mirror of https://github.com/mkerrisk/man-pages
324 lines
8.5 KiB
Groff
324 lines
8.5 KiB
Groff
.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" Modified Wed Jul 21 22:35:42 1993 by Rik Faith (faith@cs.unc.edu)
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.\" Modified 18 Mar 1996 by Martin Schulze (joey@infodrom.north.de):
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.\" Corrected description of getwd().
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.\" Modified Sat Aug 21 12:32:12 MET 1999 by aeb - applied fix by aj
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.\" Modified Mon Dec 11 13:32:51 MET 2000 by aeb
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.\" Modified Thu Apr 22 03:49:15 CEST 2002 by Roger Luethi <rl@hellgate.ch>
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.\"
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.TH GETCWD 3 2018-04-30 "GNU" "Linux Programmer's Manual"
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.SH NAME
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getcwd, getwd, get_current_dir_name \- get current working directory
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.SH SYNOPSIS
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.nf
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.B #include <unistd.h>
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.PP
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.BI "char *getcwd(char *" buf ", size_t " size );
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.PP
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.BI "char *getwd(char *" buf );
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.PP
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.B "char *get_current_dir_name(void);"
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.fi
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.PP
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.in -4n
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.in
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.PP
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.BR get_current_dir_name ():
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.RS
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_GNU_SOURCE
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.RE
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.PP
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.BR getwd ():
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.ad l
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.RS 4
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.PD 0
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.TP 4
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Since glibc 2.12:
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.nf
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(_XOPEN_SOURCE\ >=\ 500) && ! (_POSIX_C_SOURCE\ >=\ 200809L)
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|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
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|| /* Glibc versions <= 2.19: */ _BSD_SOURCE
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.TP 4
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.fi
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Before glibc 2.12:
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_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
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.\" || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
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.PD
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.RE
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.ad b
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.SH DESCRIPTION
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These functions return a null-terminated string containing an
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absolute pathname that is the current working directory of
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the calling process.
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The pathname is returned as the function result and via the argument
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.IR buf ,
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if present.
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.PP
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The
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.BR getcwd ()
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function copies an absolute pathname of the current working directory
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to the array pointed to by
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.IR buf ,
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which is of length
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.IR size .
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.PP
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If the length of the absolute pathname of the current working directory,
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including the terminating null byte, exceeds
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.I size
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bytes, NULL is returned, and
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.I errno
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is set to
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.BR ERANGE ;
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an application should check for this error, and allocate a larger
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buffer if necessary.
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.PP
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As an extension to the POSIX.1-2001 standard, glibc's
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.BR getcwd ()
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allocates the buffer dynamically using
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.BR malloc (3)
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if
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.I buf
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is NULL.
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In this case, the allocated buffer has the length
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.I size
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unless
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.I size
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is zero, when
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.I buf
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is allocated as big as necessary.
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The caller should
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.BR free (3)
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the returned buffer.
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.PP
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.BR get_current_dir_name ()
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will
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.BR malloc (3)
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an array big enough to hold the absolute pathname of
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the current working directory.
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If the environment
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variable
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.B PWD
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is set, and its value is correct, then that value will be returned.
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The caller should
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.BR free (3)
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the returned buffer.
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.PP
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.BR getwd ()
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does not
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.BR malloc (3)
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any memory.
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The
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.I buf
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argument should be a pointer to an array at least
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.B PATH_MAX
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bytes long.
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If the length of the absolute pathname of the current working directory,
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including the terminating null byte, exceeds
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.B PATH_MAX
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bytes, NULL is returned, and
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.I errno
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is set to
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.BR ENAMETOOLONG .
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(Note that on some systems,
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.B PATH_MAX
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may not be a compile-time constant;
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furthermore, its value may depend on the filesystem, see
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.BR pathconf (3).)
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For portability and security reasons, use of
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.BR getwd ()
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is deprecated.
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.SH RETURN VALUE
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On success, these functions return a pointer to a string containing
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the pathname of the current working directory.
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In the case of
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.BR getcwd ()
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and
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.BR getwd ()
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this is the same value as
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.IR buf .
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.PP
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On failure, these functions return NULL, and
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.I errno
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is set to indicate the error.
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The contents of the array pointed to by
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.I buf
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are undefined on error.
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.SH ERRORS
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.TP
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.B EACCES
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Permission to read or search a component of the filename was denied.
