Some of the functionality described on this reference page
extends the ISO C standard. Applications shall define the
appropriate feature test macro (see the System Interfaces volume
of POSIX.1‐2017, Section 2.2, The Compilation Environment) to
enable the visibility of these symbols in this header.
Many of the symbols listed here are not defined by the
ISO/IEC 9899:1999 standard. Such symbols are not shown as CX
shaded, except under the heading ``Numerical Limits''.
The <limits.h> header shall define macros and symbolic constants
for various limits. Different categories of limits are described
below, representing various limits on resources that the
implementation imposes on applications. All macros and symbolic
constants defined in this header shall be suitable for use in #if
preprocessing directives.
Implementations may choose any appropriate value for each limit,
provided it is not more restrictive than the Minimum Acceptable
Values listed below. Symbolic constant names beginning with
_POSIX may be found in <unistd.h>.
Applications should not assume any particular value for a limit.
To achieve maximum portability, an application should not require
more resource than the Minimum Acceptable Value quantity.
However, an application wishing to avail itself of the full
amount of a resource available on an implementation may make use
of the value given in <limits.h> on that particular
implementation, by using the macros and symbolic constants listed
below. It should be noted, however, that many of the listed
limits are not invariant, and at runtime, the value of the limit
may differ from those given in this header, for the following
reasons:
* The limit is pathname-dependent.
* The limit differs between the compile and runtime machines.
For these reasons, an application may use the fpathconf(),
pathconf(), and sysconf() functions to determine the actual value
of a limit at runtime.
The items in the list ending in _MIN give the most negative
values that the mathematical types are guaranteed to be capable
of representing. Numbers of a more negative value may be
supported on some implementations, as indicated by the <limits.h>
header on the implementation, but applications requiring such
numbers are not guaranteed to be portable to all implementations.
For positive constants ending in _MIN, this indicates the minimum
acceptable value.
Runtime Invariant Values (Possibly Indeterminate)
A definition of one of the symbolic constants in the following
list shall be omitted from <limits.h> on specific implementations
where the corresponding value is equal to or greater than the
stated minimum, but is unspecified.
This indetermination might depend on the amount of available
memory space on a specific instance of a specific implementation.
The actual value supported by a specific instance shall be
provided by the sysconf() function.
{AIO_LISTIO_MAX}
Maximum number of I/O operations in a single list I/O call
supported by the implementation.
Minimum Acceptable Value: {_POSIX_AIO_LISTIO_MAX}
{AIO_MAX}
Maximum number of outstanding asynchronous I/O operations
supported by the implementation.
Minimum Acceptable Value: {_POSIX_AIO_MAX}
{AIO_PRIO_DELTA_MAX}
The maximum amount by which a process can decrease its
asynchronous I/O priority level from its own scheduling
priority.
Minimum Acceptable Value: 0
{ARG_MAX}
Maximum length of argument to the exec functions including
environment data.
Minimum Acceptable Value: {_POSIX_ARG_MAX}
{ATEXIT_MAX}
Maximum number of functions that may be registered with
atexit().
Minimum Acceptable Value: 32
{CHILD_MAX}
Maximum number of simultaneous processes per real user ID.
Minimum Acceptable Value: {_POSIX_CHILD_MAX}
{DELAYTIMER_MAX}
Maximum number of timer expiration overruns.
Minimum Acceptable Value: {_POSIX_DELAYTIMER_MAX}
{HOST_NAME_MAX}
Maximum length of a host name (not including the
terminating null) as returned from the gethostname()
function.
Minimum Acceptable Value: {_POSIX_HOST_NAME_MAX}
{IOV_MAX}
Maximum number of iovec structures that one process has
available for use with readv() or writev().
Minimum Acceptable Value: {_XOPEN_IOV_MAX}
{LOGIN_NAME_MAX}
Maximum length of a login name.
Minimum Acceptable Value: {_POSIX_LOGIN_NAME_MAX}
{MQ_OPEN_MAX}
The maximum number of open message queue descriptors a
process may hold.
Minimum Acceptable Value: {_POSIX_MQ_OPEN_MAX}
{MQ_PRIO_MAX}
The maximum number of message priorities supported by the
implementation.
