System Configuration Parameters
*******************************

The functions and macros listed in this chapter give information about configuration parameters of the operating system---for example, capacity limits, presence of optional POSIX features, and the default path for executable files (see String Parameters).

General Capacity Limits
=======================

The POSIX.1 and POSIX.2 standards specify a number of parameters that describe capacity limitations of the system.  These limits can be fixed constants for a given operating system, or they can vary from machine to machine.  For example, some limit values may be configurable by the system administrator, either at run time or by rebuilding the kernel, and this should not require recompiling application programs.

Each of the following limit parameters has a macro that is defined in limits.h only if the system has a fixed, uniform limit for the parameter in question.  If the system allows different file systems or files to have different limits, then the macro is undefined; use sysconf to find out the limit that applies at a particular time on a particular machine.  See Sysconf.

Each of these parameters also has another macro, with a name starting with _POSIX, which gives the lowest value that the limit is allowed to have on any POSIX system.  See Minimums.

 -- Macro: int ARG_MAX
     POSIX.1, limits.h If defined, the unvarying maximum combined length of the ARGV and ENVIRON arguments that can be passed to the exec functions.

 -- Macro: int CHILD_MAX
     POSIX.1, limits.h If defined, the unvarying maximum number of processes that can exist with the same real user ID at any one time.  In BSD and GNU, this is controlled by the RLIMIT_NPROC resource limit; see Limits on Resources.

 -- Macro: int OPEN_MAX
     POSIX.1, limits.h If defined, the unvarying maximum number of files that a single process can have open simultaneously.  In BSD and GNU, this is controlled by the RLIMIT_NOFILE resource limit; see Limits on Resources.

 -- Macro: int STREAM_MAX
     POSIX.1, limits.h If defined, the unvarying maximum number of streams that a single process can have open simultaneously.  See Opening Streams.

 -- Macro: int TZNAME_MAX
     POSIX.1, limits.h If defined, the unvarying maximum length of a time zone name. See Time Zone Functions.

These limit macros are always defined in limits.h.

 -- Macro: int NGROUPS_MAX
     POSIX.1, limits.h The maximum number of supplementary group IDs that one process can have.

     The value of this macro is actually a lower bound for the maximum.  That is, you can count on being able to have that many supplementary group IDs, but a particular machine might let you have even more.  You can use sysconf to see whether a particular machine will let you have more (see Sysconf).

 -- Macro: ssize_t SSIZE_MAX
     POSIX.1, limits.h The largest value that can fit in an object of type ssize_t. Effectively, this is the limit on the number of bytes that can be read or written in a single operation.

     This macro is defined in all POSIX systems because this limit is never configurable.

 -- Macro: int RE_DUP_MAX
     POSIX.2, limits.h The largest number of repetitions you are guaranteed is allowed in the construct \{MIN,MAX\} in a regular expression.

     The value of this macro is actually a lower bound for the maximum.  That is, you can count on being able to have that many repetitions, but a particular machine might let you have even more.  You can use sysconf to see whether a particular machine will let you have more (see Sysconf).  And even the value that sysconf tells you is just a lower bound---larger values might work.

     This macro is defined in all POSIX.2 systems, because POSIX.2 says it should always be defined even if there is no specific imposed limit.

Overall System Options
======================

POSIX defines certain system-specific options that not all POSIX systems support.  Since these options are provided in the kernel, not in the library, simply using theglibc does not guarantee any of these features are supported; it depends on the system you are using.

You can test for the availability of a given option using the macros in this section, together with the function sysconf.  The macros are defined only if you include unistd.h.

For the following macros, if the macro is defined in unistd.h, then the option is supported.  Otherwise, the option may or may not be supported; use sysconf to find out.  See Sysconf.

 -- Macro: int _POSIX_JOB_CONTROL
     POSIX.1, unistd.h If this symbol is defined, it indicates that the system supports job control.  Otherwise, the implementation behaves as if all processes within a session belong to a single process group.  See Job Control.

 -- Macro: int _POSIX_SAVED_IDS
     POSIX.1, unistd.h If this symbol is defined, it indicates that the system remembers the effective user and group IDs of a process before it executes an executable file with the set-user-ID or set-group-ID bits set, and that explicitly changing the effective user or group IDs back to these values is permitted.  If this option is not defined, then if a nonprivileged process changes its effective user or group ID to the real user or group ID of the process, it can't change it back again.  See Enable/Disable Setuid.

For the following macros, if the macro is defined in unistd.h, then its value indicates whether the option is supported.  A value of -1 means no, and any other value means yes.  If the macro is not defined, then the option may or may not be supported; use sysconf to find out.  See Sysconf.

 -- Macro: int _POSIX2_C_DEV
     POSIX.2, unistd.h If this symbol is defined, it indicates that the system has the POSIX.2 C compiler command, c89.  Theglibc always defines this as 1, on the assumption that you would not have installed it if you didn't have a C compiler.

 -- Macro: int _POSIX2_FORT_DEV
     POSIX.2, unistd.h If this symbol is defined, it indicates that the system has the POSIX.2 Fortran compiler command, fort77.  Theglibc never defines this, because we don't know what the system has.

 -- Macro: int _POSIX2_FORT_RUN
     POSIX.2, unistd.h If this symbol is defined, it indicates that the system has the POSIX.2 asa command to interpret Fortran carriage control.  Theglibc never defines this, because we don't know what the system has.

