Your code seems to be mostly fine. You are only really mallocing the wrong amount. Here it is wth the corrections:

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

char* stringToBinary(char* s) {
    if(s == NULL) return 0; /* no input string */
    size_t len = strlen(s);
    char *binary = malloc(len*8 + 1); // each char is one byte (8 bits) and + 1 at the end for null terminator
    binary[0] = '\0';
    for(size_t i = 0; i < len; ++i) {
        char ch = s[i];
        for(int j = 7; j >= 0; --j){
            if(ch & (1 << j)) {
                strcat(binary,"1");
            } else {
                strcat(binary,"0");
            }
        }
    }
    return binary;
}

Sample runs:

"asdf"           => 01100001011100110110010001100110
"tester"         => 011101000110010101110011011101000110010101110010
"Happy New Year" => 0100100001100001011100000111000001111001001000000100111001100101011101110010000001011001011001010110000101110010

Or did you mean how to convert C string to binary representation?

Here is one solution which can convert strings to binary representation. It can be easily altered to save the binary strings into array of strings.

#include <stdio.h>

int main(int argc, char *argv[])
{
    if(argv[1] == NULL) return 0; /* no input string */

    char *ptr = argv[1];
    int i;

    for(; *ptr != 0; ++ptr)
    {
        printf("%c => ", *ptr);

        /* perform bitwise AND for every bit of the character */
        for(i = 7; i >= 0; --i) 
            (*ptr & 1 << i) ? putchar('1') : putchar('0');

        putchar('\n');
    }

    return 0;
}

Example input & output:

./ascii2bin hello

h => 01101000
e => 01100101
l => 01101100
l => 01101100
o => 01101111

The easiest way will be using a bit field:

struct code {
    unsigned opcode : 6;
    unisgned operand1 : 5;
    unisgned operand2 : 5;
    unisgned operand2 : 5;
} test_code;

Now you can simply assign to the different members:

test_code.opcode = 0x02;
test_code.operator1 = 0x01;
test_code.operator2 = 0x02;
test_code.operator3 = 0x03;

We show up two functions that prints a SINGLE character to binary.

void printbinchar(char character)
{
    char output[9];
    itoa(character, output, 2);
    printf("%s\n", output);
}

printbinchar(10) will write into the console

    1010

itoa is a library function that converts a single integer value to a string with the specified base. For example... itoa(1341, output, 10) will write in output string "1341". And of course itoa(9, output, 2) will write in the output string "1001".

The next function will print into the standard output the full binary representation of a character, that is, it will print all 8 bits, also if the higher bits are zero.

void printbincharpad(char c)
{
    for (int i = 7; i >= 0; --i)
    {
        putchar( (c & (1 << i)) ? '1' : '0' );
    }
    putchar('\n');
}

printbincharpad(10) will write into the console

    00001010

Now i present a function that prints out an entire string (without last null character).

void printstringasbinary(char* s)
{
    // A small 9 characters buffer we use to perform the conversion
    char output[9];

    // Until the first character pointed by s is not a null character
    // that indicates end of string...
    while (*s)
    {
        // Convert the first character of the string to binary using itoa.
        // Characters in c are just 8 bit integers, at least, in noawdays computers.
        itoa(*s, output, 2);

        // print out our string and let's write a new line.
        puts(output);

        // we advance our string by one character,
        // If our original string was "ABC" now we are pointing at "BC".
        ++s;
    }
}

Consider however that itoa don't adds padding zeroes, so printstringasbinary("AB1") will print something like:

1000001
1000010
110001

I have tried to modify your solution with minimal changes to make it work. There are elegant solutions to convert Integer to Binary for example using shift operators.

One of the main issue in the code was you were using character instead of character array. i.e char str; instead of char str[SIZE]; Also you were performing string operations on a single character. Additionally, iostream header file is for C++.

There is room for lot of improvements in the solution posted below (I only made your code work with minimal changes).

My suggestion is to make your C basics strong and approach this problem again.

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

void converttobinary(int n, char *op)
{
    int i;
    int a[8];
    for (i = 0; i < 8; i++)
    {
        a[i] = n % 2;
        n = (n - a[i]) / 2;
    }
    for (i = 7; i >= 0; i--)
    {
        op[i]=a[i];
    }

}
int main()
{
    int n,i;
    char str[8];
    n = 8;
    converttobinary(n,str);
    for (i = 7; i >= 0; i--)
    {
        printf(" %d ",str[i]);
    }
    return 0;

}