Supposed to return a pointer but compiles fine

Question

typedef struct person {
    int id;
    char* name;
} Person;

//constructor like function
Person* New_Person(int id,char *name){
    Person* p = malloc(sizeof(Person));
    p->id = id;
    p->name = name;
    //return p; 
}

int main(){
    for(int i=0;i<10;i++){
        Person* p = New_Person(i,"abcd");
        printf("id:%d name:%s \n",p->id,p->name);
    }
    printf("DONE\n");
    return 0;
}

New_Person is like a constructor and its supposed to return a pointer to the newly allocated person, if i comment the return statement in the New_Person function i still get the same behavior, can someone please explain to me why it compiles and gives the same behavior without a return statement?

You are just lucky since you are getting expected output by having undefined behavior in your code. I doubt how the compiler is not throwing warning that you are not returning from the function.. — Gopi, Mar 05 '15 at 14:14
@Gopi i really don't get it i forgot the return statement by accident and it still worked — FPGA, Mar 05 '15 at 14:15
possible duplicate of [Function returns value without return statement](http://stackoverflow.com/questions/4644860/function-returns-value-without-return-statement) — Anto Jurković, Mar 05 '15 at 14:25
Look for "undefined behavior". It means, anything can happen, and a compiler doesn't need to tell you. Using the return value of a function which doesn't return is undefined. Using proper warning settings catches some instances (`-Wall` does here, with gcc, for example). — mafso, Mar 05 '15 at 14:28

score 4 · Answer 1 · answered Mar 05 '15 at 14:29

What you have is undefined behavior.

The pointer p within main() is never initialized without return from function New_person(). So using uninitialized variable lead to undefined behavior.

You are allocating some memory on heap and the address returned by malloc() is never known to the pointer p within main(). So you are accessing some random memory and for your luck the random memory is the one allocated by you on heap.

score 1 · Accepted Answer · answered Mar 05 '15 at 14:28

I got the same behavior, so I disassembled your constructor:

0000000000000000 <New_Person>:
 0: 55                      push   %rbp
 1: 48 89 e5                mov    %rsp,%rbp
 4: 48 83 ec 20             sub    $0x20,%rsp
 8: 89 7d ec                mov    %edi,-0x14(%rbp)
 b: 48 89 75 e0             mov    %rsi,-0x20(%rbp)
 f: bf 10 00 00 00          mov    $0x10,%edi
14: e8 00 00 00 00          callq  19 <New_Person+0x19>
19: 48 89 45 f8             mov    %rax,-0x8(%rbp)
1d: 48 8b 45 f8             mov    -0x8(%rbp),%rax
21: 8b 55 ec                mov    -0x14(%rbp),%edx
24: 89 10                   mov    %edx,(%rax)
26: 48 8b 45 f8             mov    -0x8(%rbp),%rax
2a: 48 8b 55 e0             mov    -0x20(%rbp),%rdx
2e: 48 89 50 08             mov    %rdx,0x8(%rax)
32: c9                      leaveq 
33: c3                      retq

Looks like your pointer is being stored in the rax register which happens to be the register where function return values are stored (for integer and pointer types).

Supposed to return a pointer but compiles fine

2 Answers2