# Custom signal processing

The D3 libraries make extensive use of signal processing.
Because of this, the practice of reprogramming signals within a C
application using D3 C routines is strongly discouraged. However,
if it is absolutely necessary, there are several rules which must
be followed.

When calling _CP_logon, all signals
are rerouted to D3 signal handlers. When calling _CP_logoff, all signals are rerouted to UNIX defaults. Note that they are not
necessarily reset to the values they held before the _CP_logon call.

The signal SIGUSR2 must *never* be reprogrammed
as it is used by D3 for logging off and sending messages. Because
interrupts may occur within critical code, the user may not make calls
to _CP_ functions within signal handlers.

## Example(s)

```
/* Prints the name of all users in the users file. */
#include "CPuser.h"
main()
{
CPSTR * machine = _CP_mkstrl("pick0",5); /* default */
CPSTR * user = _CP_mkstrl("dm",2);
CPSTR * md = _CP_mkstrl("dm",2);
CPSTR * filename = _CP_mkstrl("users",5);
int fd = -1;
int sl = -1; /* initialize select list */
CPSTR * item_id = _CP_str_null;
/* Logon onto "pick0" as "dm" in the "dm" md */
if (_CP_logon(machine, user, _CP_str_null,
md, _CP_str_null, -1, 0) < 0)
{
printf("Logon error %d\n", _CP_errno);
exit(1);
}
/* done with machine, user, and md */
_CP_str_free(machine);
_CP_str_free(user);
_CP_str_free(md);
/* Make the break key terminate the program */
_CP_unix_break();
/* Open the users file into the file descriptor fd */
if (_CP_open(&fd, _CP_str_null, filename) < 0)
{
_CP_logoff();
printf("Could not open users file \n");
exit(2);
}
/* done with filename */
_CP_str_free(filename);
/* Do a raw select of "users" and store that select
list in the select list descriptor sl */
_CP_select(fd, &sl, 0);
/* Switch to Unix-type I/O so we can use printf */
_CP_unix_env();
/* repeatedly read item-IDs and print them as a list */
/* Note how item_id is repeatedly passed to _CP_readnext.
Each time, _CP_readnext will resize it if necessary,
and write the next item-ID into that CPSTR */
while (_CP_readnext(&item_id, &sl, 0, 0) >= 0)
{
_CP_TERM(item_id); /* terminate for printf */
printf("%s\n", _CP_SADDR(item_id));
}
/* done with item_id - Not absolutely necessary since */
/* it will be cleaned up when we exit */
_CP_str_free(item_id);
/* logoff and return */
_CP_logoff();
return 0;
}
```

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Source: https://d3codex.com/cfunctions/custom-signal-processing/ - part of the D3Codex reference.