C language - removing uninitialized space in string

Question

I have the following code:

char *string = malloc(strlen(stringa) + strlen(stringb) + 1);
strcpy(string, stringa);
strcpy(string+strlen(stringa), stringb);

the problem is, with what I'm doing, stringa is sometimes NULL, and when it is and it gets copied to string, it results in uninitialized characters at the very beginning of string. Here's an example of the output that I am getting when stringa happens to be NULL:

�����o
This is a test
To see the output
of string

I want to get rid of the characters at the beginning that are uninitialized. What I tried doing so far is:

memset(string, 0, strlen(string));

and

memset(string, 0, sizeof(string));

both have given me errors and do not solve my problem. Anyone know how I could edit 'string' to get rid of the uninitialized characters? This could either be applied to 'string' directly or to 'stringa' before it gets copied to 'string'.

How about test stringa for NULL and handle accordingly (ie, only copy if not NULL)? — kaylum
– kaylum, Commented May 9, 2016 at 3:34
strcpying from NULL is going to straight up crash on some systems. (i'm amazed it isn't on yours.) so you should just avoid doing that in the first place. like: if(stringa!=NULL) strcpy(string, stringa); — Jay Kominek
– Jay Kominek, Commented May 9, 2016 at 3:35
strlen(NULL) is undefined behaviour. You must alter your logic to avoid calling this. — M.M
– M.M, Commented May 9, 2016 at 3:39
Prior to malloc(), if (stringa == NULL) stringa = ""; OTOH, what do you want if both are NULL? — chux
– chux, Commented May 9, 2016 at 4:15

Dov Benyomin Sohacheski · Accepted Answer · 2017-05-22 14:49:28Z

1

You can always run some logical tests with a conditional statement on the value of the strings before you begin copying them:

Option 1:

char*  string;
size_t length = 0;

if (stringa != NULL) {
    length += strlen(stringa);
}

if (stringb != NULL) {
    length += strlen(stringb);
}

string = malloc(length + 1);

// Copy the strings

Option 2:

char* string = NULL;

if (stringa != NULL) {
    string = malloc(strlen(stringa) + 1);
    strcpy(string, stringa);
}

if (stringb != NULL) {
    if (string != NULL) {
        string = realloc(string, strlen(stringb) + 1);
        strcat(string, stringb);
    } else {
        string = malloc(strlen(stringb) + 1);
        strcpy(string, stringb);
    }
}

edited May 22, 2017 at 14:49

answered May 9, 2016 at 3:40

Dov Benyomin Sohacheski

7,8488 gold badges42 silver badges67 bronze badges

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

1 Comment

Carey Gregory Over a year ago

Won't crash (yet) but string remains uninitialized.

Malcolm McLean · Accepted Answer · 2017-05-22 15:15:06Z

0

In very old C, on many platforms, a nul byte happened to be hardcoded to location zero in memory, and so there wasn't a distinction between the null string and the empty string. It's a bit of an odd difference if you think of it, no string of sausages versus a string of no sausages.

You can easily resurrect the old behaviour, by wrapping strcpy

char *strcpy_oldbehaviour(char *dest, const char *src)
{
   if(src)
     return strcpy(dest, src);
   else
     return strcpy(dest, "");
}

answered May 22, 2017 at 15:15

Malcolm McLean

6,4201 gold badge19 silver badges18 bronze badges

Collectives™ on Stack Overflow

C language - removing uninitialized space in string

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related