Are consecutive calls to `errno` to be avoided?

Question

Is it safe to call errno multiple times when dealing with the same error. Or is it safer to work with a local copy?

This sample illustrates my question:

// If recvfrom() fails it returns -1 and sets errno to indicate the error.
int res = recvfrom(...);
if (res < 0)
{
    // Risky?
    printf("Error code: %d. Error message: %s\n", errno, strerror(errno));

    // Safer alternative?
    int errorNumber = errno;
    printf("Error code: %d. Error message: %s\n", errorNumber, strerror(errorNumber));
}

Answer 1

The value of errno shall be defined only after a call to a function for which it is explicitly stated to be set and until it is changed by the next function call or if the application assigns it a value.

http://www.opengroup.org/onlinepubs/009695399/functions/errno.html

However, even strerror could theoretically count as a function call that can change it (see comment by schot) so you should, theoretically, still go with your save-first form.

Answer 2

Any standard library function including printf and strerror is allowed to change errno, even if actually no error occurs:

7.5 3 The value of errno is zero at program startup, but is never set to zero by any library function. 170) The value of errno may be set to nonzero by a library function call whether or not there is an error, provided the use of errno is not documented in the description of the function in this International Standard.