15. System limitations and options

Concept

Constant value during operation

Runtime constant value refers to a certain limit , If already in <limits.h> In the definition of , be Fixed for implementation . But it's time to The value is uncertain （ Because this value may depend on the available memory space ）, So in the <limits.h> Its definition will be ignored in .

This case can be used sysconf() Get at run time .

Value can be added at run time

The value that can be added at runtime refers to a certain limit , Relative to a specific implementation Its value is fixed , And all systems running this implementation At least this minimum value should be supported , However , This minimum value may be increased when a particular system is running , have access to sysconf() To get the actual value supported by the system .

Pathname variable value

The path name variable refers to the path name （ file 、 Catalog 、 Terminals, etc ） Related restrictions , Every restriction It may be a constant relative to a system implementation , It may also vary from file system to file system . In cases where restrictions may change due to pathnames , Applications can use pathconf()、fpathconf() To get the value .

stay shell Get restrictions and options ：getconf

stay shell in , have access to getconf Command to get a specific UNIX Limitations and options already implemented in the system .
The format is generally as follows ：

$getconf ARG_MAX
2097152
$getconf NAME_MAX /boot
255

Get system limits at runtime

sysconf()、pathconf()、fpathconf() It is called by the application to check the limitations and options of the system implementation .

#include <unistd.h>
long sysconf(int name);
	Returns value of limit specified by name,
		or -1 if limit is indeterminate or an error occurred

• Parameters name Should be defined on <unistd.h> In the document _SC_ One of a series of constants , The limit value will be returned as the result of the day function .
• If a certain limit cannot be determined , be sysconf() return -1.
• If an error occurs during the call , And will return to -1（ The only specified error is EINVAL, Express name Invalid ）.

To distinguish the return -1 The situation of , You should set errno Set to 0, If the call returns -1, And take it with you errno Values are not for 0, be sysconf() An error occurred during the call .

The limit value will not change during the process .

Get file related restrictions at runtime

#include <unistd.h>
long pathconf(const char *pathname, int name);
long fpathconf(int fd, int name);
	Both returns value of limit specified by name,
		or -1 if limit is indeterminate or an error occured.

• pathconf() And fpahconf() The only difference is the way files or directories are specified , The former is the path name method , The latter is the file descriptor .
• Parameters name Is defined in <unistd.h> Medium _PC_ Series constants , The value of the limit will be returned as the result of the function call .

The limit value may change during the operation in the village , For example, when remounting the file system .

Uncertain limits

Sometimes system implementations do not define some system limitations as limiting constants （ such as PATH_MAX）, also sysconf() or pathconf() When the corresponding limit is returned, it will be classified as uncertain . Regarding this ,

1. Reference resources SUSv3 Specified minimum limit , Combined with practice .
2. Omit the restriction check , Judge according to the error result and try again / modify .
3. Write your own program or function to infer or estimate the limit value .