declare function inside fork()

**MK27** · 10-10-2008

Is there anything wrong with declaring a function inside a forked block, eg:

Code:

pid=fork();
if (pid==0) {
      void newfunc() {
              [....]
      }
      [...]
      newfunc();
}

it seems to work, but just because no one stopped you doesn't make it right -- so I thought I'd ask and find out what kind of trouble may await.

**eXeCuTeR** · 10-10-2008

EDIT: Wrong thread.

**Salem** · 10-10-2008

It's nothing to do with fork, it's just illegal C code.
C doesn't allow nested functions, you're relying on a gcc extension.

**MK27** · 10-10-2008

Originally Posted by Salem

It's nothing to do with fork, it's just illegal C code.
C doesn't allow nested functions, you're relying on a gcc extension.

So as long as it's compiled on gcc that's okay?

**Elysia** · 10-10-2008

No, don't rely on it.
Local functions are illegal both in C and C++ and frankly, I don't see it as good solution to anything - even if allowed.
You should write standard and portable code, first and foremost, without extensions.

**Codeplug** · 10-10-2008

Why not use a legal construct?

Code:

typedef void (*fork_func)(void);

pid_t fork_run(fork_func pfn)
{
    pid_t pid = fork();
    if (pid == 0)
        pfn();
    return pid;
}

gg

**MK27** · 10-10-2008

Originally Posted by Elysia

No, don't rely on it.
Local functions are illegal both in C and C++ and frankly, I don't see it as good solution to anything - even if allowed.
You should write standard and portable code, first and foremost, without extensions.

Well, gcc being my sole compiler, I wasn't aware I was using an extension (and without digging, I'm not really sure what one is). The reason for it is that the child performs operations on it's own variables, and I can't declare the function outside the child unless I globalize those variables. So perhaps it's just about tidyness. But suddenly I wonder -- is it also illegal to nest variable declarations that way?

Code:

pid=fork();
if (pid==0) {
      int x;
      char y;
}

**robwhit** · 10-10-2008

declaring variables at the top of any block is legal.

Code:

void func()
{
    /* code */
    {
        int x, y; /* legal */
        /* code */
    } /* x and y go out of scope here */
}

**Salem** · 10-10-2008

> Well, gcc being my sole compiler, I wasn't aware I was using an extension (and without digging, I'm not really sure what one is).
gcc -W -Wall -ansi -pedantic prog.c
Will tell you most things which are non-portable.

> But suddenly I wonder -- is it also illegal to nest variable declarations that way?
So long as they're the first thing inside any enclosing brace, that's just fine.

> The reason for it is that the child performs operations on it's own variables, and I can't declare the function outside the child unless I globalize those variables.
The scope of your variables doesn't matter.
As soon as you fork(), you've got your own separate copies no matter what scope they're declared in.

**Elysia** · 10-10-2008

C99 and C++ does not have the restriction that variables must be at the start of a block anyway. You don't need to follow the poor ruleset of C89. GCC fully supports C99, too.

**robwhit** · 10-10-2008

Partial support, actually. http://gcc.gnu.org/gcc-4.3/c99status.html

**Elysia** · 10-10-2008

Oh well, better than nothing, at least.

**MK27** · 10-10-2008

Originally Posted by Salem

>
gcc -W -Wall -ansi -pedantic prog.c
Will tell you most things which are non-portable.

Interesting. The nested function is just a warning.

One of the repeated warnings in this which is sort of befuddling is:

warning: too many arguments for format

Because I have three arguments to sprintf. How should I take that?

**robwhit** · 10-10-2008

> Interesting. The nested function is just a warning.

Add -Werror

Code?

**~~master5001~~** · 10-10-2008

warning: too many arguments for format

So subtle... if only gcc would output it more like this

warning: too many arguments for format

It means you are doing something like:

Code:

sprintf("&#37;d %d", &x, &y, &z);

Surely it isn't as bad as doing this:

Code:

sprintf("%d %d %d", &x, &y);

But it means you likely copied and pasted carelessly, or your logic somehow got muddled during the code writing process.

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