Difference between revisions of "C programming/Variable Lengt Arrays VLA"
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Latest revision as of 07:02, 1 December 2022
From C99 variable length arrays was introduced in c.
Problems
VLA use the stack as placeholder for the array:
- In the function read_and_process_vla() - the stack is used as storage for the array. If the array is greather than the remaining stack size - stack overflow occurs. Check stack size on Linux with the ulimit -a command
- In the function read_and_process_novla() - the heap through malloc() is used.
Be careful when using VLA
1 #include <stdio.h>
2 #include <stdlib.h>
3
4
5 float process(int n, float *vals) {
6 for (int i=0; i < n; i++) {
7 printf("%f ", vals[i]);
8 }
9 printf("\n");
10 }
11
12 float read_and_process_vla(int n) {
13 float vals[n];
14 for (int i = 0; i < n; ++i)
15 vals[i] = i;
16 return process(n, vals);
17 }
18
19 float read_and_process_novla(int n) {
20 float (*vals)[n] = malloc(sizeof(float[n]));
21 for (int i = 0; i < n; ++i)
22 (*vals)[i] = i;
23 float ret = process(n, *vals);
24 free(vals);
25 return ret;
26 }
27 int main(void) {
28 printf("========================= Using VLA =========================\n");
29 read_and_process_vla(10);
30 read_and_process_vla(20);
31 //read_and_process_vla(10000000); // Error if stacksize not big enough...
32 printf("======================= not using VLA =======================\n");
33 read_and_process_novla(10);
34 read_and_process_novla(20);
35 read_and_process_novla(10000000);
36 return(0);
37 }