From where do the initial values of the variable length array (VLA) in [c] come?

Question

example c code:

#include <stdio.h>
int main (int argc, char** args) {

    int k = 8;
    int sendbuffer[k]; // VLA

    for (int n = 0; n < k; n++) {
        printf("sendbuffer[%i]: %i \n", n, sendbuffer[n]);
    }

    return 0;
}

example output:

sendbuffer[0]: 1 
sendbuffer[1]: 0 
sendbuffer[2]: 1583871280 
sendbuffer[3]: 32767 
sendbuffer[4]: 22544384 
sendbuffer[5]: 1 
sendbuffer[6]: 1713234504 
sendbuffer[7]: 32767

From where do the numbers in sendbuffer[] come from? And why are sendbuffer[2,4,6] the only ones that change when running the code again?

I'm using clang compiler on OSX El Capitan (compiling with gcc example.c)
Apple LLVM version 7.3.0 (clang-703.0.31)
Target: x86_64-apple-darwin15.4.0
Thread model: posix

If is a different behavior with other compiler/OS, I'd like to hear about these cases too.

Undefined behavior. The values are whatever happens to be in the memory. — Michael Burr, May 08 '16 at 20:07

score 3 · Answer 1 · answered May 08 '16 at 20:23

This isn't just for VLAs; the behavior above is for any variable declaration. If you declare a variable, for example

int a;

This allocates sizeof(int) bytes on the stack, at whichever address the system chooses. The memory there is uninitalized; it is whatever happens to be there at the time. It is up to the programmer to initialize the memory.

score 3 · Answer 2 · answered May 08 '16 at 21:48

Online C 2011 standard, 6.7.9/10: "If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate..."

Unless you declare it at file scope or with the static keyword¹, a variable won't be initialized to any specific value; the contents will be whatever was last written to that memory location.

^{Neither of which apply to VLAs; they cannot be declared static or at file scope, nor can they have an explicit initializer.}

score 1 · Answer 3 · answered May 08 '16 at 20:08

These values are called garbage values, they're whatever that was in memory when the array was created interpreted as ints.

Your program just says to the OS: "gimme that part of memory pls" and so the OS does, giving it to your program without changing the contents.

From where do the initial values of the variable length array (VLA) in [c] come?

3 Answers3