.
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 section String-Valued Parameters).
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 section Using 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 section Minimum Values for General Capacity Limits.
- Macro: int ARG_MAX
-
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
-
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 section Limiting Resource Usage.
- Macro: int OPEN_MAX
-
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 section Limiting Resource Usage.
- Macro: int STREAM_MAX
-
If defined, the unvarying maximum number of streams that a single
process can have open simultaneously. See section Opening Streams.
- Macro: int TZNAME_MAX
-
If defined, the unvarying maximum length of a time zone name.
See section Functions and Variables for Time Zones.
These limit macros are always defined in `limits.h'.
- Macro: int NGROUPS_MAX
-
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 section Using sysconf).
- Macro: int SSIZE_MAX
-
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
-
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 section Using 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.
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 the GNU C library does not guarantee any of these
features is 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 section Using sysconf.
- Macro: int _POSIX_JOB_CONTROL
-
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 section Job Control.
- Macro: int _POSIX_SAVED_IDS
-
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 section Enabling and Disabling Setuid Access.
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 section Using sysconf.
- Macro: int _POSIX2_C_DEV
-
If this symbol is defined, it indicates that the system has the POSIX.2
C compiler command,
c89. The GNU C library 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
-
If this symbol is defined, it indicates that the system has the POSIX.2
Fortran compiler command,
fort77. The GNU C library never
defines this, because we don't know what the system has.
- Macro: int _POSIX2_FORT_RUN
-
If this symbol is defined, it indicates that the system has the POSIX.2
asa command to interpret Fortran carriage control. The GNU C
library never defines this, because we don't know what the system has.
- Macro: int _POSIX2_LOCALEDEF
-
If this symbol is defined, it indicates that the system has the POSIX.2
localedef command. The GNU C library never defines this, because
we don't know what the system has.
- Macro: int _POSIX2_SW_DEV
-
If this symbol is defined, it indicates that the system has the POSIX.2
commands
ar, make, and strip. The GNU C library
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.
- Macro: long int _POSIX_VERSION
-
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.
The GNU C compiler predefines the symbol __POSIX__ if the target
system is a POSIX system. Provided you do not use any other compilers
on POSIX systems, testing defined (__POSIX__) will reliably
detect such systems.
- Macro: long int _POSIX2_C_VERSION
-
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).
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'.
- Function: long int sysconf (int parameter)
-
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.
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
-
Inquire about the parameter corresponding to
ARG_MAX.
_SC_CHILD_MAX
-
Inquire about the parameter corresponding to
CHILD_MAX.
_SC_OPEN_MAX
-
Inquire about the parameter corresponding to
OPEN_MAX.
_SC_STREAM_MAX
-
Inquire about the parameter corresponding to
STREAM_MAX.
_SC_TZNAME_MAX
-
Inquire about the parameter corresponding to
TZNAME_MAX.
_SC_NGROUPS_MAX
-
Inquire about the parameter corresponding to
NGROUPS_MAX.
_SC_JOB_CONTROL
-
Inquire about the parameter corresponding to
_POSIX_JOB_CONTROL.
_SC_SAVED_IDS
-
Inquire about the parameter corresponding to
_POSIX_SAVED_IDS.
_SC_VERSION
-
Inquire about the parameter corresponding to
_POSIX_VERSION.
_SC_CLK_TCK
-
Inquire about the parameter corresponding to
CLOCKS_PER_SEC;
see section Basic CPU Time Inquiry.
_SC_CHARCLASS_NAME_MAX
-
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
-
Inquire about the parameter corresponding to
_POSIX_REALTIME_SIGNALS.
_SC_PRIORITY_SCHEDULING
-
Inquire about the parameter corresponding to
_POSIX_PRIORITY_SCHEDULING.
_SC_TIMERS
-
Inquire about the parameter corresponding to
_POSIX_TIMERS.
_SC_ASYNCHRONOUS_IO
-
Inquire about the parameter corresponding to
_POSIX_ASYNCHRONOUS_IO.
_SC_PRIORITIZED_IO
-
Inquire about the parameter corresponding to
_POSIX_PRIORITIZED_IO.
_SC_SYNCHRONIZED_IO
-
Inquire about the parameter corresponding to
_POSIX_SYNCHRONIZED_IO.