Minimum Acceptable Value: {_POSIX_MQ_PRIO_MAX}
{OPEN_MAX}
A value one greater than the maximum value that the system
may assign to a newly-created file descriptor.
Minimum Acceptable Value: {_POSIX_OPEN_MAX}
{PAGESIZE}
Size in bytes of a page.
Minimum Acceptable Value: 1
{PAGE_SIZE}
Equivalent to {PAGESIZE}. If either {PAGESIZE} or
{PAGE_SIZE} is defined, the other is defined with the same
value.
{PTHREAD_DESTRUCTOR_ITERATIONS}
Maximum number of attempts made to destroy a thread's
thread-specific data values on thread exit.
Minimum Acceptable Value:
{_POSIX_THREAD_DESTRUCTOR_ITERATIONS}
{PTHREAD_KEYS_MAX}
Maximum number of data keys that can be created by a
process.
Minimum Acceptable Value: {_POSIX_THREAD_KEYS_MAX}
{PTHREAD_STACK_MIN}
Minimum size in bytes of thread stack storage.
Minimum Acceptable Value: 0
{PTHREAD_THREADS_MAX}
Maximum number of threads that can be created per process.
Minimum Acceptable Value: {_POSIX_THREAD_THREADS_MAX}
{RTSIG_MAX}
Maximum number of realtime signals reserved for application
use in this implementation.
Minimum Acceptable Value: {_POSIX_RTSIG_MAX}
{SEM_NSEMS_MAX}
Maximum number of semaphores that a process may have.
Minimum Acceptable Value: {_POSIX_SEM_NSEMS_MAX}
{SEM_VALUE_MAX}
The maximum value a semaphore may have.
Minimum Acceptable Value: {_POSIX_SEM_VALUE_MAX}
{SIGQUEUE_MAX}
Maximum number of queued signals that a process may send
and have pending at the receiver(s) at any time.
Minimum Acceptable Value: {_POSIX_SIGQUEUE_MAX}
{SS_REPL_MAX}
The maximum number of replenishment operations that may be
simultaneously pending for a particular sporadic server
scheduler.
Minimum Acceptable Value: {_POSIX_SS_REPL_MAX}
{STREAM_MAX}
Maximum number of streams that one process can have open at
one time. If defined, it has the same value as {FOPEN_MAX}
(see <stdio.h>).
Minimum Acceptable Value: {_POSIX_STREAM_MAX}
{SYMLOOP_MAX}
Maximum number of symbolic links that can be reliably
traversed in the resolution of a pathname in the absence of
a loop.
Minimum Acceptable Value: {_POSIX_SYMLOOP_MAX}
{TIMER_MAX}
Maximum number of timers per process supported by the
implementation.
Minimum Acceptable Value: {_POSIX_TIMER_MAX}
{TRACE_EVENT_NAME_MAX}
Maximum length of the trace event name (not including the
terminating null).
Minimum Acceptable Value: {_POSIX_TRACE_EVENT_NAME_MAX}
{TRACE_NAME_MAX}
Maximum length of the trace generation version string or of
the trace stream name (not including the terminating null).
Minimum Acceptable Value: {_POSIX_TRACE_NAME_MAX}
{TRACE_SYS_MAX}
Maximum number of trace streams that may simultaneously
exist in the system.
Minimum Acceptable Value: {_POSIX_TRACE_SYS_MAX}
{TRACE_USER_EVENT_MAX}
Maximum number of user trace event type identifiers that
may simultaneously exist in a traced process, including the
predefined user trace event POSIX_TRACE_UNNAMED_USER_EVENT.
Minimum Acceptable Value: {_POSIX_TRACE_USER_EVENT_MAX}
{TTY_NAME_MAX}
Maximum length of terminal device name.
Minimum Acceptable Value: {_POSIX_TTY_NAME_MAX}
{TZNAME_MAX}
Maximum number of bytes supported for the name of a
timezone (not of the TZ variable).
Minimum Acceptable Value: {_POSIX_TZNAME_MAX}
Note: The length given by {TZNAME_MAX} does not include the
quoting characters mentioned in Section 8.3, Other
Environment Variables.
Pathname Variable Values
The values in the following list may be constants within an
implementation or may vary from one pathname to another. For
example, file systems or directories may have different
characteristics.
