Advanced Programming in the UNIX® Environment

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Section 10.2 Signal Concepts 321

SIGTERM This is the termination signal sent by thekill( 1 )command by default.

Because it can be caught by applications, usingSIGTERMgives programs

achance to terminate gracefully by cleaning up beforeexiting (in

contrast toSIGKILL,which can’t be caught or ignored).

SIGTHAW This signal is defined only by Solaris and is used to notify processes that

need to take special action when the system resumes operation after

being suspended.

SIGTHR This is a signal reserved for use by the thread library on FreeBSD. It is

defined to have the same value asSIGLWP.

SIGTRAP This signal indicates an implementation-defined hardwarefault.

The signal name comes from the PDP-11 TRAP instruction. Implementations

often use this signal to transfer control to a debugger when a breakpoint

instruction is executed.

SIGTSTP This interactive stop signal is generated by the terminal driver when we

press the terminal suspend key (often Control-Z). This signal is sent to

all processes in the foreground process group (refer to Figure9.9).

Unfortunately,the termstophas different meanings. When discussing job

control and signals, we talk about stopping and continuing jobs. The terminal

driver,however,has historically used the termstopto refer to stopping and

starting the terminal output using the Control-Sand Control-Qcharacters.

Therefore, the terminal driver calls the character that generates the interactive

stop signal the suspend character,not the stop character.

SIGTTIN This signal is generated by the terminal driver when a process in a

background process group tries to read from its controlling terminal.

(Refer to the discussion of this topic in Section 9.8.) As special cases, if

either (a) the reading process is ignoring or blocking this signal or (b) the

process group of the reading process is orphaned, then the signal is not

generated; instead, the read operation fails witherrnoset toEIO.

SIGTTOU This signal is generated by the terminal driver when a process in a

background process group tries to write to its controlling terminal. (This

is discussed in Section 9.8.) Unlike the case with background reads, a

process can choose to allow background writes to the controlling

terminal. Wedescribe how to modify this option in Chapter 18.

If background writes arenot allowed, then like the SIGTTINsignal,

thereare two special cases: if either (a) the writing process is ignoring or

blocking this signal or (b) the process group of the writing process is

orphaned, then the signal is not generated; instead, the write operation

returns an error witherrnoset toEIO.

Regardless of whether background writes areallowed, certain terminal

operations (other than writing) can also generate theSIGTTOUsignal.