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SYSCONF(3)		   Linux Programmer's Manual		    SYSCONF(3)


sysconf - Get configuration information at runtime


#include <unistd.h> long sysconf(int name);


POSIX allows an application to test at compile- or run-time whether certain options are supported, or what the value is of certain config- urable constants or limits. At compile time this is done by including <unistd.h> and/or <limits.h> and testing the value of certain macros. At run time, one can ask for numerical values using the present func- tion sysconf(). On can ask for numerical values that may depend on the filesystem a file is in using the calls fpathconf(3) and pathconf(3). One can ask for string values using confstr(3). The values obtained from these functions are system configuration con- stants. They do not change during the lifetime of a process. For options, typically, there is a constant _POSIX_FOO that may be defined in <unistd.h>. If it is undefined, one should ask at run-time. If it is defined to -1, then the option is not supported. If it is defined to 0, then relevant functions and headers exist, but one has to ask at runtime what degree of support is available. If it is defined to a value other than -1 or 0, then the option is supported. Usually the value (such as 200112L) indicates the year and month of the POSIX revision describing the option. Glibc uses the value 1 to indicate sup- port as long as the POSIX revision has not been published yet. The sysconf() argument will be _SC_FOO. For a list of options, see posixoptions(7). For variables or limits, typically, there is a constant _FOO, maybe defined in <limits.h>, or _POSIX_FOO, maybe defined in <unistd.h>. The constant will not be defined if the limit is unspecified. If the con- stant is defined, it gives a guaranteed value, and more might actually be supported. If an application wants to take advantage of values which may change between systems, a call to sysconf() can be made. The sysconf() argument will be _SC_FOO. POSIX.1 VARIABLES We give the name of the variable, the name of the sysconf() parameter used to inquire about its value, and a short description. First, the POSIX.1 compatible values. ARG_MAX - _SC_ARG_MAX The maximum length of the arguments to the exec() family of functions. Must not be less than _POSIX_ARG_MAX (4096). CHILD_MAX - _SC_CHILD_MAX The max number of simultaneous processes per user ID. Must not be less than _POSIX_CHILD_MAX (25). HOST_NAME_MAX - _SC_HOST_NAME_MAX Max length of a hostname, not including the terminating null byte, as returned by gethostname(2). Must not be less than _POSIX_HOST_NAME_MAX (255). LOGIN_NAME_MAX - _SC_LOGIN_NAME_MAX Maximum length of a login name, including the terminating null byte. Must not be less than _POSIX_LOGIN_NAME_MAX (9). clock ticks - _SC_CLK_TCK The number of clock ticks per second. The corresponding vari- able is obsolete. It was of course called CLK_TCK. (Note: the macro CLOCKS_PER_SEC does not give information: it must equal 1000000.) OPEN_MAX - _SC_OPEN_MAX The maximum number of files that a process can have open at any time. Must not be less than _POSIX_OPEN_MAX (20). PAGESIZE - _SC_PAGESIZE Size of a page in bytes. Must not be less than 1. (Some systems use PAGE_SIZE instead.) RE_DUP_MAX - _SC_RE_DUP_MAX The number of repeated occurrences of a BRE permitted by regexec(3) and regcomp(3). Must not be less than _POSIX2_RE_DUP_MAX (255). STREAM_MAX - _SC_STREAM_MAX The maximum number of streams that a process can have open at any time. If defined, it has the same value as the standard C macro FOPEN_MAX. Must not be less than _POSIX_STREAM_MAX (8). SYMLOOP_MAX The maximum number of symbolic links seen in a pathname before resolution returns ELOOP. Must not be less than _POSIX_SYM- LOOP_MAX (8). TTY_NAME_MAX - _SC_TTY_NAME_MAX The maximum length of terminal device name, including the termi- nating null byte. Must not be less than _POSIX_TTY_NAME_MAX (9). TZNAME_MAX - _SC_TZNAME_MAX The maximum number of bytes in a timezone name. Must not be less than _POSIX_TZNAME_MAX (6). _POSIX_VERSION - _SC_VERSION indicates the year and month the POSIX.1 standard was approved in the format YYYYMML;the value 199009L indicates the Sept. 1990 revision. POSIX.2 VARIABLES Next, the POSIX.2 values, giving limits for utilities. BC_BASE_MAX - _SC_BC_BASE_MAX indicates the maximum obase value accepted by the bc(1) utility. BC_DIM_MAX - _SC_BC_DIM_MAX indicates the maximum value of elements permitted in an array by bc(1). BC_SCALE_MAX - _SC_BC_SCALE_MAX indicates the maximum scale value allowed by bc(1). BC_STRING_MAX - _SC_BC_STRING_MAX indicates the maximum length of a string accepted by bc(1). COLL_WEIGHTS_MAX - _SC_COLL_WEIGHTS_MAX indicates the maximum numbers of weights that can be assigned to an entry of the LC_COLLATE order keyword in the locale definition file, EXPR_NEST_MAX - _SC_EXPR_NEST_MAX is the maximum number of expressions which can be nested within parentheses by expr(1). LINE_MAX - _SC_LINE_MAX The maximum length of a utility's input line length, either from standard input or from a file. This includes length for a trail- ing newline. RE_DUP_MAX - _SC_RE_DUP_MAX The maximum number of repeated occurrences of a regular expres- sion when the interval notation \{m,n\} is used. POSIX2_VERSION - _SC_2_VERSION indicates the version of the POSIX.2 standard in the format of YYYYMML. POSIX2_C_DEV - _SC_2_C_DEV indicates whether the POSIX.2 C language development facilities are supported. POSIX2_FORT_DEV - _SC_2_FORT_DEV indicates whether the POSIX.2 FORTRAN development utilities are supported. POSIX2_FORT_RUN - _SC_2_FORT_RUN indicates whether the POSIX.2 FORTRAN runtime utilities are sup- ported. _POSIX2_LOCALEDEF - _SC_2_LOCALEDEF indicates whether the POSIX.2 creation of locates via localedef(1) is supported. POSIX2_SW_DEV - _SC_2_SW_DEV indicates whether the POSIX.2 software development utilities option is supported. These values also exist, but may not be standard. - _SC_PHYS_PAGES The number of pages of physical memory. Note that it is possi- ble for the product of this value and the value of _SC_PAGE_SIZE to overflow. - _SC_AVPHYS_PAGES The number of currently available pages of physical memory.


If name is invalid, -1 is returned, and errno is set to EINVAL. Other- wise, the value returned is the value of the system resource and errno is not changed. In the case of options, a positive value is returned if a queried option is available, and -1 if it is not. In the case of limits, -1 means that there is no definite limit.




It is difficult to use ARG_MAX because it is not specified how much of the argument space for exec() is consumed by the user's environment variables. Some returned values may be huge; they are not suitable for allocating memory.


bc(1), expr(1), locale(1), fpathconf(3), pathconf(3), posixoptions(7) GNU 1993-04-18 SYSCONF(3)

1994 Man-cgi 1.15, Panagiotis Christias <>