4 ms·

Could you explain (or link to a good resource about) what pointers really are? I'm surprised that they aren't just ints that point to the address of a variable.

enriquto 6y ago
In all modern implementations that I know of, pointers are effectively ints. But the formal definition of the C language takes great care in not saying that, allowing for implementation to chose how to implement pointers. In some older systems, for example in 16-bit MS-DOS with "high memory", pointers were not really ints, but pairs of ints (called the segment and the offset, if I recall correctly) whose mapping to memory positions was overlapping, i.e., non unique, you could have very different representations for the same memory position. I guess the gcc port to MSDOS by DJ Delorie had to deal with these issues somehow. But it is possible to imagine completely exotic CPU architectures, where there is a separate type of register that can only be used for pointers and it is impossible to convert to int; you could still write C compilers for that architecture and be 100% conforming to the standard.
- Gibbon1 6y ago
  If I remember 16 bit C compilers let you choose a couple of different memory models. I remember 3, data and code fits in 16bits. data is 16 bits and code is segment and offset. And both pointers are segment and offset. I think there were more, especially for systems with 'high memory' Most processors a pointer just gets stuffed into a register and then that's used by a assembly instruction to read/write a memory location. But it's certainly possible to have systems where it's not so simple.
saagarjha 6y ago
They are variables that store an address. Usually they are of integer-ish thing internally, but this is in no way required, just like C has no need to run on a machine with linear memory. You could have a variable to memory hash table backing all your objects, for example, with a pointer that is a bucket or something.
m463 6y ago
This is the kind of thing that you need a significant other to straighten out. "Hey!" (smacks on head) "It's an int that points to a variable." "Now get off the computer and take out the trash without talking about garbage collection!" "but it could be an int32_t and sometimes..." (smack)
steveklabnik 6y ago
This is what you want to read: https://blog.regehr.org/archives/1621 https://blog.regehr.org/archives/1621

enriquto 6y ago

In all modern implementations that I know of, pointers are effectively ints. But the formal definition of the C language takes great care in not saying that, allowing for implementation to chose how to implement pointers. In some older systems, for example in 16-bit MS-DOS with "high memory", pointers were not really ints, but pairs of ints (called the segment and the offset, if I recall correctly) whose mapping to memory positions was overlapping, i.e., non unique, you could have very different representations for the same memory position. I guess the gcc port to MSDOS by DJ Delorie had to deal with these issues somehow. But it is possible to imagine completely exotic CPU architectures, where there is a separate type of register that can only be used for pointers and it is impossible to convert to int; you could still write C compilers for that architecture and be 100% conforming to the standard.

Gibbon1 6y ago
If I remember 16 bit C compilers let you choose a couple of different memory models. I remember 3, data and code fits in 16bits. data is 16 bits and code is segment and offset. And both pointers are segment and offset. I think there were more, especially for systems with 'high memory' Most processors a pointer just gets stuffed into a register and then that's used by a assembly instruction to read/write a memory location. But it's certainly possible to have systems where it's not so simple.

saagarjha 6y ago

They are variables that store an address. Usually they are of integer-ish thing internally, but this is in no way required, just like C has no need to run on a machine with linear memory. You could have a variable to memory hash table backing all your objects, for example, with a pointer that is a bucket or something.

m463 6y ago

This is the kind of thing that you need a significant other to straighten out. "Hey!" (smacks on head) "It's an int that points to a variable." "Now get off the computer and take out the trash without talking about garbage collection!" "but it could be an int32_t and sometimes..." (smack)

steveklabnik 6y ago

This is what you want to read: https://blog.regehr.org/archives/1621 https://blog.regehr.org/archives/1621