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cvariable-length-array

Dynamic arrays in C without malloc?

I've always wondered how I could get away with this:

int main(int argc, char **argv) {
    printf("%p %s %d\n", &argv[1], argv[1], strlen(argv[1]));
    char copy[strlen(argv[1]) + 1];
    strcpy(copy, argv[1]);
    printf("%p %s %d\n", &copy, copy, strlen(copy));
    return 0;
}

The char array copy gets allocated anyway and the program runs fine, printing out the original and the copy. And Valgrind doesn’t complain about anything.

I thought dynamic arrays weren’t possible in C without malloc. Was I wrong?

Solution

  • This is a C99 feature and could be implemented on prior versions by the compiler.

    Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C90 mode and in C++. These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited.