Parsing command line input string into char array - c

Question

I am trying to write a C program where the input from command line argument is in the form 0x1234aabb.
This definitely comes in the program as a char *[].
Now, I want to store this input in the form of char a[]={0x12, 0x34, 0xaa, 0xbb}.
Please help.

Answer 1

If you know the string is always a fixed length (e.g. 10 characters as in your example), then you can split the string into four equal parts each three characters long (two for the digits and one for the terminator), and then use strtoul on each part. If the argument is less than ten characters, only fill in the relevant parts and keep the other as "00".

Answer 2

What I would firstly do is use strtoul with the base argument of 16 (hex) to parse the string as an integer. unsigned long is guaranteed to be at least 32-bits in size, so if all your numbers have 8 hexadecimal digits (4 bytes) it should be a suitable type.
Once you have your number, bit shift and bit-wise AND the number 4 times to extract each of the 4 bytes that make up the number. Here's an example:
#include <stdlib.h>
#include <stdio.h>
#define NUM_BYTES 4
int main(void)
{
char *input = "0x1234aabb";
unsigned long num = strtoul(input, NULL /* TODO: error checking */, 16);
unsigned char bytes[NUM_BYTES];
unsigned i;
printf("The number is: 0x%lx\n", num);
/* bytes[0] = LSB, bytes[NUM_BYTES - 1] = MSB */
for (i = 0; i < NUM_BYTES; i++) {
bytes[i] = (num >> 8*i) & 0xFF; /* byte = 8 bits, 0xFF = max byte value */
printf("Byte %u: 0x%x\n", i+1, bytes[i]);
}
return 0;
}
Output:
The number is: 0x1234aabb
Byte 1: 0xbb
Byte 2: 0xaa
Byte 3: 0x34
Byte 4: 0x12

Answer 3

Try something like this to parse input of an arbitrary length:
#include <stdio.h>
int main(int argc, char** argv)
{
char *input;
int len;
int num;
char *a;
int i;
int tmp;
if (argv < 2)
{
printf("No input specified!\n");
return 0;
}
input = argv[1]; // "0x1234aabb"
if (strncmpi(input, "0x", 2) != 0)
{
printf("Bad input!\n");
return 0;
}
printf("Input: %s\n", input);
input += 2;
len = strlen(input);
num = (len / 2) + (len % 2);
if (num < 1)
{
printf("Bad input!\n");
return 0;
}
printf("Number of bytes: %d\n", num);
a = (char*) calloc(num, sizeof(char));
if (a == NULL)
{
printf("Cannot allocate memory for bytes!\n");
return 0;
}
for (i = 0; i < num; ++i)
{
if (sscanf(input, "%2x", &tmp) != 1)
{
printf("Byte %d: Illegal byte value '%02s'\n", i+1, input);
break;
}
a[i] = (char) tmp;
printf("Byte %d: 0x%02x\n", i+1, a[i]);
input += 2;
}
free(a);
return 0;
}