Default value of errno variable [closed]

Question

I want to use the errno library in order to define the return of a project (c language) functions. And I'm wondering about something...

I will do something like this :

#include <errno.h>

int myfunction (void)
{
  int res;

  /*Some actions....*/

  if(success)
    res = 0;
  else if (fail)
    res = -EIO;

  return res;
 }

I like to always initialize my local variable and I am wondering what should be the default value of a variable using the errno value (probably 0) ? But I don't really like to set by default : "SUCCESS" I prefer a "Failure" value. What is your point of view (or rule) about it ?

Thanks by advance.

Answer 1

If your question is what the default, no error, value is for errno and all the functions that return error status in errno, that value is 0 as you expect.

But the error code is usually not returned as a negative value to the caller as you have it in your snippet.

Answer 2

There is no EGENERIC/EUNKNOWN value defined as some general error indicator.

You have the following options:

• Be optimistic and set the default result to 0 indicating success.
• Use a function specific default out of the set of values defined for errno.
• If returning negative values for errno use 1 as generic error value.
• Use positive errno value and use -1 as generic error indicator.

For the last options also set errno inside your function.

---

Update on POSIX systems

If going to use the value defined for errno there is no portable way to use any other value but 0, as POSIX explicitly states:

Only the macro names ^*1 should be used in programs, since the actual value of the error number is unspecified.

^{*1 This referrs to the Exyz macros, as listed here.}