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Returning extra results from a function in C
There are three ways of returning a result from a function in C. You can use return
to pass a result back, you can use a variable declared outside all of your function (so it's global and accessible everywhere), and you can pass in a pointer to act at the target for where a result is to be saved.
If you've only one result to pass back, use the return
. It's much the easiest, and it's what users will expect.
You should use variables declared outside your functions very sparingly - they're liable to leave code maintainers coming to look after the programs later on scratching their heads, and they make the name you use a universal one withing your whole program, which can be a bit of a problem if you want to use a library that someone's provided to you as a binary that happens to use the same name. I'm not saying "never", but please be careful.
So what's this pointer business?
• Set up a variable - in my example I'll call it doggie
• Pass in the ADDRESS of doggie
to your function.
• Store the (secondary) result in your function at the ADDRESS that's been passed in.
Although the address held in the function is a copy of the address in the main code, both the variables point to the same underlying variable / memory location, so changing *dd in the function also changes doggie in the main code.
Complete example [here]
A fourth alternative - outside the scope of my example which was written on last week's C course before I covered structures, is to pass back a structure or - better - a pointer to a structure.
(written 2012-05-03, updated 2012-05-05)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesC204 - C and C based languages - Functions, Macros and programs in multiple files 
Do not duplicate your code - (2006-06-23) 
A better alternative to cutting and pasting code - (2007-04-26) 
Some new C programming examples - files, structs, unions etc - (2007-12-19) 
Function Prototypes in C - (2010-01-11) 
Sharing variables between files of code in C - extern - (2010-01-14) 
C Course exercise and sample answer - source in 2 files - (2010-06-30) 
Using functions to keep look and feel apart from calculations - simple C example - (2011-04-09) 
Naming blocks of code, structures and Object Orientation - efficient coding in manageable chunks - (2012-05-06) 
Passing arrays into functions in C - (2014-12-02) 
Passing information into functions in C - by name, by value - (2015-10-26) 
Preprocessor directives in C and C++ - what they mean - (2015-10-27) 
Function prototype - what they are and why you should use them - C and C++ - (2015-10-27)
Some other Articles
Melksham ATC - freedom of the townStrawberry Cream Teas, Well House Manor, Melksham, starting this weekendSplitting a record into individual data values in CReturning extra results from a function in CLearning C++ - a design pattern for your first classChanging face - FiltonJust 12 winners? Or 371 or more? Town Centres.Verticals ... last monthSome unpublished and historic pictures - Museum of Melksham
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