 -- Macro: int _POSIX2_LOCALEDEF
     POSIX.2, unistd.h If this symbol is defined, it indicates that the system has the POSIX.2 localedef command.  Theglibc never defines this, because we don't know what the system has.

 -- Macro: int _POSIX2_SW_DEV
     POSIX.2, unistd.h If this symbol is defined, it indicates that the system has the POSIX.2 commands ar, make, and strip.  Theglibc always defines this as 1, on the assumption that you had to have ar and make to install the library, and it's unlikely that strip would be absent when those are present.

Which Version of POSIX is Supported
===================================

 -- Macro: long int _POSIX_VERSION
     POSIX.1, unistd.h This constant represents the version of the POSIX.1 standard to which the implementation conforms.  For an implementation conforming to the 1995 POSIX.1 standard, the value is the integer 199506L.

     _POSIX_VERSION is always defined (in unistd.h) in any POSIX system.

     Usage Note: Don't try to test whether the system supports POSIX by including unistd.h and then checking whether _POSIX_VERSION is defined.  On a non-POSIX system, this will probably fail because there is no unistd.h.  We do not know of any way you can reliably test at compilation time whether your target system supports POSIX or whether unistd.h exists.

 -- Macro: long int _POSIX2_C_VERSION
     POSIX.2, unistd.h This constant represents the version of the POSIX.2 standard which the library and system kernel support.  We don't know what value this will be for the first version of the POSIX.2 standard, because the value is based on the year and month in which the standard is officially adopted.

     The value of this symbol says nothing about the utilities installed on the system.

     Usage Note: You can use this macro to tell whether a POSIX.1 system library supports POSIX.2 as well.  Any POSIX.1 system contains unistd.h, so include that file and then test defined (_POSIX2_C_VERSION).

Using sysconf
=============

When your system has configurable system limits, you can use the sysconf function to find out the value that applies to any particular machine.  The function and the associated PARAMETER constants are declared in the header file unistd.h.

Definition of sysconf
---------------------

 -- Function: long int sysconf (int PARAMETER)
     POSIX.1, unistd.h This function is used to inquire about runtime system parameters.  The PARAMETER argument should be one of the _SC_ symbols listed below.

     The normal return value from sysconf is the value you requested. A value of -1 is returned both if the implementation does not impose a limit, and in case of an error.

     The following errno error conditions are defined for this function:

     EINVAL
          The value of the PARAMETER is invalid.

Constants for sysconf Parameters
--------------------------------

Here are the symbolic constants for use as the PARAMETER argument to sysconf.  The values are all integer constants (more specifically, enumeration type values).

_SC_ARG_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to ARG_MAX.

_SC_CHILD_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to CHILD_MAX.

_SC_OPEN_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to OPEN_MAX.

_SC_STREAM_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to STREAM_MAX.

_SC_TZNAME_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to TZNAME_MAX.

_SC_NGROUPS_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to NGROUPS_MAX.

_SC_JOB_CONTROL
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_JOB_CONTROL.

_SC_SAVED_IDS
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_SAVED_IDS.

_SC_VERSION
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_VERSION.

_SC_CLK_TCK
     POSIX.1, unistd.h Inquire about the number of clock ticks per second; see CPU Time. The corresponding parameter CLK_TCK is obsolete.

_SC_CHARCLASS_NAME_MAX
     GNU, unistd.h Inquire about the parameter corresponding to maximal length allowed for a character class name in an extended locale specification.  These extensions are not yet standardized and so this option is not standardized as well.

_SC_REALTIME_SIGNALS
     POSIX.1, unistdh.h Inquire about the parameter corresponding to _POSIX_REALTIME_SIGNALS.

_SC_PRIORITY_SCHEDULING
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_PRIORITY_SCHEDULING.

_SC_TIMERS
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_TIMERS.

_SC_ASYNCHRONOUS_IO
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_ASYNCHRONOUS_IO.

_SC_PRIORITIZED_IO
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_PRIORITIZED_IO.

_SC_SYNCHRONIZED_IO
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_SYNCHRONIZED_IO.

_SC_FSYNC
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_FSYNC.

_SC_MAPPED_FILES
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_MAPPED_FILES.

_SC_MEMLOCK
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_MEMLOCK.

_SC_MEMLOCK_RANGE
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_MEMLOCK_RANGE.

_SC_MEMORY_PROTECTION
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_MEMORY_PROTECTION.

_SC_MESSAGE_PASSING
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_MESSAGE_PASSING.

_SC_SEMAPHORES
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_SEMAPHORES.

_SC_SHARED_MEMORY_OBJECTS
     POSIX.1, unistd.h Inquire about the parameter corresponding to

     _POSIX_SHARED_MEMORY_OBJECTS.

_SC_AIO_LISTIO_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_AIO_LISTIO_MAX.

_SC_AIO_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_AIO_MAX.

_SC_AIO_PRIO_DELTA_MAX
     POSIX.1, unistd.h Inquire about the value by which a process can decrease its asynchronous I/O priority level from its own scheduling priority.  This corresponds to the run-time invariant value AIO_PRIO_DELTA_MAX.

_SC_DELAYTIMER_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_DELAYTIMER_MAX.

