So include errno. You should ask yourself if errno is the best error-reporting mechanism for your purposes. Can the functions be engineered to return the error code themselves? IMO, the standard errno designed for system level. My experience is do not pollute them. If you want to simulate the C standard errno mechanism, you can do some definition like:. More information, please refer to glibc 's implementation. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. How to set errno value? Ask Question. Asked 9 years, 3 months ago. Active 8 years, 8 months ago. This macro expands to a modifiable lvalue of type int, therefore it can be both read and modified by a program. The errno is set to zero at program startup. Certain functions of the standard C library modify its value to other than zero to signal some types of error. You can also modify its value or reset to zero at your convenience.
It is one of the bullet points under "Library Functions and errno". The new rule name is better, but still doesn't match the details within the rule. I still think it should be changed to something more general like "Handle errno appropriately for the error reporting characteristics of each function".
Besides, while printf might set errno if an encoding error occurs, would it set errno if anything else happens eg what if the stdout is closed? I thought the atoi family of functions was considered 'deprecated', in lieu of the strtoul family I take it from your comment that there isn't a function in C99 that doesn't use errno to indicate errors The sentence starting with "It might be more reasonable" under Automated Detection reads funny, and I'm not sure what it's trying to say, since it seems so similar to the previous sentence.
See ERRC. Detect errors when converting a string to a number for a more comprehensive code example about the proper use of strtoul. In particular, it is also possible for strtoul to make a successful conversion but still leave garbage characters in the input string It is possible that errno is set by some older routine and so it indicates an error, but not one in strtoul.
It is also possible though unlikely that the routine set errno erroneously and so it does not actually indicate an error. So I still believe the sentence is most correct as it is currently written. I have checked C90 and C11 standards and I could not find anything describing behavior of functions strcoll , strxfrm , wcscoll , wcsxfrm with respect to errno.
What have I missed? Could you please clarify, if it is required by the rule that every call to a function that returns an in-band error indicator is followed by an errno-check. The rule does not require that a possible error is detected as this is the subject of ERRC. Could you, please, explain the reasoning behind the following part of the description in the "Library Functions that Set errno and Return an Out-of-Band Error Indicator" section :.
Standard seems to clearly state for these kind of functions that - in case of an error - they return given value and set non-zero value of errno. As I understand, that implies that the value of errno remains unchanged when the call is successful. The caveat is that a platform is free to have ftell set errno to a nonzero value even if it succeeds.
In that case your program will "handle error" even though ftell succeeded. So you should first check that pos is -1 as a signed long , before reading errno.
The value of errno in the initial thread is zero at program startup the initial value of errno in other threads is an indeterminate value , but is never set to zero by any library function. 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. If successful, the ftell function returns the current value of the file position indicator for the stream.
As I understand, it implies that the ftell function cannot change the value of errno if there is no error. Could you, please, provide some reference to the standard that allows such platform-specific behavior? Or maybe there are some known non-compliant implementations we should guard against?
Offhand, your interpretation that 7. But I could see a platform writer arguing that that section is loose enough to allow their ftell to set errno and yet return a successful value. Next to the standard, there is the C FAQ [ Summit ], whose most relevant errno information is already quoted in this rule. Pages Boards. Page tree. Browse pages. A t tachments 0 Page History People who can view.
Error Handling ERR. Jira links. 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 an out-of-band error indicator Those that set errno and return an in-band error indicator Those that do not promise to set errno Those with differing standards documentation 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.
Douglas A. Permalink Apr 18, Robert Seacord Manager. I agree with the sentiment, which is sort of captured by this recommendation: DCLC. Permalink Jul 22, David Svoboda. Permalink Jul 25, Geoff Clare. Permalink Jul 26, It says A program that uses errno for error checking should set it to zero before a library function call and then inspect it before a subsequent library function call.
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