_SC_FSYNC
-
Inquire about the parameter corresponding to
_POSIX_FSYNC.
_SC_MAPPED_FILES
-
Inquire about the parameter corresponding to
_POSIX_MAPPED_FILES.
_SC_MEMLOCK
-
Inquire about the parameter corresponding to
_POSIX_MEMLOCK.
_SC_MEMLOCK_RANGE
-
Inquire about the parameter corresponding to
_POSIX_MEMLOCK_RANGE.
_SC_MEMORY_PROTECTION
-
Inquire about the parameter corresponding to
_POSIX_MEMORY_PROTECTION.
_SC_MESSAGE_PASSING
-
Inquire about the parameter corresponding to
_POSIX_MESSAGE_PASSING.
_SC_SEMAPHORES
-
Inquire about the parameter corresponding to
_POSIX_SEMAPHORES.
_SC_SHARED_MEMORY_OBJECTS
-
Inquire about the parameter corresponding to
_POSIX_SHARED_MEMORY_OBJECTS.
_SC_AIO_LISTIO_MAX
-
Inquire about the parameter corresponding to
_POSIX_AIO_LISTIO_MAX.
_SC_AIO_MAX
-
Inquire about the parameter corresponding to
_POSIX_AIO_MAX.
_SC_AIO_PRIO_DELTA_MAX
-
Inquire 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
-
Inquire about the parameter corresponding to
_POSIX_DELAYTIMER_MAX.
_SC_MQ_OPEN_MAX
-
Inquire about the parameter corresponding to
_POSIX_MQ_OPEN_MAX.
_SC_MQ_PRIO_MAX
-
Inquire about the parameter corresponding to
_POSIX_MQ_PRIO_MAX.
_SC_RTSIG_MAX
-
Inquire about the parameter corresponding to
_POSIX_RTSIG_MAX.
_SC_SEM_NSEMS_MAX
-
Inquire about the parameter corresponding to
_POSIX_SEM_NSEMS_MAX.
_SC_SEM_VALUE_MAX
-
Inquire about the parameter corresponding to
_POSIX_SEM_VALUE_MAX.
_SC_SIGQUEUE_MAX
-
Inquire about the parameter corresponding to
_POSIX_SIGQUEUE_MAX.
_SC_TIMER_MAX
-
Inquire about the parameter corresponding to
_POSIX_TIMER_MAX.
_SC_PII
-
Inquire about the parameter corresponding to
_POSIX_PII.
_SC_PII_XTI
-
Inquire about the parameter corresponding to
_POSIX_PII_XTI.
_SC_PII_SOCKET
-
Inquire about the parameter corresponding to
_POSIX_PII_SOCKET.
_SC_PII_INTERNET
-
Inquire about the parameter corresponding to
_POSIX_PII_INTERNET.
_SC_PII_OSI
-
Inquire about the parameter corresponding to
_POSIX_PII_OSI.
_SC_SELECT
-
Inquire about the parameter corresponding to
_POSIX_SELECT.
_SC_UIO_MAXIOV
-
Inquire about the parameter corresponding to
_POSIX_UIO_MAXIOV.
_SC_PII_INTERNET_STREAM
-
Inquire about the parameter corresponding to
_POSIX_PII_INTERNET_STREAM.
_SC_PII_INTERNET_DGRAM
-
Inquire about the parameter corresponding to
_POSIX_PII_INTERNET_DGRAM.
_SC_PII_OSI_COTS
-
Inquire about the parameter corresponding to
_POSIX_PII_OSI_COTS.
_SC_PII_OSI_CLTS
-
Inquire about the parameter corresponding to
_POSIX_PII_OSI_CLTS.
_SC_PII_OSI_M
-
Inquire about the parameter corresponding to
_POSIX_PII_OSI_M.
_SC_T_IOV_MAX
-
Inquire the value of the value associated with the
T_IOV_MAX
variable.
_SC_THREADS
-
Inquire about the parameter corresponding to
_POSIX_THREADS.
_SC_THREAD_SAFE_FUNCTIONS
-
Inquire about the parameter corresponding to
_POSIX_THREAD_SAFE_FUNCTIONS.