_SC_MQ_OPEN_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_MQ_OPEN_MAX.

_SC_MQ_PRIO_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_MQ_PRIO_MAX.

_SC_RTSIG_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_RTSIG_MAX.

_SC_SEM_NSEMS_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_SEM_NSEMS_MAX.

_SC_SEM_VALUE_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_SEM_VALUE_MAX.

_SC_SIGQUEUE_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_SIGQUEUE_MAX.

_SC_TIMER_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_TIMER_MAX.

_SC_PII
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_PII.

_SC_PII_XTI
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_PII_XTI.

_SC_PII_SOCKET
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_PII_SOCKET.

_SC_PII_INTERNET
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_PII_INTERNET.

_SC_PII_OSI
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_PII_OSI.

_SC_SELECT
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_SELECT.

_SC_UIO_MAXIOV
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_UIO_MAXIOV.

_SC_PII_INTERNET_STREAM
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_PII_INTERNET_STREAM.

_SC_PII_INTERNET_DGRAM
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_PII_INTERNET_DGRAM.

_SC_PII_OSI_COTS
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_PII_OSI_COTS.

_SC_PII_OSI_CLTS
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_PII_OSI_CLTS.

_SC_PII_OSI_M
     POSIX.1g, unistd.h Inquire about the parameter corresponding to _POSIX_PII_OSI_M.

_SC_T_IOV_MAX
     POSIX.1g, unistd.h Inquire about the value associated with the T_IOV_MAX variable.

_SC_THREADS
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_THREADS.

_SC_THREAD_SAFE_FUNCTIONS
     POSIX.1, unistd.h Inquire about the parameter corresponding to

     _POSIX_THREAD_SAFE_FUNCTIONS.

_SC_GETGR_R_SIZE_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_GETGR_R_SIZE_MAX.

_SC_GETPW_R_SIZE_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_GETPW_R_SIZE_MAX.

_SC_LOGIN_NAME_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_LOGIN_NAME_MAX.

_SC_TTY_NAME_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_TTY_NAME_MAX.

_SC_THREAD_DESTRUCTOR_ITERATIONS
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_THREAD_DESTRUCTOR_ITERATIONS.

_SC_THREAD_KEYS_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_THREAD_KEYS_MAX.

_SC_THREAD_STACK_MIN
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_THREAD_STACK_MIN.

_SC_THREAD_THREADS_MAX
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_THREAD_THREADS_MAX.

_SC_THREAD_ATTR_STACKADDR
     POSIX.1, unistd.h Inquire about the parameter corresponding to
     a _POSIX_THREAD_ATTR_STACKADDR.

_SC_THREAD_ATTR_STACKSIZE
     POSIX.1, unistd.h Inquire about the parameter corresponding to

     _POSIX_THREAD_ATTR_STACKSIZE.

_SC_THREAD_PRIORITY_SCHEDULING
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_THREAD_PRIORITY_SCHEDULING.

_SC_THREAD_PRIO_INHERIT
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_THREAD_PRIO_INHERIT.

_SC_THREAD_PRIO_PROTECT
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_THREAD_PRIO_PROTECT.

_SC_THREAD_PROCESS_SHARED
     POSIX.1, unistd.h Inquire about the parameter corresponding to _POSIX_THREAD_PROCESS_SHARED.

_SC_2_C_DEV
     POSIX.2, unistd.h Inquire about whether the system has the POSIX.2 C compiler command, c89.

_SC_2_FORT_DEV
     POSIX.2, unistd.h Inquire about whether the system has the POSIX.2 Fortran compiler command, fort77.

_SC_2_FORT_RUN
     POSIX.2, unistd.h Inquire about whether the system has the POSIX.2 asa command to interpret Fortran carriage control.

_SC_2_LOCALEDEF
     POSIX.2, unistd.h Inquire about whether the system has the POSIX.2 localedef command.

_SC_2_SW_DEV
     POSIX.2, unistd.h Inquire about whether the system has the POSIX.2 commands ar, make, and strip.

_SC_BC_BASE_MAX
     POSIX.2, unistd.h Inquire about the maximum value of obase in the bc utility.

_SC_BC_DIM_MAX
     POSIX.2, unistd.h Inquire about the maximum size of an array in the bc utility.

_SC_BC_SCALE_MAX
     POSIX.2, unistd.h Inquire about the maximum value of scale in the bc utility.

_SC_BC_STRING_MAX
     POSIX.2, unistd.h Inquire about the maximum size of a string constant in the bc utility.

_SC_COLL_WEIGHTS_MAX
     POSIX.2, unistd.h Inquire about the maximum number of weights that can necessarily be used in defining the collating sequence for a locale.

_SC_EXPR_NEST_MAX
     POSIX.2, unistd.h Inquire about the maximum number of expressions nested within parentheses when using the expr utility.

_SC_LINE_MAX
     POSIX.2, unistd.h Inquire about the maximum size of a text line that the POSIX.2 text utilities can handle.

_SC_EQUIV_CLASS_MAX
     POSIX.2, unistd.h Inquire about the maximum number of weights that can be assigned to an entry of the LC_COLLATE category order keyword in a locale definition.  Theglibc does not presently support locale definitions.

_SC_VERSION
     POSIX.2, unistd.h Inquire about the version number of POSIX.1 that the library and kernel support.

