Signal handling strategy

4 Signal modes

The shell switches between 4 signal configurations depending on context:

Mode	SIGINT	SIGQUIT	Active while…	Setup
interactive	interactive_sigint_handler	SIG_IGN	waiting on readline()	setup_interactive_signals()
exec	SIG_IGN	SIG_IGN	parent waiting for child	setup_execution_signals()
child	SIG_DFL	SIG_DFL	after fork(), before execve()	reset_child_signals()
heredoc	heredoc_sigint_handler	SIG_IGN	heredoc readline() loop	setup_heredoc_signals()

Note: interactive uses SA_RESTART but heredoc does not.

The global variable

⠀
volatile sig_atomic_t g_signal = 0;
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• Set inside signal handlers
• Checked in main_loop() after readline() returns
  If g_signal == SIGINT set exit_status = 130 and clear
• Checked in read_heredoc_lines() after each readline() call
  If g_signal == SIGINT return immediately (interrupted)

Signal flow for CTRL C

Idle prompt:

• interactive_sigint_handler() sets g_signal, prints newline and calls rl_replace_line("") + rl_redisplay().
• SA_RESTART ensures readline() retries transparently.
• After readline returns the next input, main_loop sees that g_signal == SIGINT and sets exit_status = 130.

During external command:

• Parent ignores SIGINT (SIG_IGN). Child has SIG_DFL and gets killed.
• Parent's wait_for_child() sees WIFSIGNALED and returns 128 + WTERMSIG(status)

DUring heredoc:

• heredoc_sigint_handler() closes STDIN_FILENO in order to force readline() to return NULL.
• No SA_RESTART so readline does not retry.
• g_signal == SIGINT detected -> read_heredoc_lines() returns 1 (interrupted).
• handle_heredoc() closes pipe fds & calls restore_stdin() to reopen /dev/tty on fd 0.