SIGILL is the signal sent to computer programs that attempt to execute malformed, unknown, or privileged instructions on POSIX-compliant platforms. In Computer science, an instruction is a single operation of a processor defined by an Instruction set architecture. POSIX (ˈpɒzɪks or "Portable Operating System Interface" is the collective name of a family of related standards specified by the IEEE to define The symbolic constant for SIGILL is defined in the signal. The C preprocessor ( cpp) is the Preprocessor for the C programming language. h header file. In Computer programming, particularly in the C and C++ programming languages a header file or include file is a file, usually in Symbolic signal names are used because signal numbers can vary across platforms.

Etymology

SIG is a common prefix for signal names; ILL is an abbreviation for illegal instruction. A prefix is a type of Affix attached to a stem which modifies the meaning of that stem For the HTML tag see HTML element. An abbreviation (from Latin brevis "short"

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. In Computer security and programming, a buffer overflow, or buffer overrun, is an anomalous condition where a process attempts to In postal Mail, a return address is an explicit inclusion of the address of the person sending the message Debt AIDS Trade in Africa (or DATA) is a Multinational non-government organization founded in January 2002 in London by U2 's A function pointer is a type of Pointer in C, C++, D, and other C-like programming languages Other problems might involve compiler (toolchain) bugs, filesystem corruption or attempting to execute instructions that require special privileges. A compiler is a Computer program (or set of programs that translates text written in a computer language (the source language) into another In Software, a toolchain is the set of Computer programs ( tools) that are used to create a product (typically another computer program or system of programs In Computing, a file system (often also written as filesystem) is a method for storing and organizing Computer files and the data they contain to make

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 hardware that does not recognise the new opcodes. An instruction set is a list of all the instructions and all their variations that a processor can execute In Computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage In computer technology an opcode ( op eration code) is the portion of a Machine language instruction that specifies the operation to be performed An example might be attempting to use MMX instructions on an Intel 80486 processor, which didn't support the feature. MMX is a single instruction multiple data (SIMD Instruction set designed by Intel, introduced in 1997 in their Pentium line of The Intel 486, otherwise known as the 80486 i486 or just 486 was the first tightly pipelined X86 design

SIGILL can also be generated by users with the appropriate permissions, using the kill system call. In Unix and Unix-like Operating systems kill is a command used to send simple messages to processes running on the system In Computing, a system call is the mechanism used by an application program to request service from the Kernel.

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. A programmer is someone who writes Computer software. The term computer programmer can refer to a specialist in one area of computer programming or to a generalist In Computer science, a subroutine ( function, method, procedure, or subprogram) is a portion of code within a larger

Note that under certain circumstances, attempting to ignore SIGILL can result in undefined behaviour. In Computer science, undefined behavior is a feature of some programming languages &mdash most famously C.

Sometimes bad call linkage will also cause SIGILL. In C++, passing a non-POD data type into a variadic function such as printf will cause undefined behavior; in GCC this means deliberately placing an illegal instruction at that point in the assembler. Plain Old Data Structures ( PODS) are data structures that are represented only as passive collections of field values without using Encapsulation or other Object-oriented The class of printf functions (which stands for " print f ormatted" is a class of functions, typically associated with Curly bracket programming

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. ANSI C is the standard published by the American National Standards Institute (ANSI for the C programming language.

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. IA-32 ( Intel Architecture 32-bit) often generically called X86 or x86-32, is the Instruction set architecture of Intel Linux (commonly pronounced ˈlɪnəks c
$ . /sigill
Illegal instruction (core dumped)

On more recent processors and Linux versions, the program above may not receive a SIGILL because of the NX bit feature, that allows the Linux kernel to make the memory pages on the program stack non-executable by default. The NX bit, which stands for No eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions (or code On those cases, the program will receive the SIGSEGV signal. On POSIX -compliant platforms SIGSEGV is the signal thrown by Computer programs making invalid memory references or Segmentation faults The

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. A stack trace (also called stack backtrace or stack traceback) is a report of the active Stack frames instantiated by the execution of a program The GNU Debugger, usually called just GDB, is the standard Debugger for the GNU software system 
0xbfffede4 in ?? ()
(gdb) bt
#0  0xbfffede4 in ?? ()
#1  0x0804837f in main ()
(gdb) x/i $pc
0xbfffede4:        (bad)  

Note "(bad)", indicating that the debugger does not recognize the opcode at that address. A debugger is a Computer program that is used to test and Debug other programs The mnemonic representing the instruction would normally be displayed there. A mnemonic device (nəˈmɒnɪk is a Memory aid Commonly met mnemonics are often verbal something such as a very short poem or a special word used to help a person remember

Compare the output from SIGILL with that of a segmentation fault and a SIGFPE signal. A segmentation fault (often shortened to segfault) is a particular error condition that can occur during the operation of Computer software. SIGFPE is the signal sent to computer programs that perform erroneous arithmetic operations on POSIX compliant platforms

See also

• Signal (computing)
• Pentium F0 bug
• Instruction set
• Ring 0