strlen not checking for NULL

问题

Why is strlen() not checking for NULL?

if I do strlen(NULL), the program segmentation faults.

Trying to understand the rationale behind it (if any).

回答1:

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.
 }

回答2:

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.

回答3:

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

回答4:

A little macro to help your grief:

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

回答5:

size_t strlen ( const char * str );

http://www.cplusplus.com/reference/clibrary/cstring/strlen/

Strlen takes a pointer to a character array as a parameter, null is not a valid argument to this function.

回答6:

Three significant reasons:

• The standard library and the C language are designed assuming that the programmer knows what he is doing, so a null pointer isn't treated as an edge case, but rather as a programmer's mistake that results in undefined behaviour;

• It incurs runtime overhead - calling strlen thousands of times and always doing str != NULL is not reasonable unless the programmer is treated as a sissy;

• It adds up to the code size - it could only be a few instructions, but if you adopt this principle and do it everywhere it can inflate your code significantly.

来源：https://stackoverflow.com/questions/5796103/strlen-not-checking-for-null