如何在C中使用memset为Struct成员设置随机数的随机小写字符

Question

I am forced to use memset and drand48() to set a random number (2 - 7) of random characters that are lower case letters ('a' to 'z'). My code returns non ASCII characters and I am not sure why.

struct Record {
    int seqnum;
    float threat;
    unsigned int addrs[2];
    unsigned short int ports[2];
    char dns_name[NUMLTRS];
};

My code is in a for loop:

memset(rec_ptr[i].dns_name, (char)((122 * drand48()) + 97), 
((sizeof(char) * 7) * drand48()) + (sizeof(char) * 2));

Answer 1

My code returns non ASCII characters and I am not sure why.

Wrong scale used to generate lower case letters.

(122 * drand48()) + 97 converted to an integer type can readily make 122 different values. [97...218]. This is outside the ASCII range of [0...127] .

---

How do I set a random number of random lowercase character ...

drand48() provides a random value [0...1.0). Scale by 26 and truncate to get 26 different indexes.

int index = (int) (drand48()*26);  // 0...25

Pedantic code would be concerned about the few random values that may round the product to 26.0

if (index >= 26) index = 26 - 1;
int az = index + 'a';
// or look up in a table if non-ASCII encoding might be used
//        12345678901234567890123456
int az = "abcdefghijklmnopqrstuvwxyz"[index];

Selecting a random length would use the same thing, but with NUMLTRS instead of 26.

int length = (int) (drand48()*NUMLTRS); 
if (index >= NUMLTRS) index = NUMLTRS -1;

---

... to a Struct member using memset in C

It is unclear if dns_name[] should be all the same, or generally different letters.

struct Record foo;
if (all_same) [
  memset(foo.dns_name, az, length);
} else {
  for (int i = 0; i < length; i++) {
    int index = (int) (drand48()*26);  // 0...25
    if (index >= 26) index = 26 -1;
    int az = index + 'a';
    foo.dns_name[i] = az;  // Does not make sense to use memset() here
  }
}

---

Lastly, if dns_name[] is meant to be a string for ease of later use, declare with a +1 size

dns_name[NUMLTRS + 1];

// above code

foo.dns_name[NUMLTRS] = '\0'; // append null character
printf("dna_name <%s>\n", foo.dns_name);