_SC_2_VERSION
     POSIX.2, unistd.h Inquire about the version number of POSIX.2 that the system utilities support.

_SC_PAGESIZE
     GNU, unistd.h Inquire about the virtual memory page size of the machine. getpagesize returns the same value (see Query Memory Parameters).

_SC_NPROCESSORS_CONF
     GNU, unistd.h Inquire about the number of configured processors.

_SC_NPROCESSORS_ONLN
     GNU, unistd.h Inquire about the number of processors online.

_SC_PHYS_PAGES
     GNU, unistd.h Inquire about the number of physical pages in the system.

_SC_AVPHYS_PAGES
     GNU, unistd.h Inquire about the number of available physical pages in the system.

_SC_ATEXIT_MAX
     GNU, unistd.h Inquire about the number of functions which can be registered as termination functions for atexit; see Cleanups on Exit.

_SC_LEVEL1_ICACHE_SIZE
     GNU, unistd.h Inquire about the size of the Level 1 instruction cache.

_SC_LEVEL1_ICACHE_ASSOC
     GNU, unistd.h Inquire about the associativity of the Level 1 instruction cache.

_SC_LEVEL1_ICACHE_LINESIZE
     GNU, unistd.h Inquire about the line length of the Level 1 instruction cache.

     On aarch64, the cache line size returned is the minimum instruction cache line size observable by userspace.  This is typically the same as the L1 icache size but on some cores it may not be so.  However, it is specified in the architecture that operations such as cache line invalidation are consistent with the size reported with this variable.

_SC_LEVEL1_DCACHE_SIZE
     GNU, unistd.h Inquire about the size of the Level 1 data cache.

_SC_LEVEL1_DCACHE_ASSOC
     GNU, unistd.h Inquire about the associativity of the Level 1 data cache.

_SC_LEVEL1_DCACHE_LINESIZE
     GNU, unistd.h Inquire about the line length of the Level 1 data cache.

     On aarch64, the cache line size returned is the minimum data cache line size observable by userspace.  This is typically the same as the L1 dcache size but on some cores it may not be so.  However, it is specified in the architecture that operations such as cache line invalidation are consistent with the size reported with this variable.

_SC_LEVEL2_CACHE_SIZE
     GNU, unistd.h Inquire about the size of the Level 2 cache.

_SC_LEVEL2_CACHE_ASSOC
     GNU, unistd.h Inquire about the associativity of the Level 2 cache.

_SC_LEVEL2_CACHE_LINESIZE
     GNU, unistd.h Inquire about the line length of the Level 2 cache.

_SC_LEVEL3_CACHE_SIZE
     GNU, unistd.h Inquire about the size of the Level 3 cache.

_SC_LEVEL3_CACHE_ASSOC
     GNU, unistd.h Inquire about the associativity of the Level 3 cache.

_SC_LEVEL3_CACHE_LINESIZE
     GNU, unistd.h Inquire about the line length of the Level 3 cache.

_SC_LEVEL4_CACHE_SIZE
     GNU, unistd.h Inquire about the size of the Level 4 cache.

_SC_LEVEL4_CACHE_ASSOC
     GNU, unistd.h Inquire about the associativity of the Level 4 cache.

_SC_LEVEL4_CACHE_LINESIZE
     GNU, unistd.h Inquire about the line length of the Level 4 cache.

_SC_XOPEN_VERSION
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_VERSION.

_SC_XOPEN_XCU_VERSION
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_XCU_VERSION.

_SC_XOPEN_UNIX
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_UNIX.

_SC_XOPEN_REALTIME
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_REALTIME.

_SC_XOPEN_REALTIME_THREADS
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_REALTIME_THREADS.

_SC_XOPEN_LEGACY
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_LEGACY.

_SC_XOPEN_CRYPT
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_CRYPT.

_SC_XOPEN_ENH_I18N
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_ENH_I18N.

_SC_XOPEN_SHM
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_SHM.

_SC_XOPEN_XPG2
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_XPG2.

_SC_XOPEN_XPG3
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_XPG3.

_SC_XOPEN_XPG4
     X/Open, unistd.h Inquire about the parameter corresponding to _XOPEN_XPG4.

_SC_CHAR_BIT
     X/Open, unistd.h Inquire about the number of bits in a variable of type char.

_SC_CHAR_MAX
     X/Open, unistd.h Inquire about the maximum value which can be stored in a variable of type char.

_SC_CHAR_MIN
     X/Open, unistd.h Inquire about the minimum value which can be stored in a variable of type char.

_SC_INT_MAX
     X/Open, unistd.h Inquire about the maximum value which can be stored in a variable of type int.

_SC_INT_MIN
     X/Open, unistd.h Inquire about the minimum value which can be stored in a variable of type int.

_SC_LONG_BIT
     X/Open, unistd.h Inquire about the number of bits in a variable of type long int.

_SC_WORD_BIT
     X/Open, unistd.h Inquire about the number of bits in a variable of a register word.

_SC_MB_LEN_MAX
     X/Open, unistd.h Inquire about the maximum length of a multi-byte representation of a wide character value.

_SC_NZERO
     X/Open, unistd.h Inquire about the value used to internally represent the zero priority level for the process execution.

