Using malloc on a elemnt in struct in C

Question

I have this struct:

typedef struct
{
    char name[3];
    int month_num;
    int day_num;
    int day_size; // amount of days that are field in with tasks.
    char *task[40];
}

month; // each month contains a name, its num in the calendar and days.

and I need to assign memory allocation for it. I was able to assign the memory for the struct itself:

mon = (month*)malloc(12 * sizeof(month));

But I am having trouble to do it the same for the char *task[40].

I tried a lot of possibilities, but non of them worked...

char temptask[40];
mon->task=malloc(strlen(temptask)+1);

Standard Warning : Please [do not cast](http://stackoverflow.com/q/605845/2173917) the return value of `malloc()`. — Sourav Ghosh, Jan 28 '15 at 08:23
You should `malloc()` for `mon->task[0]` and/or other elements not `mon->task`. — Rohan, Jan 28 '15 at 08:23
well, since task is in the struct named month, then i'll say it's in the type month. (if in type you mean as if in int, char, double, etc.) — lili, Jan 28 '15 at 08:41
@lili No, not "what type is it inside?" but "what type is it?" — user253751, Jan 28 '15 at 08:43
`int i;` <- now `i` is an integer. `month m;` <- now `m.task` is what? (I'm testing your understanding; this is not part of an answer) — user253751, Jan 28 '15 at 08:46
well i do think it is a string (but since task has a pointer then it is an array of pointers) — lili, Jan 28 '15 at 08:48
@lil A value in C can't be "a string" and "an array of pointers" at the same time, they are not the same. — unwind, Jan 28 '15 at 08:56

Gopi · Accepted Answer · 2015-01-28T08:28:55.713

1

for(i=0;i<40;i++)
{ 
  /* Allocating memory to each pointers and later you write to this location */
  mon->task[i] = malloc(size);/* If you have a initialized string then size = strlen(tempTask) + 1 */
}

What you have is array of pointers and just access them and allocate memory to each of the pointers individually as shown above.

edited Jan 28 '15 at 08:28

answered Jan 28 '15 at 08:23

Gopi

19,784
4
24
36

i have tried that option but for some reason the compailer screms: "="- cannot convert from 'void *' to 'char *' – lili Jan 28 '15 at 08:36
1

What compiler are you using that can't convert from `void*` to `char*`? See http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc – John Zwinck Jan 28 '15 at 08:49
2

@JohnZwinck Sounds like a C++ compiler to me. :( – unwind Jan 28 '15 at 08:57
1

@unwind You should be right. The most acclaimed answer of yours not to cast `malloc()` answers the OP's question :) – Gopi Jan 28 '15 at 08:58
@lili: If you're using a C++ compiler you need to cast the result of malloc, like `(char*)malloc(...)`. – John Zwinck Jan 28 '15 at 09:00
Better: If you use a C++ compiler, you should program in C++. Use `new`, `delete` etc. If you want to program in C, use a C compiler (resp. tell your compiler to compile in C). – glglgl Jan 28 '15 at 09:18

score 0 · Answer 2 · answered Jan 28 '15 at 08:24

char *task[40]; is an array of 40 pointers. You probably want a single array of 40 characters, in which case no need to malloc it separately, or else a single pointer in which case you can malloc(40). You can't call strlen() on an uninitialized C string, that's undefined behavior (buffer overrun).

Sourav Ghosh · Answer 3 · 2015-01-28T14:05:06.583

0

I think, what you need is

char *task;

and then allocate memory as

mon[j].task=malloc(sizeof(temptask));

Next, as you've allocated memory to mon as

 mon = malloc(12 * sizeof(month));

the access should be made as

 mon[j].task  // j being the index, running from 0 to 11

Otherwise, if you really have a char *task[40];, that means, an array of 40 char *s, then you have to allocate memory for each of them, one by one.

int p = strlen(temptask);
for(i=0;i<40;i++)
{
      mon[j].task[i] = malloc(p+1);
}

edited Jan 28 '15 at 14:05

answered Jan 28 '15 at 08:24

Sourav Ghosh

133,132
16
183
261

What you're proposing is undefined behavior! – John Zwinck Jan 28 '15 at 08:26
@JohnZwinck I really appreciate leaving the comment along with the downvote. Can you please elaborate a bit, as i'm not clear on the reason behind undefined behaviour here. – Sourav Ghosh Jan 28 '15 at 08:31
It's undefined behavior to call `strlen(temptask)` when `temptask` is an uninitialized C string. This UB was in the OP as well. There's no need for the `temptask` variable at all. – John Zwinck Jan 28 '15 at 08:32
why do i need to write mon[i] if month are going to max 12? why is going from 0 to 39? – lili Jan 28 '15 at 08:43
@JohnZwinck Is it better now? – Sourav Ghosh Jan 28 '15 at 08:44
@lili `mon[0]` will be the first month, `mon[1]` will be the second and so on. each month will be represented by a structure variable in the array – Sourav Ghosh Jan 28 '15 at 08:46
@lili aah, I din't mean to have to loop for 40 times. for clarification, let's use `j` as index to `mon`. Clear? – Sourav Ghosh Jan 28 '15 at 09:09

Using malloc on a elemnt in struct in C

3 Answers3