Question concerning passing data from function using pointer

[david000]

Hi all!

I've got a question as how to pass data from one function to another using pointer. My example code:

#include <stdlib.h>
#include <string>
#include <iostream>
#include <fstream>
using namespace std;

void test(char *data, int *len);

int main(){
	char *data;
	int len;
	test(data,&len);	
	
	cout << "len="<<len<< " "<< data << "\r\n";
	
	free(data);
	return 0;
}

void test(char *data, int *len){
	*len=3;
	data=(char*) malloc(sizeof(char)* *len);
	data[0]='a';
	data[1]='b';
	data[2]='\0';
	
	cout << "\r\n" << data << "--ok\r\n";
}

Using g++ 2.cpp -o 2 && ./2 I get the following output:

ab--ok
len=3 ������������)�����t$1��E�D�E
                                    �D$��$���������
9�uރ�
                                                           [^_]Ë$Ð�U��S��������t1��Ћ��������u���[]Ð��U��S��
*** glibc detected *** ./2: munmap_chunk(): invalid pointer: 0x08048999 ***
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6(cfree+0x1bb)[0xb7cfe61b]
./2(__gxx_personality_v0+0x301)[0x8048931]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe0)[0xb7ca5450]
./2(__gxx_personality_v0+0x31)[0x8048661]
======= Memory map: ========
....
....
....
Aborted

What's wrong here?

I even tried passing the data using new/delete instead of malloc/free.
Thanks!

[jsmith]

You are passing the "data" pointer by value to test(), which means that while test() allocates memory and fills it out correctly, the pointer it created is not returned to the calling function.

You need

void test( char** data, int* len ) {
  // ...
}

int main() {
    char* data;
     int len;

    test( &data, &len );
}

[writetonsharma]

//catch it as double pointer.
void test(char **data, int *len){
	*len=3;
	*data=(char*) malloc(sizeof(char) * (*len));
	*(*data + 0)='a';
	*(*data + 1)='b';
	*(*data + 2)='\0';
	
	cout << "\r\n" << *data << "--ok\r\n";
}

[david000]

Thanks guys!!! :)