_SC_GETGR_R_SIZE_MAX
-
Inquire about the parameter corresponding to
_POSIX_GETGR_R_SIZE_MAX.
_SC_GETPW_R_SIZE_MAX
-
Inquire about the parameter corresponding to
_POSIX_GETPW_R_SIZE_MAX.
_SC_LOGIN_NAME_MAX
-
Inquire about the parameter corresponding to
_POSIX_LOGIN_NAME_MAX.
_SC_TTY_NAME_MAX
-
Inquire about the parameter corresponding to
_POSIX_TTY_NAME_MAX.
_SC_THREAD_DESTRUCTOR_ITERATIONS
-
Inquire about the parameter corresponding to
_POSIX_THREAD_DESTRUCTOR_ITERATIONS.
_SC_THREAD_KEYS_MAX
-
Inquire about the parameter corresponding to
_POSIX_THREAD_KEYS_MAX.
_SC_THREAD_STACK_MIN
-
Inquire about the parameter corresponding to
_POSIX_THREAD_STACK_MIN.
_SC_THREAD_THREADS_MAX
-
Inquire about the parameter corresponding to
_POSIX_THREAD_THREADS_MAX.
_SC_THREAD_ATTR_STACKADDR
-
Inquire about the parameter corresponding to
_POSIX_THREAD_ATTR_STACKADDR.
_SC_THREAD_ATTR_STACKSIZE
-
Inquire about the parameter corresponding to
_POSIX_THREAD_ATTR_STACKSIZE.
_SC_THREAD_PRIORITY_SCHEDULING
-
Inquire about the parameter corresponding to
_POSIX_THREAD_PRIORITY_SCHEDULING.
_SC_THREAD_PRIO_INHERIT
-
Inquire about the parameter corresponding to
_POSIX_THREAD_PRIO_INHERIT.
_SC_THREAD_PRIO_PROTECT
-
Inquire about the parameter corresponding to
_POSIX_THREAD_PRIO_PROTECT.
_SC_THREAD_PROCESS_SHARED
-
Inquire about the parameter corresponding to
_POSIX_THREAD_PROCESS_SHARED.
_SC_2_C_DEV
-
Inquire about whether the system has the POSIX.2 C compiler command,
c89.
_SC_2_FORT_DEV
-
Inquire about whether the system has the POSIX.2 Fortran compiler
command,
fort77.
_SC_2_FORT_RUN
-
Inquire about whether the system has the POSIX.2
asa command to
interpret Fortran carriage control.
_SC_2_LOCALEDEF
-
Inquire about whether the system has the POSIX.2
localedef
command.
_SC_2_SW_DEV
-
Inquire about whether the system has the POSIX.2 commands
ar,
make, and strip.
_SC_BC_BASE_MAX
-
Inquire about the maximum value of
obase in the bc
utility.
_SC_BC_DIM_MAX
-
Inquire about the maximum size of an array in the
bc
utility.
_SC_BC_SCALE_MAX
-
Inquire about the maximum value of
scale in the bc
utility.
_SC_BC_STRING_MAX
-
Inquire about the maximum size of a string constant in the
bc utility.
_SC_COLL_WEIGHTS_MAX
-
Inquire about the maximum number of weights that can necessarily
be used in defining the collating sequence for a locale.
_SC_EXPR_NEST_MAX
-
Inquire about the maximum number of expressions nested within
parentheses when using the
expr utility.
_SC_LINE_MAX
-
Inquire about the maximum size of a text line that the POSIX.2 text
utilities can handle.
_SC_EQUIV_CLASS_MAX
-
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. The GNU C library does not presently support locale
definitions.
_SC_VERSION
-
Inquire about the version number of POSIX.1 that the library and kernel
support.
_SC_2_VERSION
-
Inquire about the version number of POSIX.2 that the system utilities
support.
_SC_PAGESIZE
-
Inquire about the virtual memory page size of the machine.
getpagesize returns the same value.
_SC_NPROCESSORS_CONF
-
Inquire about number of configured processors.
_SC_NPROCESSORS_ONLN
-
Inquire about number of processors online.
_SC_PHYS_PAGES
-
Inquire about number of physical pages in the system.
_SC_AVPHYS_PAGES
-
Inquire about number of available physical pages in the system.
