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.Dd August 9, 2011
.Dt LIMITS 3
.Os
.Sh NAME
.Nm limits
.Nd standard limits
.Sh SYNOPSIS
.In limits.h
.Sh DESCRIPTION
The
.In limits.h
header defines various compile-time and runtime limits.
These can be grouped into three categories:
.Bl -enum -offset indent
.It
Compile-time limits defined in a header file.
.It
Runtime system limits that are not associated with a file or directory; see
.Xr sysconf 3 .
.It
Runtime limits that are associated with a file or directory; see
.Xr pathconf 2 .
.El
.Pp
The
.In limits.h
header has been standardized by at least three entities.
.Ss ISO Limits
The limits defined by the
.St -isoC-99
standard are all compile-time limits.
The numerical (integer) limits are:
.Bl -column -offset indent \
"Constant   " "  Type             " "either SCHAR_MAX or UCHAR_MAX  "
.It Sy "Constant" Ta Sy "Type" Ta Sy " Minimum value"
.It Dv CHAR_BIT Ta Va char Ta " 8"
.It Dv SCHAR_MAX Ta Va signed char Ta " 127"
.It Dv SCHAR_MIN Ta Va signed char Ta "\-127"
.It Dv UCHAR_MAX Ta Va unsigned char Ta " 255"
.Pp
.It Dv INT_MAX Ta Va int Ta " 32767"
.It Dv INT_MIN Ta Va int Ta "\-32767"
.It Dv UINT_MAX Ta Va unsigned int Ta " 65535"
.Pp
.It Dv SHRT_MIN Ta Va short Ta "\-32767"
.It Dv SHRT_MAX Ta Va short Ta " 32767"
.It Dv USHRT_MAX Ta Va unsigned short Ta " 65535"
.Pp
.It Dv LONG_MAX Ta Va long int Ta " 2147483647"
.It Dv LONG_MIN Ta Va long int Ta "\-2147483647"
.It Dv ULONG_MAX Ta Va unsigned long int Ta " 4294967295"
.Pp
.It Dv LLONG_MAX Ta Va long long int Ta " 9223372036854775807"
.It Dv LLONG_MIN Ta Va long long int Ta "\-9223372036854775807"
.It Dv ULLONG_MAX Ta Va unsigned long long int Ta " 18446744073709551615"
.Pp
.It Dv MB_LEN_MAX Ta - Ta 1
.El
.Pp
All listed limits may vary across machines and operating systems.
The standard guarantees only that the implementation-defined values are
equal or greater in absolute value to those shown.
The values permit a system with 16-bit integers
using one's complement arithmetic.
.Pp
Depending whether the system defines
.Va char
as signed or unsigned, the maximum and minimum values are:
.Bl -column -offset indent \
"Constant   " "  Type             " "either SCHAR_MAX or UCHAR_MAX  "
.It Sy "Constant" Ta Sy "Type" Ta Sy "Minimum value"
.It Dv CHAR_MAX Ta Va char Ta either Dv SCHAR_MAX or Dv UCHAR_MAX
.It Dv CHAR_MIN Ta Va char Ta either Dv SCHAR_MIN or 0
.El
.Pp
The two special cases,
.Dv CHAR_BIT
and
.Dv MB_LEN_MAX ,
define the number of bits in
.Va char
and the maximum number of bytes in a multibyte character constant,
respectively.
.Ss POSIX Limits
The
.Dv POSIX.1
standard specifies numerous limits related to the operating system.
For each limit, a separate constant prefixed with
.Dq Dv _POSIX_
defines the
.Em lowest
value that the limit is allowed to have on
.Em any
.Tn POSIX
compliant system.
For instance,
.Dv _POSIX_OPEN_MAX
defines the minimum upper bound permitted by
.Tn POSIX
for the number of files that a single process may have open at any time.
This ensures that a portable program can safely reach these limits without
prior knowledge about the actual limits used in a particular system.
.Pp
As the limits are not necessary invariant,
.Xr pathconf 2
and
.Xr sysconf 3
should be used to determine the actual value of a limit at runtime.
The manual pages of these two functions also contain a more detailed
description of the limits available in
.Nx .
.Ss XSI Limits
Also the X/Open System Interface Extension
.Pq Dv XSI
specifies few limits.
In
.Nx
these are limited to
.Dv LONG_BIT
(the number of bits in
.Vt long ) ,
.Dv WORD_BIT
(the number of bits in a
.Dq word ) ,
and few limits related to
.Vt float
and
.Vt double .
.Sh SEE ALSO
.Xr getconf 1 ,
.Xr pathconf 2 ,
.Xr sysconf 3 ,
.Xr types 3 ,
.Xr unistd 3
.Rs
.%A Richard W. Stevens
.%A Stephen A. Rago
.%B Advanced Programming in the UNIX Environment
.%V Second Edition
.%D 2005
.%I Addison-Wesley
.Re