получить настраиваемые переменные (get configurable variables)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
confstr — get configurable variables
Синопсис (Synopsis)
#include <unistd.h>
size_t confstr(int name, char *buf, size_t len);
Описание (Description)
The confstr() function shall return configuration-defined string
values. Its use and purpose are similar to sysconf(), but it is
used where string values rather than numeric values are returned.
The name argument represents the system variable to be queried.
The implementation shall support the following name values,
defined in <unistd.h>. It may support others:
_CS_PATH
_CS_POSIX_V7_ILP32_OFF32_CFLAGS
_CS_POSIX_V7_ILP32_OFF32_LDFLAGS
_CS_POSIX_V7_ILP32_OFF32_LIBS
_CS_POSIX_V7_ILP32_OFFBIG_CFLAGS
_CS_POSIX_V7_ILP32_OFFBIG_LDFLAGS
_CS_POSIX_V7_ILP32_OFFBIG_LIBS
_CS_POSIX_V7_LP64_OFF64_CFLAGS
_CS_POSIX_V7_LP64_OFF64_LDFLAGS
_CS_POSIX_V7_LP64_OFF64_LIBS
_CS_POSIX_V7_LPBIG_OFFBIG_CFLAGS
_CS_POSIX_V7_LPBIG_OFFBIG_LDFLAGS
_CS_POSIX_V7_LPBIG_OFFBIG_LIBS
_CS_POSIX_V7_THREADS_CFLAGS
_CS_POSIX_V7_THREADS_LDFLAGS
_CS_POSIX_V7_WIDTH_RESTRICTED_ENVS
_CS_V7_ENV
_CS_POSIX_V6_ILP32_OFF32_CFLAGS
_CS_POSIX_V6_ILP32_OFF32_LDFLAGS
_CS_POSIX_V6_ILP32_OFF32_LIBS
_CS_POSIX_V6_ILP32_OFFBIG_CFLAGS
_CS_POSIX_V6_ILP32_OFFBIG_LDFLAGS
_CS_POSIX_V6_ILP32_OFFBIG_LIBS
_CS_POSIX_V6_LP64_OFF64_CFLAGS
_CS_POSIX_V6_LP64_OFF64_LDFLAGS
_CS_POSIX_V6_LP64_OFF64_LIBS
_CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS
_CS_POSIX_V6_LPBIG_OFFBIG_LDFLAGS
_CS_POSIX_V6_LPBIG_OFFBIG_LIBS
_CS_POSIX_V6_WIDTH_RESTRICTED_ENVS
_CS_V6_ENV
If len is not 0, and if name has a configuration-defined value,
confstr() shall copy that value into the len-byte buffer pointed
to by buf. If the string to be returned is longer than len
bytes, including the terminating null, then confstr() shall
truncate the string to len-1 bytes and null-terminate the result.
The application can detect that the string was truncated by
comparing the value returned by confstr() with len.
If len is 0 and buf is a null pointer, then confstr() shall still
return the integer value as defined below, but shall not return a
string. If len is 0 but buf is not a null pointer, the result is
unspecified.
After a call to:
confstr(_CS_V7_ENV, buf, sizeof(buf))
the string stored in buf shall contain a <space>-separated list
of the variable=value environment variable pairs an
implementation requires as part of specifying a conforming
environment, as described in the implementations' conformance
documentation.
If the implementation supports the POSIX shell option, the string
stored in buf after a call to:
confstr(_CS_PATH, buf, sizeof(buf))
can be used as a value of the PATH environment variable that
accesses all of the standard utilities of POSIX.1‐2008, that are
provided in a manner accessible via the exec family of functions,
if the return value is less than or equal to sizeof(buf).
Возвращаемое значение (Return value)
If name has a configuration-defined value, confstr() shall return
the size of buffer that would be needed to hold the entire
configuration-defined value including the terminating null. If
this return value is greater than len, the string returned in buf
is truncated.
If name is invalid, confstr() shall return 0 and set errno to
indicate the error.
If name does not have a configuration-defined value, confstr()
shall return 0 and leave errno unchanged.
Ошибки (Error)
The confstr() function shall fail if:
EINVAL The value of the name argument is invalid.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
An application can distinguish between an invalid name parameter
value and one that corresponds to a configurable variable that
has no configuration-defined value by checking if errno is
modified. This mirrors the behavior of sysconf().
The original need for this function was to provide a way of
finding the configuration-defined default value for the
environment variable PATH. Since PATH can be modified by the
user to include directories that could contain utilities
replacing the standard utilities in the Shell and Utilities
volume of POSIX.1‐2017, applications need a way to determine the
system-supplied PATH environment variable value that contains the
correct search path for the standard utilities.
An application could use:
confstr(name, (char *)NULL, (size_t)0)
to find out how big a buffer is needed for the string value; use
malloc() to allocate a buffer to hold the string; and call
confstr() again to get the string. Alternately, it could allocate
a fixed, static buffer that is big enough to hold most answers
(perhaps 512 or 1024 bytes), but then use malloc() to allocate a
larger buffer if it finds that this is too small.
Обоснование (Rationale)
Application developers can normally determine any configuration
variable by means of reading from the stream opened by a call to:
popen("command -p getconf variable", "r");
The confstr() function with a name argument of _CS_PATH returns a
string that can be used as a PATH environment variable setting
that will reference the standard shell and utilities as described
in the Shell and Utilities volume of POSIX.1‐2017.
The confstr() function copies the returned string into a buffer
supplied by the application instead of returning a pointer to a
string. This allows a cleaner function in some implementations
(such as those with lightweight threads) and resolves questions
about when the application must copy the string returned.
Будущие направления (Future directions)
None.
Смотри также (See also)
exec(1p), fpathconf(3p), sysconf(3p)
The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)
The Shell and Utilities volume of POSIX.1‐2017, c99(1p)
