unlink deletes a name from the filesystem. If that name was the last link to a file and no processes have the file open the file is deleted and the space it was using is made available for reuse.
If the name was the last link to a file but any processes still have the file open the file will remain in existence until the last file descriptor referring to it is closed.
If the name referred to a symbolic link the link is removed.
Write access to the directory containing pathname is not allowed for the process's effective uid, or one of the directories in pathname did not allow search (execute) permission.
EPERM or EACCES
The directory containing pathname has the sticky-bit (S_ISVTX) set and the process's effective uid is neither the uid of the file to be deleted nor that of the directory containing it.
EPERM (Linux only)
The filesystem does not allow unlinking of files.
The system does not allow unlinking of directories, or unlinking of directories requires privileges that the current process doesn't have. (This is the POSIX prescribed error return.)
pathname refers to a directory. (This is the non-POSIX value returned by Linux since 2.1.132.)
EBUSY (not on Linux)
The file pathname cannot be unlinked because it is being used by the system or another process and the implementation considers this an error.
pathname points outside your accessible address space.
pathname was too long.
A directory component in pathname does not exist or is a dangling symbolic link.
A component used as a directory in pathname is not, in fact, a directory.
Insufficient kernel memory was available.
pathname refers to a file on a read-only filesystem.
Too many symbolic links were encountered in translating pathname.