strlen not checking for NULL

The standard does not require it, so implementations just avoid a test and potentially an expensive jump.

A little macro to help your grief:

#define strlens(s) (s==NULL?0:strlen(s))

The rational behind it is simple -- how can you check the length of something that does not exist?

Also, unlike "managed languages" there is no expectations the run time system will handle invalid data or data structures correctly. (This type of issue is exactly why more "modern" languages are more popular for non-computation or less performant requiring applications).

A standard template in c would look like this

 int someStrLen;

 if (someStr != NULL)  // or if (someStr)
    someStrLen = strlen(someStr);
 else
 {
    // handle error.
 }

The portion of the language standard that defines the string handling library states that, unless specified otherwise for the specific function, any pointer arguments must have valid values.

The philosphy behind the design of the C standard library is that the programmer is ultimately in the best position to know whether a run-time check really needs to be performed. Back in the days when your total system memory was measured in kilobytes, the overhead of performing an unnecessary runtime check could be pretty painful. So the C standard library doesn't bother doing any of those checks; it assumes that the programmer has already done it if it's really necessary. If you know you will never pass a bad pointer value to strlen (such as, you're passing in a string literal, or a locally allocated array), then there's no need to clutter up the resulting binary with an unnecessary check against NULL.