Array with undefined length in C

Question

i was watching an exercise in my textbook that says: Create a C program that take from the keyboard an array with length "N".

The question is: In C language, how can i create an undefined length array?

Thank you all.

Answer 1

Do not create an array of undefined length.

After getting the needed length N, if C99 use a VLA (Variable Length Array)

int A[N];

... or allocate memory

int *A = malloc(sizeof *A * N);
...
// use A
...
free(A);

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[Edit]

Good to add validation on N before proceeding. Example:

if (N <= 0 || N >= Some_Sane_Upper_Limit_Like_1000) return;