C99 Variable-length arrays Wikipedia example

Question

I came across this example of C99 Variable-length arrays on Wikipedia:

float read_and_process(int n)
{
    float vals[n];

    for (int i = 0; i < n; i++)
        vals[i] = read_val();
    return process(vals, n);
}

Is this incorrect? I was under the impression that variable-length arrays are still just pointers which means the above code is passing the expired pointer vals to the process(...) function.

Arrays are not pointers. They just decay to pointers -- annoyingly easily, some would say. — cHao
– cHao, Commented Dec 29, 2011 at 5:47
+1 for "Arrays are not pointers". See also section 6 of the comp.lang.c FAQ — Keith Thompson
– Keith Thompson, Commented Dec 29, 2011 at 6:36

Paul · Accepted Answer · 2011-12-29 05:47:45Z

7

The pointer hasn't expired. It is a pointer to valid memory until the end of the function read_and_process. Which means it is still defined when process is called.

This would be an example of invalid usage:

float read_and_process(int n)
{
    float vals[n];

    for (int i = 0; i < n; i++)
        vals[i] = read_val();
    return vals;
}

answered Dec 29, 2011 at 5:47

Paul

142k28 gold badges285 silver badges272 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Jeff Over a year ago

Oh ok that makes sense. As long as you're growing the stack the memory address must still be valid.

Keith Thompson Over a year ago

@Jeff: A better way to look at it: the array (or any local non-static object) continues to exist until the function that created it returns. (Actually it's tied to the enclosing block, but it's the same in this case.) The lifetime of a local object is a span of time during execution, not a region of program text. Calling process doesn't terminate the execution of read_and_process, so the array still exists.

sarnold · Accepted Answer · 2011-12-29 05:57:58Z

1

Don't forget that the stack frame that contains all of read_and_process()'s automatic variables, including float vals[n], is still valid and in memory when process() is executed.

answered Dec 29, 2011 at 5:57

sarnold

105k23 gold badges187 silver badges243 bronze badges

Collectives™ on Stack Overflow

C99 Variable-length arrays Wikipedia example

2 Answers 2

2 Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Related