The errors defined in <errno.h> can be divided into two categories: fatal and nonfatal.
[1]A fatal error has no recovery action. The best we can do is print an error message on the user ’s screen or to a log file, and then exit.
Nonfatal errors, on the other hand, can sometimes be dealt with more robustly.
Nonfatal Errors
    |___resource shortage(EAGAIN, ENFILE, ENOBUFS, ENOLCK,ENOSPC,EWOULDBLOCK)
[2]The typical recovery action for a resource-related nonfatal error is to delay and retry later.

[key] We call the user whose user ID is 0 either root or the superuser.

[FAQ]
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Q: what is the function of /etc/passwd and /etc/group
A: Map. /etc/group maps group names into numeric group IDs./etc/passwd maps user names into numeric user IDs.
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Q: why  should we need user/grounp id?
A: the file system stores both the user ID and the group ID of a
file’s owner. Storing both of these values requires only four bytes, assuming that each is stored as a two-byte integer. If the full ASCII login name and group name were used instead, additional disk space would be required. In addition, comparing strings during permission checks is more expensive than comparing integers.
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Q: whereas why does we just use the user id to instead user id+user name?
A: Users, however, work better with names than with numbers, so the password file maintains the mapping between login names and user IDs, and the group file provides the mapping between group names and group IDs.

Many conditions generate signals. 
[HardWare]
    |___Two terminal keys
    |        |_____interrupt key(DELETE key or Control-C)
    |        |_____quit key(Control-backslash)

[SoftWare]
    |___calling the kill function
Naturally, there are [limitations]: we have to be the owner of the other
process (or the superuser) to be able to send it a signal.

compile-time limits can be defined in header that any program can include at compile time. but runtime limits require the process to call a function to obtain the limit''s value.

1. compile-time limits(headers)
2. runtime limits not associated with a file or directory(the sysconf function)
3. runtime limits that are associated with a  file or directory(the pathconf and fpathconf function)

TOCTTOU
errors in the file system namespace generally deal with attempts to subvert file system
permissions by tricking a privileged program into either reducing permissions on a
privileged file or modifying a privileged file to open up a security hole.