C: How to store a string in a structure

Question

So I have a structure, and one of its members is a string.

struct Output {
     char *axis;
     int value;
};
struct Output Jsoutput;

My question is, how do I store a string in axis?

char whichaxis[4][3] = {"LX","LY","RY","RX"};
// Store which  axis and value of the joystick position in Jsoutput
Jsoutput.axis =  whichaxis[jse.number];
printf("%s\n",Jsoutput.axis);

I feel like there should be some & somewhere, but not sure where.

What is `whichaxis`'s lifetime? Is it really on the stack like that? Are you planning on writing to the string at `axis`? — Kevin, Nov 24 '13 at 18:08
It's an event processing thing, so `Jsoutput.axis` will be overwritten regularly with a new pair of characters from `whichaxis`. `whichaxis` will not change, and should stay establsihed. — theo-brown, Nov 24 '13 at 18:12
So you'll be reassigning `axis`, but never writing, e.g. `Jsoutput.axis[1] = 'x'`? — Kevin, Nov 24 '13 at 18:14
Oh ok, sorry misunderstood. In my main loop stuff happens if `Jsoutput.axis == "LX"` — theo-brown, Nov 24 '13 at 18:19

score 4 · Accepted Answer · answered Nov 24 '13 at 18:13

4

Just use strdup

Jsoutput.axis =  strdup(whichaxis[jse.number]);

answered Nov 24 '13 at 18:13

Charlie Burns

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Does this negate the need for malloc? – theo-brown Nov 24 '13 at 18:23
Yes strdup does a malloc. – woolstar Nov 24 '13 at 18:32

Florian Groetzner · Answer 2 · 2013-11-24T18:12:26.530

1

You can copy a String with the function strcpy(destination, source)from string.h

see http://www.cplusplus.com/reference/cstring/strcpy/

Jsoutput.axis =  malloc(3);
strcpy(Jsoutput.axis,whichaxis[jse.number]);

edited Nov 24 '13 at 18:12

answered Nov 24 '13 at 18:03

Florian Groetzner

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And write all over who knows what, as there's no memory there to write to. – Kevin Nov 24 '13 at 18:05
Better, but you need an extra byte for the `'\0'`, and you shouldn't cast the result of malloc. – Kevin Nov 24 '13 at 18:06
@Kevin What do you mean "cast the result of malloc"? – theo-brown Nov 24 '13 at 18:08
he means (char*) thats a cast – Florian Groetzner Nov 24 '13 at 18:08
i dont know because in the refference they use a cast too – Florian Groetzner Nov 24 '13 at 18:09
@theoB610 the proper answer depends on what exactly you intend to do with `whichaxis` and `axis`. See my comment on the question. – Kevin Nov 24 '13 at 18:09
[You should not](http://stackoverflow.com/a/605858/900873) cast in c. In c++ you should use `new` instead (though if you use malloc you must cast). (cc @theoB610) – Kevin Nov 24 '13 at 18:10

score 0 · Answer 3 · edited May 23 '17 at 12:16

0

You don't have to "store" the string a second time.

char whichaxis[4][3] = {"LX","LY","RY","RX"};

Stores the string.

char *axis;

Says "I'm going to point at a string".

If you wanted a & in there, you could do:

Jsoutput.axis =  & (whichaxis[jse.number][0]) ;

But the original designers of C were very pragmatic and let arrays turn into pointers all the time for convenience. See What is array decaying for more details.

edited May 23 '17 at 12:16

Community

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answered Nov 24 '13 at 18:19

woolstar

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Me saying I feel like there should be an & was just what I thought the solution would be. – theo-brown Nov 24 '13 at 18:22

C: How to store a string in a structure

3 Answers3