在C中返回数组：随机错误，垃圾值

Question

I'm trying to get this to work but I keep getting really weird errors, sometimes it executes without error, sometimes I get memory access violation errors, 7 of the returned values are always garbage and there's a printf that the program won't work with for some reason. I'm not good with C so I haven't the slightest clue what is going on.

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <string.h>

int gen_bp() {
  int min = 0;
  int max = 3;
  int r;
  r = (rand() % (max + 1 - min)) + min;
  return r;
}

int * gen_gene(int len) {
  int a;
  int * gene = malloc(len);
  int bp;
  srand( (unsigned)time( NULL ) );
  for( a = 0; a < len; a = a + 1 ){
    bp = gen_bp();
    printf("value of a: %i\n", bp); //if i remove this line, it crashes?!
    gene[a] = bp;
  }
  return gene;
}

int main()
{
  char codons[4] = {'G','T','A','C'};
  int genelen = 20;
  int counter;
  int * gene;
  gene = gen_gene(genelen);
  for( counter = 0; counter < genelen; counter++ ){
    printf("%i value of a: %i\n", counter, gene[counter]);
  }
  free(gene);
  return(0);
}

This is the output I get

value of a: 1
value of a: 1
value of a: 3
value of a: 0
value of a: 2
value of a: 1
value of a: 3
value of a: 3
value of a: 1
value of a: 2
value of a: 3
value of a: 0
value of a: 3
value of a: 1
value of a: 0
value of a: 2
value of a: 3
value of a: 2
value of a: 2
value of a: 0
0 value of a: 1
1 value of a: 1
2 value of a: 3
3 value of a: 0
4 value of a: 2
5 value of a: 1
6 value of a: 3
7 value of a: 3
8 value of a: 1
9 value of a: 2
10 value of a: 1635131449 // 10 to 16 are always garbage, and never change
11 value of a: 1702194273
12 value of a: 543584032
13 value of a: 891304545
14 value of a: 808661305
15 value of a: 892351281
16 value of a: 2570
17 value of a: 2
18 value of a: 2
19 value of a: 0

Sometimes it ends fine with 0 error, other times it crashes after the output. Absolutely not the slightest clue why.

Answer 1

You are reserving space for len bytes, but you want to reserve space for

int * gene = malloc(sizeof(int) * len);

or

int * gene = malloc(sizeof(*gene) * len);

And you forget to #include <time.h>

Answer 2

Using malloc directly is too error-prone; in your code you forgot to multiply with the element size.

Use a macro instead:

#define NEW_ARRAY(ptr, n) (ptr) = malloc((n) * sizeof (ptr)[0])

int *gene;
NEW_ARRAY(gene, len);