A definition of one of the symbolic constants in the following
list shall be omitted from the <limits.h> header on specific
implementations where the corresponding value is equal to or
greater than the stated minimum, but where the value can vary
depending on the file to which it is applied. The actual value
supported for a specific pathname shall be provided by the
pathconf() function.
{FILESIZEBITS}
Minimum number of bits needed to represent, as a signed
integer value, the maximum size of a regular file allowed
in the specified directory.
Minimum Acceptable Value: 32
{LINK_MAX}
Maximum number of links to a single file.
Minimum Acceptable Value: {_POSIX_LINK_MAX}
{MAX_CANON}
Maximum number of bytes in a terminal canonical input line.
Minimum Acceptable Value: {_POSIX_MAX_CANON}
{MAX_INPUT}
Minimum number of bytes for which space is available in a
terminal input queue; therefore, the maximum number of
bytes a conforming application may require to be typed as
input before reading them.
Minimum Acceptable Value: {_POSIX_MAX_INPUT}
{NAME_MAX}
Maximum number of bytes in a filename (not including the
terminating null of a filename string).
Minimum Acceptable Value: {_POSIX_NAME_MAX}
Minimum Acceptable Value: {_XOPEN_NAME_MAX}
{PATH_MAX}
Maximum number of bytes the implementation will store as a
pathname in a user-supplied buffer of unspecified size,
including the terminating null character. Minimum number
the implementation will accept as the maximum number of
bytes in a pathname.
Minimum Acceptable Value: {_POSIX_PATH_MAX}
Minimum Acceptable Value: {_XOPEN_PATH_MAX}
{PIPE_BUF}
Maximum number of bytes that is guaranteed to be atomic
when writing to a pipe.
Minimum Acceptable Value: {_POSIX_PIPE_BUF}
{POSIX_ALLOC_SIZE_MIN}
Minimum number of bytes of storage actually allocated for
any portion of a file.
Minimum Acceptable Value: Not specified.
{POSIX_REC_INCR_XFER_SIZE}
Recommended increment for file transfer sizes between the
{POSIX_REC_MIN_XFER_SIZE} and {POSIX_REC_MAX_XFER_SIZE}
values.
Minimum Acceptable Value: Not specified.
{POSIX_REC_MAX_XFER_SIZE}
Maximum recommended file transfer size.
Minimum Acceptable Value: Not specified.
{POSIX_REC_MIN_XFER_SIZE}
Minimum recommended file transfer size.
Minimum Acceptable Value: Not specified.
{POSIX_REC_XFER_ALIGN}
Recommended file transfer buffer alignment.
Minimum Acceptable Value: Not specified.
{SYMLINK_MAX}
Maximum number of bytes in a symbolic link.
Minimum Acceptable Value: {_POSIX_SYMLINK_MAX}
Runtime Increasable Values
The magnitude limitations in the following list shall be fixed by
specific implementations. An application should assume that the
value of the symbolic constant defined by <limits.h> in a
specific implementation is the minimum that pertains whenever the
application is run under that implementation. A specific instance
of a specific implementation may increase the value relative to
that supplied by <limits.h> for that implementation. The actual
value supported by a specific instance shall be provided by the
sysconf() function.
{BC_BASE_MAX}
Maximum obase values allowed by the bc utility.
Minimum Acceptable Value: {_POSIX2_BC_BASE_MAX}
{BC_DIM_MAX}
Maximum number of elements permitted in an array by the bc
utility.
Minimum Acceptable Value: {_POSIX2_BC_DIM_MAX}
{BC_SCALE_MAX}
Maximum scale value allowed by the bc utility.
Minimum Acceptable Value: {_POSIX2_BC_SCALE_MAX}
{BC_STRING_MAX}
Maximum length of a string constant accepted by the bc
utility.
Minimum Acceptable Value: {_POSIX2_BC_STRING_MAX}
{CHARCLASS_NAME_MAX}
Maximum number of bytes in a character class name.
Minimum Acceptable Value: {_POSIX2_CHARCLASS_NAME_MAX}
{COLL_WEIGHTS_MAX}
Maximum number of weights that can be assigned to an entry
of the LC_COLLATE order keyword in the locale definition
file; see Chapter 7, Locale.
