SIGILL
From Wikipedia, the free encyclopedia
| Description: | Illegal instruction |
|---|---|
| Default action: | Abnormal termination of the process |
| SA_SIGINFO macros | |
ILL_ILLOPC |
illegal opcode |
ILL_ILLOPN |
illegal operand |
ILL_ADR |
illegal addressing mode |
ILL_ILLTRP |
illegal trap |
ILL_PRVOPC |
privileged opcode |
ILL_PRVREG |
privileged register |
ILL_COPROC |
coprocessor error |
ILL_BADSTK |
internal stack error |
SIGILL is the signal sent to computer programs that attempt to execute malformed, unknown, or privileged instructions on POSIX-compliant platforms. The symbolic constant for SIGILL is defined in the signal.h header file. Symbolic signal names are used because signal numbers can vary across platforms.
Contents |
[edit] Etymology
SIG is a common prefix for signal names; ILL is a contraction of illegal instruction.
[edit] Description
There are many possible reasons for receiving a SIGILL. A common mistake involves accidentally overwriting stack data with a return address that points to data not meant to be executed or trying to execute a function pointer that is not properly initialized (E.g. a Null pointer). Other problems might involve compiler (toolchain) bugs, filesystem corruption or attempting to execute instructions that require special privileges.
Many platforms implement new instructions or provide additional registers on subsequent hardware revisions, so applications compiled for more recent hardware may generate illegal instructions when run on older hardward that does not recognise the new opcodes. An example might be attempting to use MMX instructions on an Intel 80486 processor, which didn't support the feature.
SIGILL can also be generated by users with the appropriate permissions, using the kill system call.
SIGILL can be handled. That is, programmers can specify the action they would like to occur upon receiving a SIGILL, such as execute a subroutine, ignore the event, or restore the default behaviour.
Note that under certain circumstances, attempting to ignore SIGILL can result in undefined behaviour.
[edit] Example
Here is an example of an ANSI C program that attempts to execute an illegal instruction on platforms where 0xFFFFFFFF is not a valid opcode.
int main()
{
unsigned char insn[4] = { 0xff, 0xff, 0xff, 0xff };
void (*function)() = (void (*)()) insn;
function();
}
Compiling and running it on IA-32 with Linux produces the following:
$ gcc -o sigill sigill.c $ ./sigill Illegal instruction (core dumped)
A backtrace from gdb shows that the program crashed within the main function when the program tried to execute an instruction at address 0xBFFFEDE4:
Program received signal SIGILL, Illegal instruction. 0xbfffede4 in ?? () (gdb) bt #0 0xbfffede4 in ?? () #1 0x0804837f in main () (gdb) display /i $eip 1: x/i $eip 0xbfffede4: (bad)
Note "(bad)", indicating that the debugger does not recognize the opcode at that address. The mnemonic representing the instruction would normally be displayed there.
Compare the output from SIGILL with that of a segmentation fault and a SIGFPE signal.
[edit] See also
| POSIX Signals |
| SIGABRT | SIGALRM | SIGFPE | SIGHUP | SIGILL | SIGINT | SIGKILL | SIGPIPE | SIGQUIT | SIGSEGV | SIGTERM | SIGUSR1 | SIGUSR2 | SIGCHLD | SIGCONT | SIGSTOP | SIGTSTP | SIGTTIN | SIGTTOU | SIGBUS | SIGPOLL | SIGPROF | SIGSYS | SIGTRAP | SIGURG | SIGVTALRM | SIGXCPU | SIGXFSZ | Realtime Signals are user definable—SIGRTMIN+n through SIGRTMAX. |
| Common non-POSIX signals and synonyms |
| SIGIOT | SIGEMT | SIGSTKFLT | SIGIO | SIGCLD | SIGINFO | SIGPWR (SIGINFO) | SIGLOST | SIGWINCH | SIGUNUSED |
