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 processs real or effective user has appropriate privileges and indicates success for X_OK, the file may not actually have execute permission bits set. Likewise for R_OK and W_OK.
Access to the file is denied if: