Memory Allocation in c with string

Question

void Strcat(char str1[], char str2[]){
long len1 = strlen(str1);
long len2 = strlen(str2);

char* str = (char*)malloc(((len1 + len2) + 1)*sizeof(char));
if(str == NULL){
    printf("No memory");
    exit(1);
}
for (int i = 0 ; str1[i] != '\0'; i++) {
    str[i] = str1[i];
}

str[strlen(str1)] = ' ';
for (long i = 0, j = strlen(str1)+1 ; str2[i] !='\0' ; i++, j++) {
    str[j] = str2[i];
    if(str2[i+1] == '\0')
        str[j+1] = '\0';
}

//puts(str);
printf("strlen STR -> %ld\n", strlen(str));
for (int i = 0; str[i] != '\0'; i++) {
    printf("%c",str[i]);
}

free(str);

}

Ok I know the strcat function is a string between two strings. Suppose I put the input "ttt" into the first string And the second string the input "yyy". I am now using dynamic assignment using malloc Now I know we need to take the length of the first + second + 1 the 1 is for the '\0' character.

So my allocation is size 7.

but I need to make a space between the two strings Do I need my allocation to be 8? because when I do only sizeLength + 1 the program is still working and it still puts a space between the two strings and I feel like the compiler forgives me.

Welcome to SO. "Do I need my allocation to be 8?" Of course, who else should take that space into account if it isn't you? — Gerhardh, Sep 28 '21 at 10:43
"and I feel like the compiler forgives me." That is just bad luck. You are causing undefined behaviour and anything can happen. It can evel look to work as expected while desaster is waiting to break free.... — Gerhardh, Sep 28 '21 at 10:45
You don't return the new string, so there is no reason to create it. You could just do printf. — stark, Sep 28 '21 at 10:51
@Gerhardh But can you explain to me why this happens even though I assign a size 7 and I still insert the space character and the \ 0 character — Roni Jack Vituli, Sep 28 '21 at 10:52
The C compiler does not ensure that you manage your dynamic memory correctly. That's totally up to you. If you do it incorrectly, your program may or may not result in an error when it runs depending upon what part of memory you erroneously write/access and what that other memory is used for. — lurker, Sep 28 '21 at 10:53
@omerbenyair, because `malloc()` usually gives you **more** memory than you asked for (like rounding up to multiple of 32 bytes). This often hides many errors. — tstanisl, Sep 28 '21 at 10:54
Does this answer your question? [How dangerous is it to access an array out of bounds?](https://stackoverflow.com/questions/15646973/how-dangerous-is-it-to-access-an-array-out-of-bounds) — Gerhardh, Sep 28 '21 at 11:09
Another dupe: https://stackoverflow.com/questions/1239938/accessing-an-array-out-of-bounds-gives-no-error-why While it is for C++ the message is valid for C as well. — Gerhardh, Sep 28 '21 at 11:10

score 2 · Accepted Answer · answered Sep 28 '21 at 10:55

I know the strcat function is a string between two strings

Umm, no it isn't? I don't know what that's even supposed to mean. strcat stands for string concatenation (and not "strangle the cat" as one may assume at a glance :) ). Adding a space between them is not how standard library strcat works.

So my allocation is size 7

Yes, that is correct for implementing strcat with input lengths of 3 + 3, with space for the null terminator at the end.

but I need to make a space between the two strings

Not sure why you'd want to do that, but if you want a space character in between them you need to allocate one extra byte indeed.

Do I need my allocation to be 8?

Only if you need to make room for the space.

because when I do only sizeLength + 1 the program is still working

It only works by (bad) luck, this time. What is undefined behavior and how does it work?

I feel like the compiler forgives me

It's not the programmer's job to keep track of correct amounts of memory allocated. C doesn't have bounds-checking of arrays, nor does it have automatic (re)allocation etc as higher level languages might have.

And as someone said in comments, malloc likely allocates larger chunks of aligned memory. Size 8 bytes seems like a very likely candidate for one such aligned chunk, regardless if you actually just use 7 bytes. But this is by no means guaranteed behavior and not something you can rely on.

Memory Allocation in c with string

1 Answers1