Minimum Acceptable Value: {_POSIX2_COLL_WEIGHTS_MAX}
{EXPR_NEST_MAX}
Maximum number of expressions that can be nested within
parentheses by the expr utility.
Minimum Acceptable Value: {_POSIX2_EXPR_NEST_MAX}
{LINE_MAX}
Unless otherwise noted, the maximum length, in bytes, of a
utility's input line (either standard input or another
file), when the utility is described as processing text
files. The length includes room for the trailing <newline>.
Minimum Acceptable Value: {_POSIX2_LINE_MAX}
{NGROUPS_MAX}
Maximum number of simultaneous supplementary group IDs per
process.
Minimum Acceptable Value: {_POSIX_NGROUPS_MAX}
{RE_DUP_MAX}
Maximum number of repeated occurrences of a BRE or ERE
interval expression; see Section 9.3.6, BREs Matching
Multiple Characters and Section 9.4.6, EREs Matching
Multiple Characters.
Minimum Acceptable Value: {_POSIX_RE_DUP_MAX}
Maximum Values
The <limits.h> header shall define the following symbolic
constants with the values shown. These are the most restrictive
values for certain features on an implementation. A conforming
implementation shall provide values no larger than these values.
A conforming application must not require a smaller value for
correct operation.
{_POSIX_CLOCKRES_MIN}
The resolution of the CLOCK_REALTIME clock, in nanoseconds.
Value: 20 000 000
If the Monotonic Clock option is supported, the resolution
of the CLOCK_MONOTONIC clock, in nanoseconds, is
represented by {_POSIX_CLOCKRES_MIN}.
Minimum Values
The <limits.h> header shall define the following symbolic
constants with the values shown. These are the most restrictive
values for certain features on an implementation conforming to
this volume of POSIX.1‐2017. Related symbolic constants are
defined elsewhere in this volume of POSIX.1‐2017 which reflect
the actual implementation and which need not be as restrictive.
For each of these limits, a conforming implementation shall
provide a value at least this large or shall have no limit. A
strictly conforming application must not require a larger value
for correct operation.
{_POSIX_AIO_LISTIO_MAX}
The number of I/O operations that can be specified in a
list I/O call.
Value: 2
{_POSIX_AIO_MAX}
The number of outstanding asynchronous I/O operations.
Value: 1
{_POSIX_ARG_MAX}
Maximum length of argument to the exec functions including
environment data.
Value: 4 096
{_POSIX_CHILD_MAX}
Maximum number of simultaneous processes per real user ID.
Value: 25
{_POSIX_DELAYTIMER_MAX}
The number of timer expiration overruns.
Value: 32
{_POSIX_HOST_NAME_MAX}
Maximum length of a host name (not including the
terminating null) as returned from the gethostname()
function.
Value: 255
{_POSIX_LINK_MAX}
Maximum number of links to a single file.
Value: 8
{_POSIX_LOGIN_NAME_MAX}
The size of the storage required for a login name, in bytes
(including the terminating null).
Value: 9
{_POSIX_MAX_CANON}
Maximum number of bytes in a terminal canonical input
queue.
Value: 255
{_POSIX_MAX_INPUT}
Maximum number of bytes allowed in a terminal input queue.
Value: 255
{_POSIX_MQ_OPEN_MAX}
The number of message queues that can be open for a single
process.
Value: 8
{_POSIX_MQ_PRIO_MAX}
The maximum number of message priorities supported by the
implementation.
Value: 32
{_POSIX_NAME_MAX}
Maximum number of bytes in a filename (not including the
terminating null of a filename string).
Value: 14
{_POSIX_NGROUPS_MAX}
Maximum number of simultaneous supplementary group IDs per
process.
Value: 8
{_POSIX_OPEN_MAX}
A value one greater than the maximum value that the system
may assign to a newly-created file descriptor.
Value: 20
{_POSIX_PATH_MAX}
Minimum number the implementation will accept as the
maximum number of bytes in a pathname.
Value: 256
{_POSIX_PIPE_BUF}
Maximum number of bytes that is guaranteed to be atomic
when writing to a pipe.
