4 ms·

It’s a weakness of the fork()+exec() model, for sure. However, creating a fork_and_execve() API is extremely tricky. Just think of all the innumerable setup opt

It’s a weakness of the fork()+exec() model, for sure. However, creating a fork_and_execve() API is extremely tricky. Just think of all the innumerable setup options you would need to give it, e.g. what file handles should be closed or left open? What directory should it start in? What environment variables should be set - or cleared? And on and on and on… the flexibility of a separate fork() then exec() means you can set up the initial state of a new process exactly as you want, by doing whatever work is needed between the two calls. If you merge them into one, then you will never be able to encapsulate all of that.

saagarjha 5y ago
I mean, posix_spawn exists. It's a messy function, but its job is messy for exactly the reasons you describe. (FWIW, there are very few things you can legally perform between fork and exec.)
- marcan_42 5y ago
  Are there any things that are illegal between fork and exec? It is perfectly legal to exec whenever you want, and it it perfectly legal to fork whenever you want. I'm not aware of any requirements around the fork/exec sequence.
  - tsimionescu 5y ago
    For a single-threaded process, sure. But in a multi threaded context, almost any operation done in the child after fork() can royally mess up the system. There are many versions of libcs where malloc() after fork() is likely to deadlock between the parent and child processes, as they share the internal malloc() locks.
    - marcan_42 5y ago
      Ah, yes, multithreading. That always interacts poorly with old UNIX APIs...
  - jart 5y ago
    There's no restrictions in that sense. However the computer might not do what you intended if you were to do something like fputc('x',stdout);fork();fflush(0);
  - spacechild1 5y ago
    From the fork() man page: “After a fork() in a multithreaded program, the child can safely call only async-signal-safe functions (see signal-safety(7)) until such time as it calls execve(2)“ For example, you can't do any dynamic memory allocation or call a function that may allocate.
- nextaccountic 5y ago
  I think you mean between vfork and exec. The Linux manpage says: Standard description (From POSIX.1) The vfork() function has the same effect as fork(2), except that the behavior is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork(), or returns from the function in which vfork() was called, or calls any other function before successfully calling _exit(2) or one of the exec(3) family of functions.
  - jart 5y ago
    Note that modern POSIX recommends just calling fork() because "[vfork] was previously under-specified". The glibc man pages shouldn't say undefined since that's too strong a word. There is so much you can actually do in practice during vfork(). Pretty much all system calls are just as safe. It's mostly a question of whether or not your userspace libraries might clobber some global, due to how vfork puts the process in a state where implementation details that are normally private become part of the public surface area of these apis, and not many library authors are disposed to offer those kinds of assurances.
wmf 5y ago
A third option would be the ability to create a blank process, configure it, then exec within it. I think Mach allows this.

saagarjha 5y ago

I mean, posix_spawn exists. It's a messy function, but its job is messy for exactly the reasons you describe. (FWIW, there are very few things you can legally perform between fork and exec.)

marcan_42 5y ago
Are there any things that are illegal between fork and exec? It is perfectly legal to exec whenever you want, and it it perfectly legal to fork whenever you want. I'm not aware of any requirements around the fork/exec sequence.
- tsimionescu 5y ago
  For a single-threaded process, sure. But in a multi threaded context, almost any operation done in the child after fork() can royally mess up the system. There are many versions of libcs where malloc() after fork() is likely to deadlock between the parent and child processes, as they share the internal malloc() locks.
  - marcan_42 5y ago
    Ah, yes, multithreading. That always interacts poorly with old UNIX APIs...
- jart 5y ago
  There's no restrictions in that sense. However the computer might not do what you intended if you were to do something like fputc('x',stdout);fork();fflush(0);
- spacechild1 5y ago
  From the fork() man page: “After a fork() in a multithreaded program, the child can safely call only async-signal-safe functions (see signal-safety(7)) until such time as it calls execve(2)“ For example, you can't do any dynamic memory allocation or call a function that may allocate.
nextaccountic 5y ago
I think you mean between vfork and exec. The Linux manpage says: Standard description (From POSIX.1) The vfork() function has the same effect as fork(2), except that the behavior is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork(), or returns from the function in which vfork() was called, or calls any other function before successfully calling _exit(2) or one of the exec(3) family of functions.
- jart 5y ago
  Note that modern POSIX recommends just calling fork() because "[vfork] was previously under-specified". The glibc man pages shouldn't say undefined since that's too strong a word. There is so much you can actually do in practice during vfork(). Pretty much all system calls are just as safe. It's mostly a question of whether or not your userspace libraries might clobber some global, due to how vfork puts the process in a state where implementation details that are normally private become part of the public surface area of these apis, and not many library authors are disposed to offer those kinds of assurances.

wmf 5y ago

A third option would be the ability to create a blank process, configure it, then exec within it. I think Mach allows this.