_SC_ATEXIT_MAX
-
Inquire about number of functions which can be registered as termination
functions for
atexit; see section Cleanups on Exit.
_SC_XOPEN_VERSION
-
Inquire about the parameter corresponding to
_XOPEN_VERSION.
_SC_XOPEN_XCU_VERSION
-
Inquire about the parameter corresponding to
_XOPEN_XCU_VERSION.
_SC_XOPEN_UNIX
-
Inquire about the parameter corresponding to
_XOPEN_UNIX.
_SC_XOPEN_CRYPT
-
Inquire about the parameter corresponding to
_XOPEN_CRYPT.
_SC_XOPEN_ENH_I18N
-
Inquire about the parameter corresponding to
_XOPEN_ENH_I18N.
_SC_XOPEN_SHM
-
Inquire about the parameter corresponding to
_XOPEN_SHM.
_SC_XOPEN_XPG2
-
Inquire about the parameter corresponding to
_XOPEN_XPG2.
_SC_XOPEN_XPG3
-
Inquire about the parameter corresponding to
_XOPEN_XPG3.
_SC_XOPEN_XPG4
-
Inquire about the parameter corresponding to
_XOPEN_XPG4.
_SC_CHAR_BIT
-
Inquire about number of bits in a variable of type
char.
_SC_CHAR_MAX
-
Inquire about maximum value which can be stored in a variable of type
char.
_SC_CHAR_MIN
-
Inquire about minimum value which can be stored in a variable of type
char.
_SC_INT_MAX
-
Inquire about maximum value which can be stored in a variable of type
int.
_SC_INT_MIN
-
Inquire about minimum value which can be stored in a variable of type
int.
_SC_LONG_BIT
-
Inquire about number of bits in a variable of type
long int.
_SC_WORD_BIT
-
Inquire about number of bits in a variable of a register word.
_SC_MB_LEN_MAX
-
Inquire the maximum length of a multi-byte representation of a wide
character value.
_SC_NZERO
-
Inquire value used to internally represent the zero priority level for
the process execution.
SC_SSIZE_MAX
-
Inquire about maximum value which can be stored in a variable of type
ssize_t.
_SC_SCHAR_MAX
-
Inquire about maximum value which can be stored in a variable of type
signed char.
_SC_SCHAR_MIN
-
Inquire about minimum value which can be stored in a variable of type
signed char.
_SC_SHRT_MAX
-
Inquire about maximum value which can be stored in a variable of type
short int.
_SC_SHRT_MIN
-
Inquire about minimum value which can be stored in a variable of type
short int.
_SC_UCHAR_MAX
-
Inquire about maximum value which can be stored in a variable of type
unsigned char.
_SC_UINT_MAX
-
Inquire about maximum value which can be stored in a variable of type
unsigned int.
_SC_ULONG_MAX
-
Inquire about maximum value which can be stored in a variable of type
unsigned long int.
_SC_USHRT_MAX
-
Inquire about maximum value which can be stored in a variable of type
unsigned short int.
_SC_NL_ARGMAX
-
Inquire about the parameter corresponding to
NL_ARGMAX.
_SC_NL_LANGMAX
-
Inquire about the parameter corresponding to
NL_LANGMAX.
_SC_NL_MSGMAX
-
Inquire about the parameter corresponding to
NL_MSGMAX.
_SC_NL_NMAX
-
Inquire about the parameter corresponding to
NL_NMAX.
_SC_NL_SETMAX
-
Inquire about the parameter corresponding to
NL_SETMAX.
_SC_NL_TEXTMAX
-
Inquire about the parameter corresponding to
NL_TEXTMAX.
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
}
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
-
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 outstandard operations.
_POSIX_AIO_MAX
-
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
-
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
-
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
-
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
-
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
-
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
-
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
-
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
-
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.
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 section Using 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 section Minimum Values for File System Limits.
- Macro: int LINK_MAX
-
The uniform system limit (if any) for the number of names for a given
file. See section Hard Links.
- Macro: int MAX_CANON
-
The uniform system limit (if any) for the amount of text in a line of
input when input editing is enabled. See section Two Styles of Input: Canonical or Not.
- Macro: int MAX_INPUT
-
The uniform system limit (if any) for the total number of characters
typed ahead as input. See section I/O Queues.
