SEI CERT C Coding Standard

Error Handling (ERR) - ERR30-C. Set errno to zero before calling a library function known to set errno, and check errno only
after the function returns a value indicating failure
13 Error Handling (ERR)
13.1 ERR30-C. Set errno to zero before calling a library function known
to set errno, and check errno only after the function returns a value
indicating failure
The value of errno is initialized to zero at program startup, but it is never subsequently set to
zero by any C standard library function. The value of errno may be set to nonzero by a C standard
library function call whether or not there is an error, provided the use of errno is not documented
in the description of the function. It is meaningful for a program to inspect the contents of
errno only after an error might have occurred. More precisely, errno is meaningful only after a
library function that sets errno on error has returned an error code.
According to Question 20.4 of C-FAQ [Summit 2005]
In general, you should detect errors by checking return values, and use errno only to
distinguish among the various causes of an error, such as “File not found” or “Permission
denied.” (Typically, you use perror or strerror to print these discriminating error
messages.) It’s only necessary to detect errors with errno when a function does not
have a unique, unambiguous, out-of-band error return (that is, because all of its possible
return values are valid; one example is atoi [sic]). In these cases (and in these
cases only; check the documentation to be sure whether a function allows this), you can
detect errors by setting errno to 0, calling the function, and then testing errno. (Setting
errno to 0 first is important, as no library function ever does that for you.)
Note that atoi() is not required to set the value of errno.
Library functions fall into the following categories:
• Those that set errno and return and out-of-band error indicator
• Those that set errno and return and in-band error indicator
• Those that do not promise to set errno
• Those with differing standards documentation
13.1.1 Library Functions that Set errno and Return an Out-of-Band Error
Indicator
The C Standard specifies that the functions listed in the following table set errno and return an
out-of-band error indicator. That is, their return value on error can never be returned by a successful
call.
A program may set and check errno for these library functions but is not required to do so. The
program should not check the value of errno without first verifying that the function returned an
SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems 374
Software Engineering Institute | Carnegie Mellon University
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