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sysconf: získat konfigurovatelné systémové proměnné

Originální popis anglicky: sysconf - get configurable system variables

Návod, kniha: POSIX Programmer's Manual


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


The sysconf() function provides a method for the application to determine the current value of a configurable system limit or option ( variable). The implementation shall support all of the variables listed in the following table and may support others.
The name argument represents the system variable to be queried. The following table lists the minimal set of system variables from <limits.h> or <unistd.h> that can be returned by sysconf(), and the symbolic constants defined in <unistd.h> that are the corresponding values used for name.
Variable Value of Name
Clock ticks/second _SC_CLK_TCK
Maximum size of getgrgid_r() and _SC_GETGR_R_SIZE_MAX
getgrnam_r() data buffers  
Maximum size of getpwuid_r() and _SC_GETPW_R_SIZE_MAX
getpwnam_r() data buffers  
_POSIX_V6_LP64_OFF64 _SC_V6_LP64_OFF64


If name is an invalid value, sysconf() shall return -1 and set errno to indicate the error. If the variable corresponding to name has no limit, sysconf() shall return -1 without changing the value of errno. Note that indefinite limits do not imply infinite limits; see <limits.h>.
Otherwise, sysconf() shall return the current variable value on the system. The value returned shall not be more restrictive than the corresponding value described to the application when it was compiled with the implementation's <limits.h> or <unistd.h>. The value shall not change during the lifetime of the calling process,  except that sysconf(_SC_OPEN_MAX) may return different values before and after a call to setrlimit() which changes the RLIMIT_NOFILE soft limit.


The sysconf() function shall fail if:
The value of the name argument is invalid.
The following sections are informative.




As -1 is a permissible return value in a successful situation, an application wishing to check for error situations should set errno to 0, then call sysconf(), and, if it returns -1, check to see if errno is non-zero.
If the value of sysconf(_SC_2_VERSION) is not equal to the value of the _POSIX2_VERSION symbolic constant, the utilities available via system() or popen() might not behave as described in the Shell and Utilities volume of IEEE Std 1003.1-2001. This would mean that the application is not running in an environment that conforms to the Shell and Utilities volume of IEEE Std 1003.1-2001. Some applications might be able to deal with this, others might not. However, the functions defined in this volume of IEEE Std 1003.1-2001 continue to operate as specified, even if sysconf(_SC_2_VERSION) reports that the utilities no longer perform as specified.


This functionality was added in response to requirements of application developers and of system vendors who deal with many international system configurations. It is closely related to pathconf() and fpathconf().
Although a conforming application can run on all systems by never demanding more resources than the minimum values published in this volume of IEEE Std 1003.1-2001, it is useful for that application to be able to use the actual value for the quantity of a resource available on any given system. To do this, the application makes use of the value of a symbolic constant in <limits.h> or <unistd.h>.
However, once compiled, the application must still be able to cope if the amount of resource available is increased. To that end, an application may need a means of determining the quantity of a resource, or the presence of an option, at execution time.
Two examples are offered:
Applications may wish to act differently on systems with or without job control. Applications vendors who wish to distribute only a single binary package to all instances of a computer architecture would be forced to assume job control is never available if it were to rely solely on the <unistd.h> value published in this volume of IEEE Std 1003.1-2001.
International applications vendors occasionally require knowledge of the number of clock ticks per second. Without these facilities, they would be required to either distribute their applications partially in source form or to have 50 Hz and 60 Hz versions for the various countries in which they operate.
It is the knowledge that many applications are actually distributed widely in executable form that leads to this facility. If limited to the most restrictive values in the headers, such applications would have to be prepared to accept the most limited environments offered by the smallest microcomputers. Although this is entirely portable, there was a consensus that they should be able to take advantage of the facilities offered by large systems, without the restrictions associated with source and object distributions.
During the discussions of this feature, it was pointed out that it is almost always possible for an application to discern what a value might be at runtime by suitably testing the various functions themselves. And, in any event, it could always be written to adequately deal with error returns from the various functions. In the end, it was felt that this imposed an unreasonable level of complication and sophistication on the application writer.
This runtime facility is not meant to provide ever-changing values that applications have to check multiple times. The values are seen as changing no more frequently than once per system initialization, such as by a system administrator or operator with an automatic configuration program. This volume of IEEE Std 1003.1-2001 specifies that they shall not change within the lifetime of the process.
Some values apply to the system overall and others vary at the file system or directory level. The latter are described in pathconf() .
Note that all values returned must be expressible as integers. String values were considered, but the additional flexibility of this approach was rejected due to its added complexity of implementation and use.
Some values, such as {PATH_MAX}, are sometimes so large that they must not be used to, say, allocate arrays. The sysconf() function returns a negative value to show that this symbolic constant is not even defined in this case.
Similar to pathconf(), this permits the implementation not to have a limit. When one resource is infinite, returning an error indicating that some other resource limit has been reached is conforming behavior.




confstr() , pathconf() , the Base Definitions volume of IEEE Std 1003.1-2001, <limits.h>, <unistd.h>, the Shell and Utilities volume of IEEE Std 1003.1-2001, getconf Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
2003 IEEE/The Open Group
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