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PROLOG |NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |EXAMPLES |APPLICATION USAGE |RATIONALE |FUTURE DIRECTIONS |SEE ALSO |COPYRIGHT | |
READLINK(3P) POSIX Programmer's ManualREADLINK(3P)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.
readlink, readlinkat — read the contents of a symbolic link
#include <unistd.h> ssize_t readlink(const char *restrictpath, char *restrictbuf, size_tbufsize); #include <fcntl.h> ssize_t readlinkat(intfd, const char *restrictpath, char *restrictbuf, size_tbufsize);
Thereadlink() function shall place the contents of the symbolic link referred to bypath in the bufferbuf which has sizebufsize. If the number of bytes in the symbolic link is less thanbufsize, the contents of the remainder ofbuf are unspecified. If thebuf argument is not large enough to contain the link content, the firstbufsize bytes shall be placed inbuf. If the value ofbufsize is greater than {SSIZE_MAX}, the result is implementation-defined. Upon successful completion,readlink() shall mark for update the last data access timestamp of the symbolic link. Thereadlinkat() function shall be equivalent to thereadlink() function except in the case wherepath specifies a relative path. In this case the symbolic link whose content is read is relative to the directory associated with the file descriptorfd instead of the current working directory. If the access mode of the open file description associated with the file descriptor is not O_SEARCH, the function shall check whether directory searches are permitted using the current permissions of the directory underlying the file descriptor. If the access mode is O_SEARCH, the function shall not perform the check. Ifreadlinkat() is passed the special value AT_FDCWD in thefd parameter, the current working directory shall be used and the behavior shall be identical to a call toreadlink().Upon successful completion, these functions shall return the count of bytes placed in the buffer. Otherwise, these functions shall return a value of -1, leave the buffer unchanged, and seterrno to indicate the error.
These functions shall fail if:EACCESSearch permission is denied for a component of the path prefix ofpath.EINVALThepath argument names a file that is not a symbolic link.EIOAn I/O error occurred while reading from the file system.ELOOPA loop exists in symbolic links encountered during resolution of thepath argument.ENAMETOOLONG The length of a component of a pathname is longer than {NAME_MAX}.ENOENTA component ofpath does not name an existing file orpath is an empty string.ENOTDIR A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory, or thepath argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory. Thereadlinkat() function shall fail if:EACCESThe access mode of the open file description associated withfd is not O_SEARCH and the permissions of the directory underlyingfd do not permit directory searches.EBADFThepath argument does not specify an absolute path and thefd argument is neither AT_FDCWD nor a valid file descriptor open for reading or searching.ENOTDIR Thepath argument is not an absolute path andfd is a file descriptor associated with a non-directory file. These functions may fail if:ELOOPMore than {SYMLOOP_MAX} symbolic links were encountered during resolution of thepath argument.ENAMETOOLONG The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.The following sections are informative.Reading the Name of a Symbolic Link The following example shows how to read the name of a symbolic link named/modules/pass1. #include <unistd.h> char buf[1024]; ssize_t len; ... if ((len = readlink("/modules/pass1", buf, sizeof(buf)-1)) != -1) buf[len] = '\0';Conforming applications should not assume that the returned contents of the symbolic link are null-terminated.
The type associated withbufsiz is asize_tin order to be consistent with both the ISO C standard and the definition ofread(). The behavior specified forreadlink() whenbufsiz is zero represents historical practice. For this case, the standard developers considered a change wherebyreadlink() would return the number of non-null bytes contained in the symbolic link with the bufferbuf remaining unchanged; however, since thestatstructure memberst_size value can be used to determine the size of buffer necessary to contain the contents of the symbolic link as returned byreadlink(), this proposal was rejected, and the historical practice retained. The purpose of thereadlinkat() function is to read the content of symbolic links in directories other than the current working directory without exposure to race conditions. Any part of the path of a file could be changed in parallel to a call toreadlink(), resulting in unspecified behavior. By opening a file descriptor for the target directory and using thereadlinkat() function it can be guaranteed that the symbolic link read is located relative to the desired directory.
None.
fstatat(3p),symlink(3p) The Base Definitions volume of POSIX.1‐2017,fcntl.h(0p),unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online athttp://www.opengroup.org/unix/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group 2017READLINK(3P)Pages that refer to this page:unistd.h(0p), fstatat(3p), symlink(3p)
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