Getting char array value from a method by using pointer. - c

Question

I have been trying to get the following to work:
My goal is to use pointers in main() to access elements created in a method().
// takes in address of pointer
int method(char** input) {
char *buffer = malloc(sizeof(char)*10);
buffer[0] = 0x12;
buffer[1] = 0x34;
buffer[2] = 0xab;
*input = & buffer;
printf("%x\n", *buffer); // this prints 0x12
printf("%x\n", &buffer); // this prints address of buffer example: 0x7fffbd98bf78
printf("%x\n", *input); // this prints address of buffer
return 0;
}
int main(){
char *ptr;
method(&ptr);
printf(%p\n", ptr); // this prints address of buffer
//this does not seem to print out buffer[0]
printf(%x\n", *ptr);
}
I want to print each element of buffer values, as created by the method() by using ptr. Any suggestions on how I can go about doing this?
I am not sure if I am misunderstanding something, but I thought ptr points to address of buffer. Thus, dereferencing would give me buffer[0]?
Thank you.

Answer 1

This a fixed & commented version of your code. Ask in the comments if there is smth. you don't understand.
#include <stdio.h>
#include <stdlib.h>
// takes in address of pointer
//Hex: 0xab is larger than the max value of a signed char.
//Most comilers default to signed char if you don't specify unsigned.
//So you need to use unsigned for the values you chose
int method(unsigned char** input) { //<<< changed
unsigned char *buffer = malloc(sizeof(char)*10);
//Check for malloc success <<< added
if(!buffer)
exit(EXIT_FAILURE);
buffer[0] = 0x12;
buffer[1] = 0x34;
buffer[2] = 0xab;
//I recommend not to mix array notation and pointer notation on the same object.
//Alternatively, you could write:
*buffer = 0x12;
*(buffer + 1) = 0x34;
*(buffer + 2) = 0xab;
//buffer already contains the address of your "array".
//You don't want the address of that address
*input = buffer; //<<< changed (removed &)
printf("%x\n", *buffer); // this prints 0x12
//Not casting &buffer will likely work (with compiler warnings
//But it is better to conform. Either use (char *) or (void *)
//<<< added the cast for printf()
printf("%p\n", (char *)&buffer); // this prints address of buffer example: 0x7fffbd98bf78
printf("%p\n", *input); // this prints address of buffer
return 0;
}
int main(){
unsigned char *ptr;
method(&ptr);
printf("%p\n", ptr); // this prints address of buffer
//this does not seem to print out buffer[0]
for(int i = 0; i < 3; i++){
//<<< changed to obtain content of buffer via ptr for loop.
unsigned char buf_elem = *(ptr + i);
printf("buffer[%d] in hex: %x\t in decimal: %d\n", i, buf_elem, buf_elem);
}
// Don't forget to free the memory. //<<< changed
free(ptr);
}