- Macro: int NAME_MAX
-
The uniform system limit (if any) for the length of a file name component.
- Macro: int PATH_MAX
-
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).
- Macro: int PIPE_BUF
-
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 section Pipes and FIFOs.
These are alternative macro names for some of the same information.
- Macro: int MAXNAMLEN
-
This is the BSD name for
NAME_MAX. It is defined in
`dirent.h'.
- Macro: int FILENAME_MAX
-
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 the GNU system.
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 section Memory Allocation) instead.
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 the GNU C library does not guarantee 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 the GNU C library 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 the GNU C
library.
- Macro: int _POSIX_CHOWN_RESTRICTED
-
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 section File Owner.
- Macro: int _POSIX_NO_TRUNC
-
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
-
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 section 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 section Using pathconf.
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 GNU systems do not
have these strict limitations. The actual limit should be requested if
necessary.
_POSIX_LINK_MAX
-
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
-
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
-
The most restrictive limit permitted by POSIX for the maximum number of
bytes in a terminal device input queue (or typeahead buffer).
See section Input Modes. The value of this constant is
255.
_POSIX_NAME_MAX
-
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
-
The most restrictive limit permitted by POSIX for the maximum number of
bytes in a file name. The value of this constant is
255.
_POSIX_PIPE_BUF
-
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.
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)
-
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 section 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)
-
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
-
Inquire about the value of
LINK_MAX.
_PC_MAX_CANON
-
Inquire about the value of
MAX_CANON.
_PC_MAX_INPUT
-
Inquire about the value of
MAX_INPUT.
_PC_NAME_MAX
-
Inquire about the value of
NAME_MAX.
_PC_PATH_MAX
-
Inquire about the value of
PATH_MAX.
_PC_PIPE_BUF
-
Inquire about the value of
PIPE_BUF.
_PC_CHOWN_RESTRICTED
-
Inquire about the value of
_POSIX_CHOWN_RESTRICTED.
_PC_NO_TRUNC
-
Inquire about the value of
_POSIX_NO_TRUNC.
_PC_VDISABLE
-
Inquire about the value of
_POSIX_VDISABLE.
_PC_SYNC_IO
-
Inquire about the value of
_POSIX_SYNC_IO.
_PC_ASYNC_IO
-
Inquire about the value of
_POSIX_ASYNC_IO.
_PC_PRIO_IO
-
Inquire about the value of
_POSIX_PRIO_IO.
_PC_SOCK_MAXBUF
-
Inquire about the value of
_POSIX_PIPE_BUF.
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.
The GNU C library 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
-
The largest value of
obase that the bc utility is
guaranteed to support.
- Macro: int BC_DIM_MAX
-
The largest number of elements in one array that the
bc utility
is guaranteed to support.
- Macro: int BC_SCALE_MAX
-
The largest value of
scale that the bc utility is
guaranteed to support.
- Macro: int BC_STRING_MAX
-
The largest number of characters in one string constant that the
bc utility is guaranteed to support.
- Macro: int COLL_WEIGHTS_MAX
-
The largest number of weights that can necessarily be used in defining
the collating sequence for a locale.
- Macro: int EXPR_NEST_MAX
-
The maximum number of expressions that can be nested within parenthesis
by the
expr utility.
- Macro: int LINE_MAX
-
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
-
The maximum number of weights that can be assigned to an entry of the
LC_COLLATE category `order' keyword in a locale definition.
The GNU C library does not presently support locale definitions.
_POSIX2_BC_BASE_MAX
-
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
-
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
-
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
-
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
-
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
-
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
-
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
-
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. The GNU C library does not presently support locale
definitions.
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)
-
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
-
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
-
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 section Feature Test Macros.
_CS_LFS_LDFLAGS
-
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 section Feature Test Macros.
_CS_LFS_LIBS
-
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 section Feature Test Macros.
_CS_LFS_LINTFLAGS
-
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 section Feature Test Macros.
_CS_LFS64_CFLAGS
-
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 section Feature Test Macros.
_CS_LFS64_LDFLAGS
-
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 section Feature Test Macros.
_CS_LFS64_LIBS
-
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 section Feature Test Macros.
_CS_LFS64_LINTFLAGS
-
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 section 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;
}
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