CODEWITHSUNDEEP
cmacrospreprocessor
ComplicatedPhenomenon
ComplicatedPhenomenon

Reputation: 4199

if else shorthand in C

Is the statement below a kind of shorthand? i need someone help me understand with it. 

#define clean_errno()(errno == 0? "None": strerror(errno))

From the execution result, I guess it means once I confront clean_errno(),if errno ==0, I replace clearn_errno() with None, if not, I replace clean__errno() with strerror(errno). but I don't know how to analyse this statement logically?

Upvotes: 0

Views: 404

Answers (2)

Some programmer dude
Some programmer dude

Reputation: 409442

Preprocessor macros are replaced in their call-sites.

That means a statement like

printf("Error = %s\n", clean_errno());

will be replaced by

printf("Error = %s\n", (errno == 0? "None": strerror(errno)));

That will then at run-time print either "None" if errno == 0, otherwise print the result of strerror(errno).

As for the ?: expression itself, it is the conditional (a.k.a. the ternary) expression, and it works similar to a if-else.

However, using a macro like this is not something I would recommend, as the value of errno is usually undefined if a function doesn't fail. You need to make sure the previous function actually did fail before checking errno, in which case it will never be zero. There are very few places where errno will be reset to zero unless you specifically do it beforehand and know that the function you call will not modify it unless there's an error.

Upvotes: 6

Myst
Myst

Reputation: 19221

The code is evaluated during runtime and follows the shorthand:

condition ? if_true : if_false

This shorthand is very similar to a regular if else statement.

However, unlike normal if else in C, the shorthand can be used as an expression as well as a statement. i.e.:

char * str = 1 ? "true" : "false";

... which doesn't work so well with if else (char * str = if ... probably wouldn't work).

Try it with 0 ? "true" : "false" and test it out.

Good luck!

Upvotes: 2

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