conversion to ‘size_t’ from ‘int’ may change the sign of the result - GCC , C

Question

In my project I have turned on treat warnings as errors and compiling using the -pedantic and -ansi tags. I am using GCC compiler. In this project I have to use a third party source code which has got lot of warnings. Since I treat warnings as errors, I am having a tough time in fixing their code.

Most of the warnings are about invalid conversion from int to size_t or viceversa. In some cases, I won't be able to make both the variables same type, I mean I won't be able to change something to size_t. In such cases I am doing an explicit cast. Something like,

size_t a = (size_t) atoi(val);

I am wondering is this the correct approach? Is there any problem in doing cast like this?

If these warnings are minor, can I suppress it only on their files? How do I do the same on MSVC?

Answer 1

Have a look at the OpenOffice wiki on the best practices for error-free code: http://wiki.services.openoffice.org/wiki/Writing_warning-free_code

They suggest static casts for these conversions, and then supply a pragma to disable warnings for a particular section of code.

Answer 2

I personally consider this kind of warning idiotic and would turn it off, but the fact that you're asking about it suggests that you might be sufficiently unfamiliar with conversions between integer types and the differences in signed and unsigned behavior that the warning could be useful to you.

Back on the other hand again, I really despise [explicit] casts. The suggestion to use strtoul instead of atoi is probably a very good one. I see you commented that atoi was only an example, but the same principle applies in general: use functions that return the type you want rather than forcing a different type into the type you want. If the function is one you wrote yourself rather than a library function, this may just mean fixing your functions to return size_t for sizes rather than int.