получить путь к текущему рабочему каталогу (get the pathname of the current working directory)
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)
getcwd — get the pathname of the current working directory
Синопсис (Synopsis)
#include <unistd.h>
char *getcwd(char *buf, size_t size);
Описание (Description)
The getcwd() function shall place an absolute pathname of the
current working directory in the array pointed to by buf, and
return buf. The pathname shall contain no components that are
dot or dot-dot, or are symbolic links.
If there are multiple pathnames that getcwd() could place in the
array pointed to by buf, one beginning with a single <slash>
character and one or more beginning with two <slash> characters,
then getcwd() shall place the pathname beginning with a single
<slash> character in the array. The pathname shall not contain
any unnecessary <slash> characters after the leading one or two
<slash> characters.
The size argument is the size in bytes of the character array
pointed to by the buf argument. If buf is a null pointer, the
behavior of getcwd() is unspecified.
Возвращаемое значение (Return value)
Upon successful completion, getcwd() shall return the buf
argument. Otherwise, getcwd() shall return a null pointer and set
errno to indicate the error. The contents of the array pointed to
by buf are then undefined.
Ошибки (Error)
The getcwd() function shall fail if:
EINVAL The size argument is 0.
ERANGE The size argument is greater than 0, but is smaller than
the length of the string +1.
The getcwd() function may fail if:
EACCES Search permission was denied for the current directory, or
read or search permission was denied for a directory above
the current directory in the file hierarchy.
ENOMEM Insufficient storage space is available.
The following sections are informative.
Примеры (Examples)
The following example uses {PATH_MAX} as the initial buffer size
(unless it is indeterminate or very large), and calls getcwd()
with progressively larger buffers until it does not give an
[ERANGE] error.
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
...
long path_max;
size_t size;
char *buf;
char *ptr;
path_max = pathconf(".", _PC_PATH_MAX);
if (path_max == -1)
size = 1024;
else if (path_max > 10240)
size = 10240;
else
size = path_max;
for (buf = ptr = NULL; ptr == NULL; size *= 2)
{
if ((buf = realloc(buf, size)) == NULL)
{
... handle error ...
}
ptr = getcwd(buf, size);
if (ptr == NULL && errno != ERANGE)
{
... handle error ...
}
}
...
free (buf);
Использование в приложениях (Application usage)
If the pathname obtained from getcwd() is longer than {PATH_MAX}
bytes, it could produce an [ENAMETOOLONG] error if passed to
chdir(). Therefore, in order to return to that directory it may
be necessary to break the pathname into sections shorter than
{PATH_MAX} bytes and call chdir() on each section in turn (the
first section being an absolute pathname and subsequent sections
being relative pathnames). A simpler way to handle saving and
restoring the working directory when it may be deeper than
{PATH_MAX} bytes in the file hierarchy is to use a file
descriptor and fchdir(), rather than getcwd() and chdir().
However, the two methods do have some differences. The fchdir()
approach causes the program to restore a working directory even
if it has been renamed in the meantime, whereas the chdir()
approach restores to a directory with the same name as the
original, even if the directories were renamed in the meantime.
Since the fchdir() approach does not access parent directories,
it can succeed when getcwd() would fail due to permissions
problems. In applications conforming to earlier versions of this
standard, it was not possible to use the fchdir() approach when
the working directory is searchable but not readable, as the only
way to open a directory was with O_RDONLY, whereas the getcwd()
approach can succeed in this case.
Обоснование (Rationale)
Having getcwd() take no arguments and instead use the malloc()
function to produce space for the returned argument was
considered. The advantage is that getcwd() knows how big the
working directory pathname is and can allocate an appropriate
amount of space. But the programmer would have to use the free()
function to free the resulting object, or each use of getcwd()
would further reduce the available memory. Finally, getcwd() is
taken from the SVID where it has the two arguments used in this
volume of POSIX.1‐2017.
The older function getwd() was rejected for use in this context
because it had only a buffer argument and no size argument, and
thus had no way to prevent overwriting the buffer, except to
depend on the programmer to provide a large enough buffer.
On some implementations, if buf is a null pointer, getcwd() may
obtain size bytes of memory using malloc(). In this case, the
pointer returned by getcwd() may be used as the argument in a
subsequent call to free(). Invoking getcwd() with buf as a null
pointer is not recommended in conforming applications.
Earlier implementations of getcwd() sometimes generated pathnames
like "../../../subdirname" internally, using them to explore the
path of ancestor directories back to the root. If one of these
internal pathnames exceeded {PATH_MAX} in length, the
implementation could fail with errno set to [ENAMETOOLONG]. This
is no longer allowed.
If a program is operating in a directory where some (grand)parent
directory does not permit reading, getcwd() may fail, as in most
implementations it must read the directory to determine the name
of the file. This can occur if search, but not read, permission
is granted in an intermediate directory, or if the program is
placed in that directory by some more privileged process (for
example, login). Including the [EACCES] error condition makes the
reporting of the error consistent and warns the application
developer that getcwd() can fail for reasons beyond the control
of the application developer or user. Some implementations can
avoid this occurrence (for example, by implementing getcwd()
using pwd, where pwd is a set-user-root process), thus the error
was made optional. Since this volume of POSIX.1‐2017 permits the
addition of other errors, this would be a common addition and yet
one that applications could not be expected to deal with without
this addition.
Будущие направления (Future directions)
None.
Смотри также (See also)
malloc(3p)
The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)