_SC_SSIZE_MAX
     X/Open, unistd.h Inquire about the maximum value which can be stored in a variable of type ssize_t.

_SC_SCHAR_MAX
     X/Open, unistd.h Inquire about the maximum value which can be stored in a variable of type signed char.

_SC_SCHAR_MIN
     X/Open, unistd.h Inquire about the minimum value which can be stored in a variable of type signed char.

_SC_SHRT_MAX
     X/Open, unistd.h Inquire about the maximum value which can be stored in a variable of type short int.

_SC_SHRT_MIN
     X/Open, unistd.h Inquire about the minimum value which can be stored in a variable of type short int.

_SC_UCHAR_MAX
     X/Open, unistd.h Inquire about the maximum value which can be stored in a variable of type unsigned char.

_SC_UINT_MAX
     X/Open, unistd.h Inquire about the maximum value which can be stored in a variable of type unsigned int.

_SC_ULONG_MAX
     X/Open, unistd.h Inquire about the maximum value which can be stored in a variable of type unsigned long int.

_SC_USHRT_MAX
     X/Open, unistd.h Inquire about the maximum value which can be stored in a variable of type unsigned short int.

_SC_NL_ARGMAX
     X/Open, unistd.h Inquire about the parameter corresponding to NL_ARGMAX.

_SC_NL_LANGMAX
     X/Open, unistd.h Inquire about the parameter corresponding to NL_LANGMAX.

_SC_NL_MSGMAX
     X/Open, unistd.h Inquire about the parameter corresponding to NL_MSGMAX.

_SC_NL_NMAX
     X/Open, unistd.h Inquire about  the parameter corresponding to NL_NMAX.

_SC_NL_SETMAX
     X/Open, unistd.h Inquire about the parameter corresponding to NL_SETMAX.

_SC_NL_TEXTMAX
     X/Open, unistd.h Inquire about the parameter corresponding to NL_TEXTMAX.

Examples of sysconf
-------------------

We recommend that you first test for a macro definition for the parameter you are interested in, and call sysconf only if the macro is not defined.  For example, here is how to test whether job control is supported:

     int
     have_job_control (void)
     {
     #ifdef _POSIX_JOB_CONTROL
       return 1;
     #else
       int value = sysconf (_SC_JOB_CONTROL);
       if (value < 0)
         /* If the system is that badly wedged,
            there's no use trying to go on.  */
         fatal (strerror (errno));
       return value;
     #endif
     }

Here is how to get the value of a numeric limit:

     int
     get_child_max ()
     {
     #ifdef CHILD_MAX
       return CHILD_MAX;
     #else
       int value = sysconf (_SC_CHILD_MAX);
       if (value < 0)
         fatal (strerror (errno));
       return value;
     #endif
     }

Minimum Values for General Capacity Limits
==========================================

Here are the names for the POSIX minimum upper bounds for the system limit parameters.  The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far.

_POSIX_AIO_LISTIO_MAX
     POSIX.1, limits.h The most restrictive limit permitted by POSIX for the maximum number of I/O operations that can be specified in a list I/O call.  The value of this constant is 2; thus you can add up to two new entries of the list of outstanding operations.

_POSIX_AIO_MAX
     POSIX.1, limits.h The most restrictive limit permitted by POSIX for the maximum number of outstanding asynchronous I/O operations.  The value of this constant is 1.  So you cannot expect that you can issue more than one operation and immediately continue with the normal work, receiving the notifications asynchronously.

_POSIX_ARG_MAX
     POSIX.1, limits.h The value of this macro is the most restrictive limit permitted by POSIX for the maximum combined length of the ARGV and ENVIRON arguments that can be passed to the exec functions. Its value is 4096.

_POSIX_CHILD_MAX
     POSIX.1, limits.h The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of simultaneous processes per real user ID.  Its value is 6.

_POSIX_NGROUPS_MAX
     POSIX.1, limits.h The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of supplementary group IDs per process.  Its value is 0.

_POSIX_OPEN_MAX
     POSIX.1, limits.h The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of files that a single process can have open simultaneously.  Its value is 16.

_POSIX_SSIZE_MAX
     POSIX.1, limits.h The value of this macro is the most restrictive limit permitted by POSIX for the maximum value that can be stored in an object of type ssize_t.  Its value is 32767.

_POSIX_STREAM_MAX
     POSIX.1, limits.h The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of streams that a single process can have open simultaneously.  Its value is 8.

_POSIX_TZNAME_MAX
     POSIX.1, limits.h The value of this macro is the most restrictive limit permitted by POSIX for the maximum length of a time zone name.  Its value is 3.

_POSIX2_RE_DUP_MAX
     POSIX.2, limits.h The value of this macro is the most restrictive limit permitted by POSIX for the numbers used in the \{MIN,MAX\} construct in a regular expression.  Its value is 255.

Limits on File System Capacity
==============================

The POSIX.1 standard specifies a number of parameters that describe the limitations of the file system.  It's possible for the system to have a fixed, uniform limit for a parameter, but this isn't the usual case.  On most systems, it's possible for different file systems (and, for some parameters, even different files) to have different maximum limits.  For example, this is very likely if you use NFS to mount some of the file systems from other machines.

