Write and read an integer in a char * - c

Question

I'm coding a little server in c (a chat server) and i want to write and read an integer (and other type of variable like short int, unsigned int blablabla) in my char *data.
I have a structure DataOutput :
typedef struct t_dataoutput
{
char *data;
unsigned int pos;
} DataOutput;
And i have a function to write an int :
int writeInt(DataOutput *out, int i)
{
// here i resize my char *data
out->data[out->pos] = (i >> 24);
out->data[out->pos + 1] = (i >> 16) & 0xff;
out->data[out->pos + 2] = (i >> 8) & 0xff;
out->data[out->pos + 3] = i & 0xff;
out->pos += 4;
}
In my main function i want to try my code :
int main()
{
DataOutput out;
out.writeInt(&out, 9000);
printf("%d\n", (out.data[0] << 24) | (out.data[1] << 16) | (out.data[2] << 8) | (out.data[3]));
}
But the result is not good ... Why ? I don't understand :(
Sorry for my english i'm french ^^ !
Thx for your help !

Answer 1

The DataOutput.data should be forced to unsigned char because char are sometimes signed, depending of the compiler, and shifting signed chars or casting them to int (even implicitly) will propagate the sign bit:
typedef struct t_dataoutput
{
unsigned char *data;
unsigned int pos;
} DataOutput;
The memory for out->data has to be allocated before filled.
You can use realloc in the writeInt function like that:
int writeInt(DataOutput *out, int i)
{
// here i resize my char *data
out->data = realloc(out->data, out->pos + 4);
// TODO: test if out->data == NULL --> not enough memory!
out->data[out->pos] = (i >> 24) & 0xff;
out->data[out->pos + 1] = (i >> 16) & 0xff;
out->data[out->pos + 2] = (i >> 8) & 0xff;
out->data[out->pos + 3] = i & 0xff;
out->pos += 4;
}

Answer 2

Of course I initialize my structure with this function :
void initDataOutput(DataOutput *out)
{
out->data = NULL;
out->pos = 0;
}
But i can't do that :
DataOutput out;
char tmp[4];
out.data = tmp;
out.pos = 0;
Because in my code i could write things like that for example :
DataOutput out;
writeInt(&out, 1);
writeString(&out, "Hello");
sendData(&out);
where the int is a kind of packet id and "Hello" is the connexion message, but if it's an other id it's not the same information in out->data
Oh sorry and I don't understand what you mean when you say : avoid shifting signed integers and chars
I must use an unsigned char *data in my DataOutput structure ?