Thread: Does C have a GOTO function

I'm trying to learn C, then I'll learn C++. What I know now is some VB and QB.

I know that in QB, I could do something like

if a < b then goto AisSmaller

AisSmaller:
Print "A is smaller than B"
END

Is there anyway in C, to tell it to go to a specific place if a certain condition is TRUE? I know you can always evaluate a condition and then use brackets, but this could get repetive you know?

Hey thanks for the syntax for goto, but maybe I won't use it if it's so bad

I am just starting, and you are right, I want to learn the right way up front....but those other examples you give...I don't get.

For instance, you know how you can use brackets right, like:

Code:

if a < b 
          {
               printf ("blah blah")
                a = j + k
           }

Ok, now that seems fine, now lets say that later in the program, I want it to do something like this:

Code:

if d < c
         {
              printf ("blah blah")
              a = j + k
           }

In other words, if either A is less than b, or d is less than c, I want it to execute the same exact code, but it seems foolish to have to type the exact same code ("printf and a=j+k") over and over in brackets everytime, when I need a way to simply refer it to a certain place that already has the specified code, know what I mean?

As usual, you guys are very helpfull.

Ok, I think that I will stick to brackets most times, but can you please tell me how I can "call a function". I know I should know this, but I don't as of yet!

That way, I can do it in the future if I need to (call a function that is)

You shouldn't assume 'goto' is evil.

In some cases, the use of a goto can make code more efficient, and more readable.

If you want to create a single, local exception handler within a C function (not C++), the most practical way is with the use of goto. Consider the following code segment:

Code:

int create_client_socket(struct sockaddr_in *addr)
{
  int fd;

  if ((fd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
    goto exception;

  if (connect(fd, addr, sizeof(*addr)) == -1)
    goto exception;

  return fd;

  exception:
    fprintf(stderr, "%s: %s", __func__, strerror(errno));
    return -1;
}

Remember this is only an example (true error handling not shown). I would argue that this is more readable than the alternative version; which would include redundant fprintf's at each possible error stage.

You could have a default error handling routine - but sometimes you need to handle error's specifically. The above code really represents what you would call an 'exception handler' in other languages.

The generated code for the above routine may also be less than that of the redundant fprintf counterpart.

Goto is not a purist's recommendation - but sometimes it does have its uses. If you want further proof, try viewing the linux kernel source.

> Yes, that's what blackrat said :P You don't need to use gotos, and they make code slower and harder to read.

It does not make it necessarily slower. It is also possible to produce very slow code without using even one goto in your code. But it can make your code hard to read, debug and maintain if you use a lot of goto's in your code. As already said, it is not necessary to use goto in C.

> The following is quite readable for most C programmers and does not rely on a goto...

Personally I'd prefer to have only one place in a function where a return is done, at the end of the function. In smaller functions it may probably be not a big problem to have more than one return, but I found that especially in large functions it is hard to read a function with lots of returns. Often in such situation the number of returns can be reduced by breaking the function op in smaller functions. It may be a bit more inefficient in run-time, but if the function is not called very often, then the inefficiency is very small and can be ignored in the total process.

Code:

int create_client_socket(struct sockaddr_in *addr)
{
    int fd = -1;
    int retval = -1;

    fd = socket (AF_INET, SOCK_STREAM, 0);
    if (fd != -1)
    {
        retval = connect (fd, addr, sizeof (*addr);
    }
    
    if (-1 == retval)
    {
        fprintf (stderr, "%s: %s", __func__, strerror (errno));
    }

    return retval;
}

Originally posted by WaltP
The following is quite readable for most C programmers and does not rely on a goto...

Code:

int create_client_socket(struct sockaddr_in *addr)
{
    int fd;

    if ((fd = socket(AF_INET, SOCK_STREAM, 0)) != -1)
    {
        if (connect(fd, addr, sizeof(*addr)) != -1)
        {
            return fd;
        }
    }
    fprintf(stderr, "%s: %s", __func__, strerror(errno));
    return -1;
}

Yuk. As I said, what I posted was purely an example. Who wants four levels of nesting when it comes to writing the server routine? Nesting reduces readability.

Also, #def'ing ERROR_VAL is superfluous. Error values from system calls don't change - constants are acceptable.

Code:

int create_client_socket(struct sockaddr_in *addr)
{
    int fd = -1;
    int retval = -1;

    fd = socket (AF_INET, SOCK_STREAM, 0);
    if (fd != -1)
    {
        retval = connect (fd, addr, sizeof (*addr);
    }
    
    if (-1 == retval)
    {
        fprintf (stderr, "%s: %s", __func__, strerror (errno));
    }

    return retval;
}

Why on earth would you delay the obvious? You're making an extra comparison that you don't need. If you're ready to return, RETURN. If you find it difficult to comprehend multiple returns in a function, you probably shouldn't be writing code in C.

P.S. If you're trying to avoid the old (var=num) expression in a test, don't you think that by writing a constant before an equality operator you're already completely aware of what you're trying to avoid?