Setting char array length with strlen() returns wrong size

Question

I am trying to create a new c-string with the length of a passed in c-string. I set a variable to the length of the old string and then try to set a new c-string with that length but checking that length returns 1. Why does this happen?

//This function will return a pointer to a char array
char *encrypt(char plainText[], char code[]){
    int i = 0,j = 0;
    //length of plainText
    int len = strlen(plainText);
    printf("len %i\n",len); //8
    //I am declaring a pointer here
    char *x;
    //make a string for the cipherText
    char cipher[len];
    printf("cipher %lu\n",strlen(cipher)); //1
    return "Nothing";
}

It's probably better to use char* in preference to char[] for consistency's sake here. — tadman
– tadman, Commented Jun 12, 2018 at 22:58
You define x; you never use it. You define cipher; you never initialize it. You don't make cipher big enough to hold the null byte at the end of a string. The length of a string does not include the null byte, though the "value" of the string does include it (at least, the string ends at the first null byte). — Jonathan Leffler
– Jonathan Leffler, Commented Jun 12, 2018 at 23:27
@JonathanLeffler Thank you. I apologize, this was from a big function I cut down to try and isolate this question and in haste left some mess. I've been really confused on how to initialize a c-string of the same size as the string I pass in. — NoleloN
– NoleloN, Commented Jun 12, 2018 at 23:38

Some programmer dude · Accepted Answer · 2018-06-12 22:56:10Z

2

The array cipher is uninitialized. It's contents is indeterminate (and will seem almost random). It might not contain the string-terminator at all, which means strlen will go out of bounds in its search for it. In your specific case you seem to be "lucky" that the second element in your array just happens to be the terminator.

And you seem to forget that char strings are really called null-terminated byte strings. That null-terminator also needs space. And exist of course.

All in all plenty of chances for undefined behavior.

answered Jun 12, 2018 at 22:56

Some programmer dude

411k36 gold badges420 silver badges655 bronze badges

Sign up to request clarification or add additional context in comments.

18 Comments

tadman Over a year ago

That and cipher goes out of scope the moment that function ends, so it can't be returned.

Ctx Over a year ago

There is no "chance" for undefined behaviour. Either it is UB or it isn't.

Eric Postpischil Over a year ago

@Ctx: While a particular code path with particular data might deterministically have behavior not defined by the C standard or not, it can be non-deterministic whether the code goes down that path with that data. For example, if a jump is conditional on indeterminate data, it might or might not go down a path with behavior not defined by the C standard.

NoleloN Over a year ago

Thank you for your answer. I'm confused now because I've seen code like char myString[10]; That makes space for 10 characters, with the last being a null terminated string. I thought that since I was passing 8 into the brackets I'd be making space in memory for an 8 char array?

Eric Postpischil Over a year ago

@Someprogrammerdude: Yes, but the likely fix is not to use strlen, rather than to try appending a null terminator to data that may contain internal zeros.

|

Collectives™ on Stack Overflow

Setting char array length with strlen() returns wrong size

1 Answer 1

18 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

18 Comments

Your Answer

Sign up or log in

Post as a guest

Related