximum limit value, and the caller is not the super-user.
.El
.Sh SEE ALSO
.Xr csh 1 ,
.Xr quota 1 ,
.Xr quotactl 2 ,
.Xr sigaction 2 ,
.Xr sigaltstack 2 ,
.Xr sysctl 3 ,
.Xr ulimit 3
.Sh HISTORY
The
.Fn getrlimit
system call appeared in
.Bx 4.2 .