Each of the following macros is defined in limits.h only if the system has a fixed, uniform limit for the parameter in question.  If the system allows different file systems or files to have different limits, then the macro is undefined; use pathconf or fpathconf to find out the limit that applies to a particular file.  See Pathconf.

Each parameter also has another macro, with a name starting with _POSIX, which gives the lowest value that the limit is allowed to have on any POSIX system.  See File Minimums.

 -- Macro: int LINK_MAX
     POSIX.1, limits.h (optional) The uniform system limit (if any) for the number of names for a given file.  See Hard Links.

 -- Macro: int MAX_CANON
     POSIX.1, limits.h The uniform system limit (if any) for the amount of text in a line of input when input editing is enabled.  See Canonical or Not.

 -- Macro: int MAX_INPUT
     POSIX.1, limits.h The uniform system limit (if any) for the total number of characters typed ahead as input.  See I/O Queues.

 -- Macro: int NAME_MAX
     POSIX.1, limits.h The uniform system limit (if any) for the length of a file name component, not including the terminating null character.

     Portability Note: On some systems, theglibc defines NAME_MAX, but does not actually enforce this limit.

 -- Macro: int PATH_MAX
     POSIX.1, limits.h The uniform system limit (if any) for the length of an entire file name (that is, the argument given to system calls such as open), including the terminating null character.

     Portability Note: Theglibc does not enforce this limit even if PATH_MAX is defined.

 -- Macro: int PIPE_BUF
     POSIX.1, limits.h The uniform system limit (if any) for the number of bytes that can be written atomically to a pipe.  If multiple processes are writing to the same pipe simultaneously, output from different processes might be interleaved in chunks of this size.  See Pipes and FIFOs.

These are alternative macro names for some of the same information.

 -- Macro: int MAXNAMLEN
     BSD, dirent.h This is the BSD name for NAME_MAX.  It is defined in dirent.h.

 -- Macro: int FILENAME_MAX
     ISO, stdio.h The value of this macro is an integer constant expression that represents the maximum length of a file name string.  It is defined in stdio.h.

     Unlike PATH_MAX, this macro is defined even if there is no actual limit imposed.  In such a case, its value is typically a very large number.  This is always the case on gnuhurdsystems.

     Usage Note: Don't use FILENAME_MAX as the size of an array in which to store a file name!  You can't possibly make an array that big!  Use dynamic allocation (see Memory Allocation) instead.

Optional Features in File Support
=================================

POSIX defines certain system-specific options in the system calls for operating on files.  Some systems support these options and others do not.  Since these options are provided in the kernel, not in the library, simply using theglibc does not guarantee that any of these features is supported; it depends on the system you are using.  They can also vary between file systems on a single machine.

This section describes the macros you can test to determine whether a particular option is supported on your machine.  If a given macro is defined in unistd.h, then its value says whether the corresponding feature is supported.  (A value of -1 indicates no; any other value indicates yes.)  If the macro is undefined, it means particular files may or may not support the feature.

Since all the machines that support theglibc also support NFS, one can never make a general statement about whether all file systems support the _POSIX_CHOWN_RESTRICTED and _POSIX_NO_TRUNC features.  So these names are never defined as macros in theglibc.

 -- Macro: int _POSIX_CHOWN_RESTRICTED
     POSIX.1, unistd.h If this option is in effect, the chown function is restricted so that the only changes permitted to nonprivileged processes is to change the group owner of a file to either be the effective group ID of the process, or one of its supplementary group IDs.  See File Owner.

 -- Macro: int _POSIX_NO_TRUNC
     POSIX.1, unistd.h If this option is in effect, file name components longer than NAME_MAX generate an ENAMETOOLONG error.  Otherwise, file name components that are too long are silently truncated.

 -- Macro: unsigned char _POSIX_VDISABLE
     POSIX.1, unistd.h This option is only meaningful for files that are terminal devices. If it is enabled, then handling for special control characters can be disabled individually.  See Special Characters.

If one of these macros is undefined, that means that the option might be in effect for some files and not for others.  To inquire about a particular file, call pathconf or fpathconf. See Pathconf.

Minimum Values for File System Limits
=====================================

Here are the names for the POSIX minimum upper bounds for some of the above parameters.  The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far.  In most cases gnusystems do not have these strict limitations.  The actual limit should be requested if necessary.

_POSIX_LINK_MAX
     POSIX.1, limits.h The most restrictive limit permitted by POSIX for the maximum value of a file's link count.  The value of this constant is 8; thus, you can always make up to eight names for a file without running into a system limit.

_POSIX_MAX_CANON
     POSIX.1, limits.h The most restrictive limit permitted by POSIX for the maximum number of bytes in a canonical input line from a terminal device.  The value of this constant is 255.

_POSIX_MAX_INPUT
     POSIX.1, limits.h The most restrictive limit permitted by POSIX for the maximum number of bytes in a terminal device input queue (or typeahead buffer). See Input Modes.  The value of this constant is 255.

_POSIX_NAME_MAX
     POSIX.1, limits.h The most restrictive limit permitted by POSIX for the maximum number of bytes in a file name component.  The value of this constant is 14.

_POSIX_PATH_MAX
     POSIX.1, limits.h The most restrictive limit permitted by POSIX for the maximum number of bytes in a file name.  The value of this constant is 256.