Value: 512
{_POSIX_RE_DUP_MAX}
Maximum number of repeated occurrences of a BRE or ERE
interval expression; see Section 9.3.6, BREs Matching
Multiple Characters and Section 9.4.6, EREs Matching
Multiple Characters.
Value: 255
{_POSIX_RTSIG_MAX}
The number of realtime signal numbers reserved for
application use.
Value: 8
{_POSIX_SEM_NSEMS_MAX}
The number of semaphores that a process may have.
Value: 256
{_POSIX_SEM_VALUE_MAX}
The maximum value a semaphore may have.
Value: 32 767
{_POSIX_SIGQUEUE_MAX}
The number of queued signals that a process may send and
have pending at the receiver(s) at any time.
Value: 32
{_POSIX_SSIZE_MAX}
The value that can be stored in an object of type ssize_t.
Value: 32 767
{_POSIX_SS_REPL_MAX}
The number of replenishment operations that may be
simultaneously pending for a particular sporadic server
scheduler.
Value: 4
{_POSIX_STREAM_MAX}
The number of streams that one process can have open at one
time.
Value: 8
{_POSIX_SYMLINK_MAX}
The number of bytes in a symbolic link.
Value: 255
{_POSIX_SYMLOOP_MAX}
The number of symbolic links that can be traversed in the
resolution of a pathname in the absence of a loop.
Value: 8
{_POSIX_THREAD_DESTRUCTOR_ITERATIONS}
The number of attempts made to destroy a thread's thread-
specific data values on thread exit.
Value: 4
{_POSIX_THREAD_KEYS_MAX}
The number of data keys per process.
Value: 128
{_POSIX_THREAD_THREADS_MAX}
The number of threads per process.
Value: 64
{_POSIX_TIMER_MAX}
The per-process number of timers.
Value: 32
{_POSIX_TRACE_EVENT_NAME_MAX}
The length in bytes of a trace event name (not including
the terminating null).
Value: 30
{_POSIX_TRACE_NAME_MAX}
The length in bytes of a trace generation version string or
a trace stream name (not including the terminating null).
Value: 8
{_POSIX_TRACE_SYS_MAX}
The number of trace streams that may simultaneously exist
in the system.
Value: 8
{_POSIX_TRACE_USER_EVENT_MAX}
The number of user trace event type identifiers that may
simultaneously exist in a traced process, including the
predefined user trace event POSIX_TRACE_UNNAMED_USER_EVENT.
Value: 32
{_POSIX_TTY_NAME_MAX}
The size of the storage required for a terminal device
name, in bytes (including the terminating null).
Value: 9
{_POSIX_TZNAME_MAX}
Maximum number of bytes supported for the name of a
timezone (not of the TZ variable).
Value: 6
Note: The length given by {_POSIX_TZNAME_MAX} does not
include the quoting characters mentioned in Section
8.3, Other Environment Variables.
{_POSIX2_BC_BASE_MAX}
Maximum obase values allowed by the bc utility.
Value: 99
{_POSIX2_BC_DIM_MAX}
Maximum number of elements permitted in an array by the bc
utility.
Value: 2 048
{_POSIX2_BC_SCALE_MAX}
Maximum scale value allowed by the bc utility.
Value: 99
{_POSIX2_BC_STRING_MAX}
Maximum length of a string constant accepted by the bc
utility.
Value: 1 000
{_POSIX2_CHARCLASS_NAME_MAX}
Maximum number of bytes in a character class name.
Value: 14
{_POSIX2_COLL_WEIGHTS_MAX}
Maximum number of weights that can be assigned to an entry
of the LC_COLLATE order keyword in the locale definition
file; see Chapter 7, Locale.
Value: 2
{_POSIX2_EXPR_NEST_MAX}
Maximum number of expressions that can be nested within
parentheses by the expr utility.
Value: 32
{_POSIX2_LINE_MAX}
Unless otherwise noted, the maximum length, in bytes, of a
utility's input line (either standard input or another
file), when the utility is described as processing text
files. The length includes room for the trailing <newline>.
Value: 2 048
{_POSIX2_RE_DUP_MAX}
Maximum number of repeated occurrences of a BRE or ERE
interval expression; see Section 9.3.6, BREs Matching
Multiple Characters and Section 9.4.6, EREs Matching
Multiple Characters.
