Stack trace
In computing, a stack trace (also called stack backtrace[1] or stack traceback[2]) is a report of the active stack frames at a certain point in time during the execution of a program. When a program is run, memory is often dynamically allocated in two places; the stack and the heap. Memory is continuously allocated on a stack but not on a heap, thus reflective of their names. Stack also refers to a programming construct, thus to differentiate it, this stack is referred to as the program's runtime stack. Technically, once a block of memory has been allocated on the stack, it cannot be easily removed as there can be other blocks of memory that were allocated before it. Each time a function is called in a program, a block of memory is allocated on top of the runtime stack called the activation record (or stack pointer.) At a high level, an activation record allocates memory for the function's parameters and local variables declared in the function.
Programmers commonly use stack tracing during interactive and post-mortem debugging. End-users may see a stack trace displayed as part of an error message, which the user can then report to a programmer.
A stack trace allows tracking the sequence of nested functions called - up to the point where the stack trace is generated. In a post-mortem scenario this extends up to the function where the failure occurred (but was not necessarily caused). Sibling calls do not appear in a stack trace.
As an example, the following Python program contains an error.
1 def a():
2 i = 0
3 j = b(i)
4 return j
5
6 def b(z):
7 k = 5
8 if z == 0:
9 c()
10 return k + z
11
12 def c():
13 error()
14
15 a()
Running the program under the standard Python interpreter produces the following error message.
Traceback (most recent call last):
File "tb.py", line 15, in <module>
a()
File "tb.py", line 3, in a
j = b(i)
File "tb.py", line 9, in b
c()
File "tb.py", line 13, in c
error()
NameError: name 'error' is not defined
The stack trace shows where the error occurs, namely in the c
function. It also shows that the c
function was called by b
, which was called by a
, which was in turn called by the code on line 15 (the last line) of the program. The activation records for each of these three functions would be arranged in a stack such that the a
function would occupy the bottom of the stack and the c
function would occupy the top of the stack.
Language support
Many programming languages, including Java and C#, have built-in support for retrieving the current stack trace via system calls. C++ has no built-in support for doing this, but C++ users can retrieve stack traces with (for example) the stacktrace library. In JavaScript, exceptions hold a stack
property that contain the stack from the place where it was thrown.
References
- "libc manual: backtraces". gnu.org. Retrieved 8 July 2014.
- "traceback — Print or retrieve a stack traceback". python.org. Retrieved 8 July 2014.