_POSIX_PIPE_BUF
     POSIX.1, limits.h The most restrictive limit permitted by POSIX for the maximum number of bytes that can be written atomically to a pipe.  The value of this constant is 512.

SYMLINK_MAX
     POSIX.1, limits.h Maximum number of bytes in a symbolic link.

POSIX_REC_INCR_XFER_SIZE
     POSIX.1, limits.h Recommended increment for file transfer sizes between the POSIX_REC_MIN_XFER_SIZE and POSIX_REC_MAX_XFER_SIZE values.

POSIX_REC_MAX_XFER_SIZE
     POSIX.1, limits.h Maximum recommended file transfer size.

POSIX_REC_MIN_XFER_SIZE
     POSIX.1, limits.h Minimum recommended file transfer size.

POSIX_REC_XFER_ALIGN
     POSIX.1, limits.h Recommended file transfer buffer alignment.

Using pathconf
==============

When your machine allows different files to have different values for a file system parameter, you can use the functions in this section to find out the value that applies to any particular file.

These functions and the associated constants for the PARAMETER argument are declared in the header file unistd.h.

 -- Function: long int pathconf (const char *FILENAME, int PARAMETER)
     POSIX.1, unistd.h This function is used to inquire about the limits that apply to the file named FILENAME.

     The PARAMETER argument should be one of the _PC_ constants listed below.

     The normal return value from pathconf is the value you requested. A value of -1 is returned both if the implementation does not impose a limit, and in case of an error.  In the former case, errno is not set, while in the latter case, errno is set to indicate the cause of the problem.  So the only way to use this function robustly is to store 0 into errno just before calling it.

     Besides the usual file name errors (see File Name Errors), the following error condition is defined for this function:

     EINVAL
          The value of PARAMETER is invalid, or the implementation doesn't support the PARAMETER for the specific file.

 -- Function: long int fpathconf (int FILEDES, int PARAMETER)
     POSIX.1, unistd.h This is just like pathconf except that an open file descriptor is used to specify the file for which information is requested, instead of a file name.

     The following errno error conditions are defined for this function:

     EBADF
          The FILEDES argument is not a valid file descriptor.

     EINVAL
          The value of PARAMETER is invalid, or the implementation doesn't support the PARAMETER for the specific file.

Here are the symbolic constants that you can use as the PARAMETER argument to pathconf and fpathconf.  The values are all integer constants.

_PC_LINK_MAX
     POSIX.1, unistd.h Inquire about the value of LINK_MAX.

_PC_MAX_CANON
     POSIX.1, unistd.h Inquire about the value of MAX_CANON.

_PC_MAX_INPUT
     POSIX.1, unistd.h Inquire about the value of MAX_INPUT.

_PC_NAME_MAX
     POSIX.1, unistd.h Inquire about the value of NAME_MAX.

_PC_PATH_MAX
     POSIX.1, unistd.h Inquire about the value of PATH_MAX.

_PC_PIPE_BUF
     POSIX.1, unistd.h Inquire about the value of PIPE_BUF.

_PC_CHOWN_RESTRICTED
     POSIX.1, unistd.h Inquire about the value of _POSIX_CHOWN_RESTRICTED.

_PC_NO_TRUNC
     POSIX.1, unistd.h Inquire about the value of _POSIX_NO_TRUNC.

_PC_VDISABLE
     POSIX.1, unistd.h Inquire about the value of _POSIX_VDISABLE.

_PC_SYNC_IO
     POSIX.1, unistd.h Inquire about the value of _POSIX_SYNC_IO.

_PC_ASYNC_IO
     POSIX.1, unistd.h Inquire about the value of _POSIX_ASYNC_IO.

_PC_PRIO_IO
     POSIX.1, unistd.h Inquire about the value of _POSIX_PRIO_IO.

_PC_FILESIZEBITS
     LFS, unistd.h Inquire about the availability of large files on the filesystem.

_PC_REC_INCR_XFER_SIZE
     POSIX.1, unistd.h Inquire about the value of POSIX_REC_INCR_XFER_SIZE.

_PC_REC_MAX_XFER_SIZE
     POSIX.1, unistd.h Inquire about the value of POSIX_REC_MAX_XFER_SIZE.

_PC_REC_MIN_XFER_SIZE
     POSIX.1, unistd.h Inquire about the value of POSIX_REC_MIN_XFER_SIZE.

_PC_REC_XFER_ALIGN
     POSIX.1, unistd.h Inquire about the value of POSIX_REC_XFER_ALIGN.

Portability Note: On some systems, theglibc does not enforce _PC_NAME_MAX or _PC_PATH_MAX limits.

Utility Program Capacity Limits
===============================

The POSIX.2 standard specifies certain system limits that you can access through sysconf that apply to utility behavior rather than the behavior of the library or the operating system.

Theglibc defines macros for these limits, and sysconf returns values for them if you ask; but these values convey no meaningful information.  They are simply the smallest values that POSIX.2 permits.

 -- Macro: int BC_BASE_MAX
     POSIX.2, limits.h The largest value of obase that the bc utility is guaranteed to support.

 -- Macro: int BC_DIM_MAX
     POSIX.2, limits.h The largest number of elements in one array that the bc utility is guaranteed to support.

 -- Macro: int BC_SCALE_MAX
     POSIX.2, limits.h The largest value of scale that the bc utility is guaranteed to support.

