return multiple values from a function in C [duplicate]

Answer 1

If you want to malloc inside the function, you need to pass the address to the outside pointer since ch inside the function is only a local variable, and changing it doesn't affect the outside cm variable

void build(char **ch){
   *ch = malloc(30*sizeof(char));
   strcpy(*ch, "I am a good guy");
}

void main(){
   char *cm;
   build(&cm);
   printf("%s\n", cm);
}

But better don't malloc inside the function, instead just write what you want to the area pointed to by the pointer. This is the common way when you need to provide a buffer to get data in C. In this way users will have the choice to allocate memory their own, or just use a local buffer and don't need to free memory after that like you've just forgotten in your example

void build(char *ch){
   strcpy(ch, "I am a good guy");
}

void main(){
   char *cm1;
   cm1 = malloc(30*sizeof(char));
   build(cm1);
   printf("%s\n", cm1);

   char cm2[30];
   build(cm2);

   printf("%s\n", cm2);   
   free(cm1);           // don't forget this
}

Answer 2

This is because in C, you cannot reassign the memory address of the pointer because it is passed by value.

If you really want to do this, you must pass the address of the pointer into the build function.

See: Passing pointer argument by reference under C?

Answer 3

build() take the pointer ch by value, i.e. a copy of the pointer is passed to the function. So any modifications you make to that value are lost when the function exits. Since you want your modification to the pointer to be visible in the caller's context, you need to pass a pointer to pointer.

void build(char **ch){
   *ch = malloc(30*sizeof(char));
   strcpy(*ch, "I am a good guy");
}

Also, you don't need to pass pointers to a function because you need to return multiple values. Another option is to create a struct that contains the values you wish to return as members, and then return an instance of said struct. I'd recommend this approach over the first if the values are related and it makes sense to package them together.

---

Here's a re-listing of your code after fixing bugs:

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

void build(char **ch){
   *ch = malloc(30 * sizeof(char));
   strcpy(*ch, "I am a good guy");
}

int main() {     // main must return int
   char *cm;
   build(&cm);   // pass pointer to pointer
   printf("%s\n", cm);
   free(cm);     // free allocated memory
   return 0;     // main's return value, not required C99 onward
}