On POSIX

POSIX

POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...

-compliant platforms, SIGTRAP is the signal

Signal (computing)

A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...

 sent to a process

Process (computing)

In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...

 when a condition arises that a debugger

Debugger

A debugger or debugging tool is a computer program that is used to test and debug other programs . The code to be examined might alternatively be running on an instruction set simulator , a technique that allows great power in its ability to halt when specific conditions are encountered but which...

 has requested to be informed of. In source code

Source code

In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...

, SIGTRAP is a symbolic constant

C preprocessor

The C preprocessor is the preprocessor for the C and C++ computer programming languages. The preprocessor handles directives for source file inclusion , macro definitions , and conditional inclusion ....

 defined in the header file

Header file

Some programming languages use header files. These files allow programmers to separate certain elements of a program's source code into reusable files. Header files commonly contain forward declarations of classes, subroutines, variables, and other identifiers...

 signal.h

Signal.h

signal.h is a header file defined in the C Standard Library to specify how a program handles signals while it executes. A signal can report some exceptional behavior within the program , or a signal can report some asynchronous event outside the program .A signal can be generated...

. Signal names are used instead of bare numbers because signal numbers can vary across platforms.

Etymology

SIG is a common prefix for signal names. TRAP refers to the intent of a debugger to trap

Trap (computing)

In computing and operating systems, a trap, also known as an exception or a fault, is typicallyThere is a wide variation in the nomenclature...

 (detect) an interesting condition.

Usage

When debugging

Debugging

Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...

 a program, it is often useful to run the program until a particular condition arises — for example, when a particular function

Subroutine

In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....

 is executed, or when a particular variable

Variable (programming)

In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...

 changes value. Although the debugger could repeatedly pause the program, checking the condition at each instruction, to do so would be very slow. More efficient is to request that the hardware itself determine when a condition obtains (e.g. when a particular memory segment is read, written or executed) and signal when this happens; this signal is then converted by the kernel into SIGTRAP, sent to the process being debugged. The debugger catches the signal, confirms for itself whether the condition has occurred, and if so pauses execution of the process and informs the developer.