Value: 255
{_XOPEN_IOV_MAX}
Maximum number of iovec structures that one process has
available for use with readv() or writev().
Value: 16
{_XOPEN_NAME_MAX}
Maximum number of bytes in a filename (not including the
terminating null of a filename string).
Value: 255
{_XOPEN_PATH_MAX}
Minimum number the implementation will accept as the
maximum number of bytes in a pathname.
Value: 1024
Numerical Limits
The <limits.h> header shall define the following macros and,
except for {CHAR_BIT}, {LONG_BIT}, {MB_LEN_MAX}, and {WORD_BIT},
they shall be replaced by expressions that have the same type as
would an expression that is an object of the corresponding type
converted according to the integer promotions.
If the value of an object of type char is treated as a signed
integer when used in an expression, the value of {CHAR_MIN} is
the same as that of {SCHAR_MIN} and the value of {CHAR_MAX} is
the same as that of {SCHAR_MAX}. Otherwise, the value of
{CHAR_MIN} is 0 and the value of {CHAR_MAX} is the same as that
of {UCHAR_MAX}.
{CHAR_BIT}
Number of bits in a type char.
Value: 8
{CHAR_MAX}
Maximum value for an object of type char.
Value: {UCHAR_MAX} or {SCHAR_MAX}
{CHAR_MIN}
Minimum value for an object of type char.
Value: {SCHAR_MIN} or 0
{INT_MAX}
Maximum value for an object of type int.
Minimum Acceptable Value: 2 147 483 647
{INT_MIN}
Minimum value for an object of type int.
Maximum Acceptable Value: -2 147 483 647
{LLONG_MAX}
Maximum value for an object of type long long.
Minimum Acceptable Value: +9223372036854775807
{LLONG_MIN}
Minimum value for an object of type long long.
Maximum Acceptable Value: -9223372036854775807
{LONG_BIT}
Number of bits in an object of type long.
Minimum Acceptable Value: 32
{LONG_MAX}
Maximum value for an object of type long.
Minimum Acceptable Value: +2 147 483 647
{LONG_MIN}
Minimum value for an object of type long.
Maximum Acceptable Value: -2 147 483 647
{MB_LEN_MAX}
Maximum number of bytes in a character, for any supported
locale.
Minimum Acceptable Value: 1
{SCHAR_MAX}
Maximum value for an object of type signed char.
Value: +127
{SCHAR_MIN}
Minimum value for an object of type signed char.
Value: -128
{SHRT_MAX}
Maximum value for an object of type short.
Minimum Acceptable Value: +32 767
{SHRT_MIN}
Minimum value for an object of type short.
Maximum Acceptable Value: -32 767
{SSIZE_MAX}
Maximum value for an object of type ssize_t.
Minimum Acceptable Value: {_POSIX_SSIZE_MAX}
{UCHAR_MAX}
Maximum value for an object of type unsigned char.
Value: 255
{UINT_MAX}
Maximum value for an object of type unsigned.
Minimum Acceptable Value: 4 294 967 295
{ULLONG_MAX}
Maximum value for an object of type unsigned long long.
Minimum Acceptable Value: 18446744073709551615
{ULONG_MAX}
Maximum value for an object of type unsigned long.
Minimum Acceptable Value: 4 294 967 295
{USHRT_MAX}
Maximum value for an object of type unsigned short.
Minimum Acceptable Value: 65 535
{WORD_BIT}
Number of bits in an object of type int.
Minimum Acceptable Value: 32
Other Invariant Values
The <limits.h> header shall define the following symbolic
constants:
{NL_ARGMAX}
Maximum value of n in conversion specifications using the
"%n$" sequence in calls to the printf() and scanf()
families of functions.
Minimum Acceptable Value: 9
{NL_LANGMAX}
Maximum number of bytes in a LANG name.
Minimum Acceptable Value: 14
{NL_MSGMAX}
Maximum message number.
Minimum Acceptable Value: 32 767
{NL_SETMAX}
Maximum set number.
Minimum Acceptable Value: 255
{NL_TEXTMAX}
Maximum number of bytes in a message string.
Minimum Acceptable Value: {_POSIX2_LINE_MAX}
{NZERO}
Default process priority.
Minimum Acceptable Value: 20
The following sections are informative.