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.TP
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.B EFAULT
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.I buf
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points to a bad address.
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.TP
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.B EINVAL
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The
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.I size
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argument is zero and
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.I buf
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is not a null pointer.
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.TP
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.B EINVAL
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.BR getwd ():
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.I buf
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is NULL.
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.TP
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.B ENAMETOOLONG
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.BR getwd ():
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The size of the null-terminated absolute pathname string exceeds
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.B PATH_MAX
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bytes.
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.TP
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.B ENOENT
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The current working directory has been unlinked.
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.TP
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.B ENOMEM
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Out of memory.
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.TP
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.B ERANGE
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The
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.I size
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argument is less than the length of the absolute pathname of the
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working directory, including the terminating null byte.
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You need to allocate a bigger array and try again.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lbw22 lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR getcwd (),
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.BR getwd ()
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T} Thread safety MT-Safe
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T{
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.BR get_current_dir_name ()
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T} Thread safety MT-Safe env
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.TE
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.SH CONFORMING TO
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.BR getcwd ()
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conforms to POSIX.1-2001.
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Note however that POSIX.1-2001 leaves the behavior of
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.BR getcwd ()
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unspecified if
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.I buf
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is NULL.
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.PP
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.BR getwd ()
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is present in POSIX.1-2001, but marked LEGACY.
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POSIX.1-2008 removes the specification of
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.BR getwd ().
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Use
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.BR getcwd ()
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instead.
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POSIX.1-2001
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does not define any errors for
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.BR getwd ().
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.PP
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.BR get_current_dir_name ()
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is a GNU extension.
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.SH NOTES
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Under Linux, these functions make use of the
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.BR getcwd ()
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system call (available since Linux 2.1.92).
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On older systems they would query
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.IR /proc/self/cwd .
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If both system call and proc filesystem are missing, a
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generic implementation is called.
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Only in that case can
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these calls fail under Linux with
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.BR EACCES .
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.PP
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These functions are often used to save the location of the current working
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directory for the purpose of returning to it later.
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Opening the current
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directory (".") and calling
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.BR fchdir (2)
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to return is usually a faster and more reliable alternative when sufficiently
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many file descriptors are available, especially on platforms other than Linux.
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.\"
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.SS C library/kernel differences
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On Linux, the kernel provides a
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.BR getcwd ()
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system call, which the functions described in this page will use if possible.
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The system call takes the same arguments as the library function
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of the same name, but is limited to returning at most
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.BR PATH_MAX
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bytes.
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(Before Linux 3.12,
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.\" commit 3272c544da48f8915a0e34189182aed029bd0f2b
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the limit on the size of the returned pathname was the system page size.
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On many architectures,
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.BR PATH_MAX
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and the system page size are both 4096 bytes,
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but a few architectures have a larger page size.)
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If the length of the pathname of the current working directory
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exceeds this limit, then the system call fails with the error
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.BR ENAMETOOLONG .
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In this case, the library functions fall back to
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a (slower) alternative implementation that returns the full pathname.
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.PP
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Following a change in Linux 2.6.36,
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.\" commit 8df9d1a4142311c084ffeeacb67cd34d190eff74
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the pathname returned by the
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.BR getcwd ()
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system call will be prefixed with the string "(unreachable)"
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if the current directory is not below the root directory of the current
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process (e.g., because the process set a new filesystem root using
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.BR chroot (2)
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without changing its current directory into the new root).
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Such behavior can also be caused by an unprivileged user by changing
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the current directory into another mount namespace.
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When dealing with pathname from untrusted sources, callers of the
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functions described in this page
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should consider checking whether the returned pathname starts
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with '/' or '(' to avoid misinterpreting an unreachable path
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as a relative pathname.
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.SH BUGS
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Since the Linux 2.6.36 change that added "(unreachable)" in the
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circumstances described above, the glibc implementation of
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.BR getcwd ()
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has failed to conform to POSIX and returned a relative pathname when the API
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contract requires an absolute pathname.
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With glibc 2.27 onwards this is corrected;
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calling
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.BR getcwd ()
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from such a pathname will now result in failure with
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.BR ENOENT .
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.SH SEE ALSO
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.BR pwd (1),
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.BR chdir (2),
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.BR fchdir (2),
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.BR open (2),
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.BR unlink (2),
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.BR free (3),
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.BR malloc (3)
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