Malloc strings?

**Mentallic** · 10-06-2013

I have an array of strings, and I just want to malloc room for my first string which is messages[0] and store hello world in it. This is what I have, which is failing to compile because of the malloc line. I've tried many different things but they're all swings in the dark, so could anyone please lend me a hand?

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MESSAGE_SIZE 100

typedef char playerMessage[MESSAGE_SIZE];

int main(void) {
	playerMessage messages[] = {};
	char *s = "hello world";
	
	messages[0] = malloc(sizeof(playerMessage));
	
	printf("%s\n", messages[0]);
	
    return 0;	
}

**std10093** · 10-06-2013

Get a paper and draw what you want.

Let me make an example, but without the typedef, which in my opinion is not helping.

You need to allocate space, equal to the length of the string, plus one for the null terminator of strings!

Code:

sizeof(strlen(s)+1)

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
#define MESSAGE_SIZE 100
 
int main(void) {
    
    /* Declare an array of char pointers.
       MESSAGE_SIZE is the number of these pointers.
       Every pointer is to point to a string.
     */
    char* messages[MESSAGE_SIZE];
    
    char *s = "hello world";
    
    /* Make the first pointer of the array, to point
       to 's'.
     */
    
    /* First, you need to make room for the string, 
       plus the null terminator.                     
     */
    messages[0] = malloc(sizeof(strlen(s)+1));
     
    /* Now copy the content of 's' to the array (at position 0). */
    strcpy(messages[0], s);
    
    printf("%s\n", messages[0]);
     
    return 0;   
}

Or, if you are sure that 's' will be in scope as long as your program executes, you can simply make the pointer to point to s, like this

Code:

messages[0] = s;

**Salem** · 10-06-2013

> messages[0] = malloc(sizeof(strlen(s)+1));
Now, do you know what sizeof is going to tell you about the return result of strlen() ?

**std10093** · 10-06-2013

Damn, missed that. Sorry guys!

**cyberfish** · 10-06-2013

Code:

typedef char playerMessage[MESSAGE_SIZE];

Probably the most confusing typedef I've seen in my entire life.

**SirPrattlepod** · 10-06-2013

Originally Posted by cyberfish

Code:

typedef char playerMessage[MESSAGE_SIZE];

Probably the most confusing typedef I've seen in my entire life.

I agree. Dracula probably wrote it.

**Mentallic** · 10-06-2013

std10093, I tried what you had posted, but I'm still getting the same error message at the malloc line:

incompatible types when assigning to type ‘playerMessage’ from type ‘void *’

Salem, I guess I don't need the sizeof function because strlen returns the size of the string, so we basically already have an integer, right?

And as for the typedef, the way it's set out in my assignment (which actually has to do with dracula

) is the typedef is stored away in a .h file that we aren't allowed to alter, so I need to work with it.

**Sebastiani** · 10-07-2013

Originally Posted by Mentallic

I'm still getting the same error message at the malloc line

Are you compiling this with a C or C++ compiler? If the latter, it's because C++ doesn't allow implicit casts...

Originally Posted by Mentallic

I guess I don't need the sizeof function because strlen returns the size of the string, so we basically already have an integer, right?.

You don't need it because it's complete nonsense. Consider this: sizeof(strlen(s)+1) == sizeof(0).

****
EDIT: Post your current code. The error message could be related to the typedef...

**Mentallic** · 10-07-2013

Originally Posted by Sebastiani

Are you compiling this with a C or C++ compiler? If the latter, it's because C++ doesn't allow implicit casts...

I'm unsure to be honest. I'm working on the terminal that comes with ubuntu and I'm using gcc with no flags to compile the file.

Originally Posted by Sebastiani

You don't need it because it's complete nonsense. Consider this: sizeof(strlen(s)+1) == sizeof(0).

****
EDIT: Post your current code. The error message could be related to the typedef...

My current code isn't the issue (at least it shouldn't be). I'm just trying to figure out how I would insert the string s into an array of strings messages[] so that I can continue with my assignment.

In my assignment, I have a test file that has the messages array and a string. It passes messages and the string into a function. In that function, I determine what the round is in the game that's being played, and so I need to insert the string into messages[round] so that I can refer back to it in the future.

**MutantJohn** · 10-07-2013

Why not just use strcpy?

**Sebastiani** · 10-07-2013

Originally Posted by Mentallic

My current code isn't the issue (at least it shouldn't be).

If your code wasn't the issue, it'd compile.

**Mentallic** · 10-07-2013

Originally Posted by MutantJohn

Why not just use strcpy?

Does the array have room? The messages[] that doesn't actually have a size associated with it at compile time is confusing me. How much room is there? Can I put as many strings in as I want?

Originally Posted by Sebastiani

If your code wasn't the issue, it'd compile.

Sorry, I thought you were referring to my assignment files, not this code. What I currently have is what std10093 posted in post #2.

**MutantJohn** · 10-07-2013

First, give messages a size. I am assuming what you are using is a 2d char array where each element of messages is playerMessage and as such already has a size.

If that is the case, just strcpy into an element of messages.

**Sebastiani** · 10-07-2013

Originally Posted by Mentallic

What I currently have is what std10093 posted in post #2.

Dandy, then it should compile just fine.

Originally Posted by Mentallic

The messages[] that doesn't actually have a size associated with it at compile time is confusing me.

Sure it does. You've declared it as MESSAGE_SIZE, right?

**Mentallic** · 10-07-2013

Originally Posted by MutantJohn

First, give messages a size. I am assuming what you are using is a 2d char array where each element of messages is playerMessage and as such already has a size.

If that is the case, just strcpy into an element of messages.

But I don't know how many strings I'll need to put into messages. If I make it messages[n] then I'm set on having only up to n strings, but may need more.

Originally Posted by Sebastiani

Dandy, then it should compile just fine.

But my assignment has a .h file that the c file has included, and it has playerMessage defined as
typedef char playerMessage[MESSAGE_SIZE];

so I need to incorporate that into the code somehow, don't I?

Originally Posted by Sebastiani

Sure it does. You've declared it as MESSAGE_SIZE, right?

playerMessage[MESSAGE_SIZE] is each string, but I have to store all of the strings in messages[].

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