Generating random string unsigned char in C

Question

I want to generate a random string text of length 100 with the code below, then to verify that I print the length of the variable text but sometimes that is less than 100. How can I fix that?

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

int main() {
    int i, LEN = 100;
    srandom(time(NULL));
    unsigned char text[LEN];
    memset(text, 1, LEN);
    for (i = 0; i < LEN; i++) {
        text[i] = (unsigned char) rand() & 0xfff;
    }
    printf("plain-text:");
    printf("strlen(text)=%zd\n", strlen(text));

}

You may want to read up on ascii if you are not already familiar, there are a lot of ascii codes which are not atypical of a string — Joshua Byer, Apr 26 '15 at 23:32
Note: Is not `text[i] = (unsigned char) rand() & 0xfff;` the same as `text[i] = ((unsigned char) rand()) & 0xfff;`? So why the `& 0xfff`? — chux - Reinstate Monica, Apr 26 '15 at 23:33
Small points: 1) Unix `srandom()` goes with [`random()`](http://linux.die.net/man/3/srandom) and std C `srand()` works with `rand()`. 2) Consider `size_t LEN` rather than `int`. — chux - Reinstate Monica, Apr 26 '15 at 23:45

AlexD · Accepted Answer · 2015-04-26T23:30:10.440

5

Perhaps a random character 0 was added to the string, and then it is considered as the end of string by strlen.

You can generate random characters as (rand() % 255) + 1 to avoid zeros.

And at the end you have to zero-terminate the string.

LEN = 101; // 100 + 1
....
for (i = 0; i < LEN - 1; i++) {
    text[i] = (unsigned char) (rand() % 255 + 1);
}
text[LEN-1] = 0;

edited Apr 26 '15 at 23:30

answered Apr 26 '15 at 23:23

AlexD

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score 3 · Answer 2 · edited Apr 27 '15 at 00:58

I want to generate a random string text of length 100 with the code below, then to verify that I print the length of the variable text but sometimes that is less than 100. How can I fix that?

First of all, if you want to generate a string of length 100, you'll need to declare an array of size 101.
```
int i, LEN = 101;
srandom(time(NULL));
unsigned char text[LEN];
```

When you are assigning the characters from the call to rand, make sure that it is not 0, which is usually the null terminator for strings.

for (i = 0; i < LEN - 1; /* Don't increment i here */) {
    c = (unsigned char) rand() & 0xfff;
    if ( c != '\0' )
    {
       text[i] = c;

       // Increment i only for this case.
       ++i
    }
}

and don't forget to null terminate the string.

text[LEN-1] = '\0';

By not using `% some non-power-of-2`, this approach avoids a slight [bias](http://stackoverflow.com/a/13104569/2410359). +1. — chux - Reinstate Monica, Apr 26 '15 at 23:40

Generating random string unsigned char in C

2 Answers2