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xpcall

Summary

Calls a function with a custom error handler

Prototype

ok, result = xpcall (f, err)


Description

Calls function f with err as the custom error handler.

If an error occurs in f it is caught and the error-handler 'err' is called. Then xpcall returns false, and whatever the error handler returned.

If there is no error in f, then xpcall returns true, followed by the function results from f.

Note that the supplied error function is called before the stack is unwound (in case of error) so this is a good time to find what functions were on the stack leading up to the error. In the example below we use the debug.traceback function, which shows a stack trace as at the time of the error.


function f ()
  return "a" + 2  -- will cause error
end -- f

function err (x)
  print ("err called", x)
  return "oh no!"
end -- err

print (xpcall (f, err))

 -->
 
err called [string "Immediate"]:2: attempt to perform arithmetic on a string value
false oh no!

function f2 ()
  return 2 + 2
end -- f

print (xpcall (f, err))  --> true 4

function f ()
return "a" + 2
end -- f

print (xpcall (f, debug.traceback))

 -->

false   stdin:2: attempt to perform arithmetic on a string value
stack traceback:
        stdin:2: in function `f'
        [C]: in function `xpcall'
        stdin:1: in main chunk
        [C]: ?


See Also ...

Lua functions

assert - Asserts that condition is not nil and not false
collectgarbage - Collects garbage
dofile - Executes a Lua file
error - Raises an error message
gcinfo - Returns amount of dynamic memory in use
getfenv - Returns the current environment table
getmetatable - Returns the metatable for the object
ipairs - Iterates over a numerically keyed table
load - Loads a chunk by calling a function repeatedly
loadfile - Loads a Lua file and parses it
loadlib - Loads a DLL (obsolete in Lua 5.1)
loadstring - Compiles a string of Lua code
module - Creates a Lua module
next - Returns next key / value pair in a table
pairs - Traverse all items in a table
pcall - Calls a function in protected mode
print - Prints its arguments
rawequal - Compares two values for equality without invoking metamethods
rawget - Gets the value of a table item without invoking metamethods
rawset - Sets the value of a table item without invoking metamethods
require - Loads a module
select - Returns items in a list
setfenv - Sets a function's environment
setmetatable - Sets the metatable for a table
tonumber - Converts a string (of the given base) to a number
tostring - Converts its argument to a string
type - Returns the type of a variable
unpack - Unpacks a table into individual items

Topics

Lua base functions
Lua bc (big number) functions
Lua bit manipulation functions
Lua coroutine functions
Lua debug functions
Lua io functions
Lua math functions
Lua os functions
Lua package functions
Lua PCRE regular expression functions
Lua script extensions
Lua string functions
Lua syntax
Lua table functions
Lua utilities
Scripting
Scripting callbacks - plugins

(Help topic: lua=xpcall)

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