string size, and concatenation

[Bladtman242]

Obviously I have some wrong assumptions, because this prints "strsize = 8" no matter what.

I realize my use of system() might be misguided, so if you have comments on that don't hold back, just please help out with the string-thing aswell : )

BTW it's a 'wrapper' for shell one-liners, so it is actually supposed to run whatever input it gets in the shell it was called from.

if( system(command) == 0) {
		char* buff = malloc(sizeof(command)+42);
		strcat(buff, "notify-send \"notif finished\" \"Command: '");
		strcat(buff, command);
		strcat(buff, "'\"");
		system(buff);
		printf("strsize = %li\n", sizeof(buff));
	}

[Narue]

Obviously I have some wrong assumptions, because this prints "strsize = 8" no matter what.

Good call. buff is a pointer to char, so sizeof(buff) will always tell you what the size of a pointer to char is, not the length of the string it points to (if it points to a string at all, which isn't a safe assumption). What you want is the strlen() function, not sizeof.

Also note that strcat() expects the destination to be a valid string, which means you code is broken. This will fix it:

char* buff = malloc(sizeof(command)+42);

buff[0] = '\0'; /* Make buff a valid string */

strcat(buff, "notify-send \"notif finished\" \"Command: '");

Finally, it's good practice to check malloc() for failure. Otherwise you'll attempt to dereference a null pointer.

[Bladtman242]

Thanks, that makes sense.
(can't believe the size '8' didn't ring a bell)
There is another thing though,
as far as I can tell, strcat depends on the string pointed to by its first parameter to be large enough to hold the two strings. but if I change the code to buff = malloc(sizeof(command)+2); (or pretty much any size at all, smaller than 42) the 'string' is expanded to be 42 characters long?

[Narue]

as far as I can tell, strcat depends on the string pointed to by its first parameter to be large enough to hold the two strings.

Correct, it's your job to ensure that the destination has enough room.but if I change the code to buff = malloc(sizeof(command)+2); (or pretty much any size at all, smaller than 42) the 'string' is expanded to be 42 characters long?
Expanded? No. C doesn't stop you from writing beyond the boundaries of an array. In Java, for example, you'll get an exception immediately, but C will happily write to memory you don't own and silently corrupt things. If you're lucky, you'll corrupt something that causes a fatal error, but in this case it seems as if you weren't lucky.

[Bladtman242]

But the printf() function still says the string length is 42? (44 when run with 'ls' as a parameter)
(heh - 42)
The call it later makes to system() also seems perfectly fine?

[Narue]

I understand your confusion, but please don't be tricked into thinking that the code isn't broken when you provide insufficient space for strcat().

[Bladtman242]

OK. I just don't understand why it is actually working? Shouldn't the strlen only return 2 (or 4 with 'ls'), shouln't the string in the system(); call be cut of, and shouln't I get a segmentation fault?
You're right, Im pretty confused by this : )

[Narue]

Shouldn't the strlen only return 2 (or 4 with 'ls'), shouln't the string in the system(); call be cut of, and shouln't I get a segmentation fault?

Yes, yes, and yes. Or no, no, and no. Or any combination thereof. Undefined behavior meansany damn thing can happen, which includes working perfectly.

[Bladtman242]

Ahh, well. at least that makes debugging easy..
How the hell do you find errors then? Only thanks to you I just realized the +42 should be + 43 (to include the string terminator).

BTW, if I do a if(buff) {*print errormessage and exit(1)*} to check if malloc returned NULL, do I still need to do a buff[0] = '\0' because malloc() doesn't guarantee anything about the data it returns a pointer to, or are we all good on the string when malloc succeeds?

[WaltP]

If "malloc() doesn't guarantee anything about the data it returns a pointer to" what do you think that means about your buffer?

[Bladtman242]

That it could contain anything?
The reference I'm using says strcat will append string2 to the end of string1, so we add a string terminator to the beginning of buff, to make sure strcat will write from buff[0], even if other elements contain a terminator. So yes, buff[0] = '\0' is necessary.
Guess I could figured that out : )

[WaltP]

That it could contain anything? The reference I'm using says strcat will append string2 to the end of string1, so we add a string terminator to the beginning of buff, to make sure strcat will write from buff[0], even if other elements contain a terminator. So yes, buff[0] = '\0' is necessary. Guess I could figured that out : )

And you did, too! :)

[Bladtman242]

Well thanks, both of you : )