How to initialize a structure within a structure in C?

Question

I have these two structures :

struct member {
  char *nickname;
  char *group;
};

struct node {
   struct member mbr;
   struct node   *next;
};

Further in my code I do this :

struct node* n = (struct node*)malloc(sizeof(struct node));

And get a "Segmentation fault" error at this line when I run the program :

strcopy(n->mbr.nickname, temp->nickname);

I've been trying for a while to solve this problem and I've searched on the web, but I didn't find any solution yet. I'm guessing the structure inside 'n' isn't initialized. I did a few test that looked like :

n->mbr = (struct member*)malloc(sizeof(struct member));

But then I get another error : "incompatible types when assigning to type 'struct member' from type 'struct member *'"...

Can anyone tell me what I'm doing wrong? Thanks.

Is strcopy() a typo or your own function?

Jonathan Leffler
– Jonathan Leffler

2013-10-20 23:05:52 +00:00
Commented Oct 20, 2013 at 23:05 — Jonathan Leffler
– Jonathan Leffler, Commented Oct 20, 2013 at 23:05

Ry- · Accepted Answer · 2013-10-20 23:02:20Z

2

You don’t need to allocate mbr; you need to allocate mbr.nickname.

struct node* n = malloc(sizeof (struct node));
n->mbr.nickname = malloc(some number of characters);

Then use strncpy. Alternatively,

n->mbr.nickname = strdup(temp->nickname);

which is the same as doing that, but using strlen(temp->nickname) + 1 as the size.

answered Oct 20, 2013 at 23:02

Ry-♦

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5 Comments

Jens Gustedt Over a year ago

Hm, strncpy is not such a good idea when the length of the string is known, anyhow. Better use memcpy, then.

Ry- Over a year ago

@JensGustedt: Is the length of the string known? If the idea is to find it using temp->nickname, I’ve added strdup.

jpw Over a year ago

Might be worth mentioning that strdup is POSIX and not standard C.

ryyker Over a year ago

OP tagged C, not posix - stick with strlen(x) (no strndup or strdup)

SimCity Over a year ago

It worked! Thank you so much, it's been a while since the last time I did some C programming and I think I'm not quite back into it ;)

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