mirror of https://github.com/mkerrisk/man-pages
160 lines
5.2 KiB
Plaintext
160 lines
5.2 KiB
Plaintext
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "ACCESS" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" access
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.SH NAME
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access \- determine accessibility of a file
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.SH SYNOPSIS
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.LP
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\fB#include <unistd.h>
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.br
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.sp
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int access(const char *\fP\fIpath\fP\fB, int\fP \fIamode\fP\fB);
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The \fIaccess\fP() function shall check the file named by the pathname
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pointed to by the \fIpath\fP argument for accessibility
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according to the bit pattern contained in \fIamode\fP, using the real
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user ID in place of the effective user ID and the real group
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ID in place of the effective group ID.
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.LP
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The value of \fIamode\fP is either the bitwise-inclusive OR of the
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access permissions to be checked (R_OK, W_OK, X_OK) or the
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existence test (F_OK).
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.LP
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If any access permissions are checked, each shall be checked individually,
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as described in the Base Definitions volume of
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IEEE\ Std\ 1003.1-2001, Chapter 3, Definitions. If the process has
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appropriate
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privileges, an implementation may indicate success for X_OK even if
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none of the execute file permission bits are set.
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.SH RETURN VALUE
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.LP
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If the requested access is permitted, \fIaccess\fP() succeeds and
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shall return 0; otherwise, -1 shall be returned and
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\fIerrno\fP shall be set to indicate the error.
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.SH ERRORS
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.LP
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The \fIaccess\fP() function shall fail if:
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.TP 7
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.B EACCES
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Permission bits of the file mode do not permit the requested access,
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or search permission is denied on a component of the path
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prefix.
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.TP 7
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.B ELOOP
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A loop exists in symbolic links encountered during resolution of the
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\fIpath\fP argument.
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.TP 7
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.B ENAMETOOLONG
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The length of the \fIpath\fP argument exceeds {PATH_MAX} or a pathname
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component is longer than {NAME_MAX}.
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.TP 7
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.B ENOENT
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A component of \fIpath\fP does not name an existing file or \fIpath\fP
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is an empty string.
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.TP 7
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.B ENOTDIR
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A component of the path prefix is not a directory.
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.TP 7
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.B EROFS
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Write access is requested for a file on a read-only file system.
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.sp
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.LP
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The \fIaccess\fP() function may fail if:
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.TP 7
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.B EINVAL
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The value of the \fIamode\fP argument is invalid.
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.TP 7
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.B ELOOP
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More than {SYMLOOP_MAX} symbolic links were encountered during resolution
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of the \fIpath\fP argument.
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.TP 7
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.B ENAMETOOLONG
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As a result of encountering a symbolic link in resolution of the \fIpath\fP
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argument, the length of the substituted pathname
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string exceeded {PATH_MAX}.
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.TP 7
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.B ETXTBSY
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Write access is requested for a pure procedure (shared text) file
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that is being executed.
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.sp
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.LP
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\fIThe following sections are informative.\fP
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.SH EXAMPLES
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.SS Testing for the Existence of a File
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.LP
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The following example tests whether a file named \fBmyfile\fP exists
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in the \fB/tmp\fP directory.
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.sp
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.RS
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.nf
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\fB#include <unistd.h>
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\&...
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int result;
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const char *filename = "/tmp/myfile";
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.sp
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result = access (filename, F_OK);
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\fP
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.fi
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.RE
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.SH APPLICATION USAGE
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.LP
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Additional values of \fIamode\fP other than the set defined in the
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description may be valid; for example, if a system has
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extended access controls.
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.SH RATIONALE
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.LP
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In early proposals, some inadequacies in the \fIaccess\fP() function
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led to the creation of an \fIeaccess\fP() function
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because:
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.IP " 1." 4
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Historical implementations of \fIaccess\fP() do not test file access
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correctly when the process' real user ID is superuser. In
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particular, they always return zero when testing execute permissions
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without regard to whether the file is executable.
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.LP
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.IP " 2." 4
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The superuser has complete access to all files on a system. As a consequence,
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programs started by the superuser and switched to
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the effective user ID with lesser privileges cannot use \fIaccess\fP()
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to test their file access permissions.
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.LP
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.LP
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However, the historical model of \fIeaccess\fP() does not resolve
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problem (1), so this volume of IEEE\ Std\ 1003.1-2001
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now allows \fIaccess\fP() to behave in the desired way because several
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implementations have corrected the problem. It was also
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argued that problem (2) is more easily solved by using \fIopen\fP(),
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\fIchdir\fP(), or one of the \fIexec\fP functions as
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appropriate and responding to the error, rather than creating a new
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function that would not be as reliable. Therefore,
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\fIeaccess\fP() is not included in this volume of IEEE\ Std\ 1003.1-2001.
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.LP
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The sentence concerning appropriate privileges and execute permission
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bits reflects the two possibilities implemented by
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historical implementations when checking superuser access for X_OK.
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.LP
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New implementations are discouraged from returning X_OK unless at
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least one execution permission bit is set.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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\fIchmod\fP() , \fIstat\fP() , the Base Definitions volume of
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IEEE\ Std\ 1003.1-2001, \fI<unistd.h>\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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