 -- Macro: int BC_STRING_MAX
     POSIX.2, limits.h The largest number of characters in one string constant that the bc utility is guaranteed to support.

 -- Macro: int COLL_WEIGHTS_MAX
     POSIX.2, limits.h The largest number of weights that can necessarily be used in defining the collating sequence for a locale.

 -- Macro: int EXPR_NEST_MAX
     POSIX.2, limits.h The maximum number of expressions that can be nested within parentheses by the expr utility.

 -- Macro: int LINE_MAX
     POSIX.2, limits.h The largest text line that the text-oriented POSIX.2 utilities can support.  (If you are using the GNU versions of these utilities, then there is no actual limit except that imposed by the available virtual memory, but there is no way that the library can tell you this.)

 -- Macro: int EQUIV_CLASS_MAX
     POSIX.2, limits.h The maximum number of weights that can be assigned to an entry of the LC_COLLATE category order keyword in a locale definition. Theglibc does not presently support locale definitions.

Minimum Values for Utility Limits
=================================

_POSIX2_BC_BASE_MAX
     POSIX.2, limits.h The most restrictive limit permitted by POSIX.2 for the maximum value of obase in the bc utility.  Its value is 99.

_POSIX2_BC_DIM_MAX
     POSIX.2, limits.h The most restrictive limit permitted by POSIX.2 for the maximum size of an array in the bc utility.  Its value is 2048.

_POSIX2_BC_SCALE_MAX
     POSIX.2, limits.h The most restrictive limit permitted by POSIX.2 for the maximum value of scale in the bc utility.  Its value is 99.

_POSIX2_BC_STRING_MAX
     POSIX.2, limits.h The most restrictive limit permitted by POSIX.2 for the maximum size of a string constant in the bc utility.  Its value is 1000.

_POSIX2_COLL_WEIGHTS_MAX
     POSIX.2, limits.h The most restrictive limit permitted by POSIX.2 for the maximum number of weights that can necessarily be used in defining the collating sequence for a locale.  Its value is 2.

_POSIX2_EXPR_NEST_MAX
     POSIX.2, limits.h The most restrictive limit permitted by POSIX.2 for the maximum number of expressions nested within parenthesis when using the expr utility. Its value is 32.

_POSIX2_LINE_MAX
     POSIX.2, limits.h The most restrictive limit permitted by POSIX.2 for the maximum size of a text line that the text utilities can handle.  Its value is 2048.

_POSIX2_EQUIV_CLASS_MAX
     POSIX.2, limits.h The most restrictive limit permitted by POSIX.2 for the maximum number of weights that can be assigned to an entry of the LC_COLLATE category order keyword in a locale definition.  Its value is 2.  Theglibc does not presently support locale definitions.

String-Valued Parameters
========================

POSIX.2 defines a way to get string-valued parameters from the operating system with the function confstr:

 -- Function: size_t confstr (int PARAMETER, char *BUF, size_t LEN)
     POSIX.2, unistd.h This function reads the value of a string-valued system parameter, storing the string into LEN bytes of memory space starting at BUF.  The PARAMETER argument should be one of the _CS_ symbols listed below.

     The normal return value from confstr is the length of the string value that you asked for.  If you supply a null pointer for BUF, then confstr does not try to store the string; it just returns its length.  A value of 0 indicates an error.

     If the string you asked for is too long for the buffer (that is, longer than LEN - 1), then confstr stores just that much (leaving room for the terminating null character).  You can tell that this has happened because confstr returns a value greater than or equal to LEN.

     The following errno error conditions are defined for this function:

     EINVAL
          The value of the PARAMETER is invalid.

Currently there is just one parameter you can read with confstr:

_CS_PATH
     POSIX.2, unistd.h This parameter's value is the recommended default path for searching for executable files.  This is the path that a user has by default just after logging in.

_CS_LFS_CFLAGS
     Unix98, unistd.h The returned string specifies which additional flags must be given to the C compiler if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.

_CS_LFS_LDFLAGS
     Unix98, unistd.h The returned string specifies which additional flags must be given to the linker if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.

_CS_LFS_LIBS
     Unix98, unistd.h The returned string specifies which additional libraries must be linked to the application if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.

_CS_LFS_LINTFLAGS
     Unix98, unistd.h The returned string specifies which additional flags must be given to the lint tool if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.

_CS_LFS64_CFLAGS
     Unix98, unistd.h The returned string specifies which additional flags must be given to the C compiler if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.

_CS_LFS64_LDFLAGS
     Unix98, unistd.h The returned string specifies which additional flags must be given to the linker if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.

_CS_LFS64_LIBS
     Unix98, unistd.h The returned string specifies which additional libraries must be linked to the application if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.

_CS_LFS64_LINTFLAGS
     Unix98, unistd.h The returned string specifies which additional flags must be given to the lint tool if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.

The way to use confstr without any arbitrary limit on string size is to call it twice: first call it to get the length, allocate the buffer accordingly, and then call confstr again to fill the buffer, like this:

     char *
     get_default_path (void)
     {
       size_t len = confstr (_CS_PATH, NULL, 0);
       char *buffer = (char *) xmalloc (len);

       if (confstr (_CS_PATH, buf, len + 1) == 0)
         {
           free (buffer);
           return NULL;
         }

       return buffer;
     }