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access, eaccess - check accessibility of a file

 

NAME

      access, eaccess - check accessibility of a file
 

LIBRARY

      Standard C Library (libc, -lc)
 

SYNOPSIS

      #include <unistd.h>
 
      int
      access(const char *path, int mode);
 
      int
      eaccess(const char *path, int mode);
 

DESCRIPTION

      The access() and eaccess() system calls check the accessibility of the
      file named by the path argument for the access permissions indicated by
      the mode argument.  The value of mode is either the bitwise-inclusive OR
      of the access permissions to be checked (R_OK for read permission, W_OK
      for write permission, and X_OK for execute/search permission), or the
      existence test (F_OK).
 
      For additional information, see the File Access Permission section of
      intro(2).
 
      The eaccess() system call uses the effective user ID and the group access
      list to authorize the request; the access() system call uses the real
      user ID in place of the effective user ID, the real group ID in place of
      the effective group ID, and the rest of the group access list.
 
      Even if a process’s real or effective user has appropriate privileges and
      indicates success for X_OK, the file may not actually have execute per‐
      mission bits set.  Likewise for R_OK and W_OK.
      Upon successful completion, the value 0 is returned; otherwise the
      value -1 is returned and the global variable errno is set to indicate the
      error.
 

ERRORS

      Access to the file is denied if:
 
      [ENOTDIR]          A component of the path prefix is not a directory.
 
      [ENAMETOOLONG]     A component of a pathname exceeded 255 characters, or
                         an entire path name exceeded 1023 characters.
 
      [ENOENT]           The named file does not exist.
 
      [ELOOP]            Too many symbolic links were encountered in translat‐
                         ing the pathname.
 
      [EROFS]            Write access is requested for a file on a read-only
                         file system.
 
      [ETXTBSY]          Write access is requested for a pure procedure (shared
                         text) file presently being executed.
 
      [EACCES]           Permission bits of the file mode do not permit the
                         requested access, or search permission is denied on a
                         component of the path prefix.
 
      [EFAULT]           The path argument points outside the process’s allo‐
                         cated address space.
 
      [EIO]              An I/O error occurred while reading from or writing to
                         the file system.
      The access() system call is a potential security hole due to race condi‐
      tions and should never be used.  Set-user-ID and set-group-ID applica‐
      tions should restore the effective user or group ID, and perform actions
      directly rather than use access() to simulate access checks for the real
      user or group ID.  The eaccess() system call likewise may be subject to
      races if used inappropriately.
 
      access() remains useful for providing clues to users as to whether opera‐
      tions make sense for particular filesystem objects (e.g. ’delete’ menu
      item only highlighted in a writable folder ... avoiding interpretation of
      the st_mode bits that the application might not understand -- e.g. in the
      case of AFS).  It also allows a cheaper file existence test than stat(2).
      chmod(2), intro(2), stat(2)
 

STANDARDS

      The access() system call is expected to conform to ISO/IEC 9945-1:1990
      (“POSIX.1”).
 

HISTORY

      The access() function appeared in Version 7 AT&T UNIX.
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.