assignment from incompatiable pointer type C

Question

Say I want to read in a list of pages name of max 19 character, e.g. 4 (Number of page) Name1 Name2 Name3 Name4

I am trying to use a global 2D array to store the page number and page name, I got an error saying assignment from incompatiable pointer type...

Thanks

static int nPages;
static char** pageName;

int main(void){
  scanf(" %d", &nPages);

  pageName = (char *)malloc(nPages*sizeof(char));
  for(int i=0; i < nPages ;i++){
    pageName[i] = (char *)malloc(20*sizeof(char));
    scanf(" %s", pageName[i]);
  }

//Free Memory Here of coz. 

  return 0;
}

http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — R. Martinho Fernandes, May 11 '11 at 14:26

score 5 · Answer 1 · edited May 23 '17 at 12:04

Never cast the return value of malloc() in C. It hides compiler warnings that actually help you, and it's never necessary so it just adds clutter. You should use:

pageName = malloc(nPages * sizeof *pageName);

Note how this is free from repetitions of the type name of pageName. Here, sizeof *pageName means "the size of the object pointed at by pageName", i.e. "the size of a character pointer". You should expect a sizeof expression as malloc()'s argument very often.

Also, sizeof (char) is always 1 in C, so that particular expression, it can be argued, adds more clutter than it helps make the code safe.

Etienne de Martel · Accepted Answer · 2011-05-11T14:33:31.837

3

The problem lies right there:

pageName = (char *)malloc(nPages*sizeof(char));

pageName is a char **, not a char *. So it should read:

pageName = malloc(nPages*sizeof(char*)); // sizeof(char *), and no need to cast

EDIT: removed the cast

edited May 11 '11 at 14:33

answered May 11 '11 at 14:07

Etienne de Martel

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Thanks but how would the char length of max 19 be expressed in this case ? – Jono May 11 '11 at 14:11
Whoops, i get it, thats just for the first malloc. Thanks =] – Jono May 11 '11 at 14:12
2

Because I disagree with what "it should read". I don't like casts in my mallocs. – R. Martinho Fernandes May 11 '11 at 14:29
@Martinho Sorry about that, I'm primarily a C++ programmer. – Etienne de Martel May 11 '11 at 14:32
@Etienne: changed to +1. FWIW, this is one of those situations that shows that C isn't strictly a subset of C++. Without the cast, it won't compile in C++. – R. Martinho Fernandes May 11 '11 at 14:38
C++ programmers using `malloc` and casting the result is roughly as offensive as English speakers using words from other languages and botching the pronunciation or meaning... :-) – R.. GitHub STOP HELPING ICE May 11 '11 at 15:57
1

@R.. I prefer using `new`, but whenever I have to write C and I have to use `malloc`, I can't help but say "oh, this returns a `void *`, better cast it, because implicit conversions are _evil_." – Etienne de Martel May 11 '11 at 17:07
On the other hand, explicit conversions are evil. They're a red flag that you're doing something you shouldn't be doing. The C language has exactly (well, almost exactly) the right set of implicit conversions so that you never need to make them explicit when you're doing something reasonable. – R.. GitHub STOP HELPING ICE May 11 '11 at 17:11

score 0 · Answer 3 · answered May 11 '11 at 14:08

0

Your defined pageName as char ** but when calling malloc to initialize the pointer, you use a cast to (char *). Maybe that is the problem.

answered May 11 '11 at 14:08

Constantinius

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score 0 · Answer 4 · answered May 11 '11 at 14:09

0

You are assigning char* to char** at your first malloc line. Just drop the (char*) cast - C handles void* to char** conversion automatically.

answered May 11 '11 at 14:09

Nikolai Fetissov

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score 0 · Answer 5 · answered May 11 '11 at 14:10

char* and char** are two different types, and you can't freely assign the first to the latter.

Having said that, you have to do a two-step initialization. First, you have to malloc() space for nPages string:

pageName = (char**) malloc(nPages * sizeof(char*));

then, you have to malloc() space for each pageName:

for (i = 0; i < nPages; ++i) {
    pageName[i] = (char*) malloc(20);
}

assignment from incompatiable pointer type C

5 Answers5