Reading/Parsing Command-Line Arguments in C; binary and decimal conversions -

i have write program in c reads , parses different command-line arguments have no idea start. have write usage:

usage: binary option size number option: -b number binary , output in decimal. -d number decimal , output in binary. size: -8 input unsigned 8-bit integer. -16 input unsigned 16-bit integer. -32 input unsigned 32-bit integer. -64 input unsigned 64-bit integer. number: number converted. 

other this, not sure how user input , go conversions. great!

you can take following code-snippet , work on there, please note:

1. i not see need make use of size argument. if argument essential exercise (or other reason behind question), you'll need think how want use it.

2. i not perform assertion on input number, code below assumes legal decimal input number when option = -d , legal binary input number when option = -d.

3. there more 1 way implement it, , code below merely example.

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#include <stdio.h> #include <string.h>  unsigned long long str_to_ull(char* str,int base) {     int i;     unsigned long long ull = 0;     (i=0; str[i] != 0; i++)     {         ull *= base;         ull += str[i]-'0';     }     return ull; }  void print_ull(unsigned long long ull,int base) {     if (ull/base > 0)         print_ull(ull/base,base);     printf("%d",ull%base); }  int main(int argc,char* argv[]) {     char* option;     char* size  ;     char* number;      unsigned long long number = 0;      if (argc < 4)     {         printf("missing input arguments\n");         return -1;     }      option = argv[1];     size   = argv[2];     number = argv[3];      if (strcmp(option,"-b") == 0)     {         number = str_to_ull(number,2);         print_ull(number,10);         return 0;     }      if (strcmp(option,"-d") == 0)     {         number = str_to_ull(number,10);         print_ull(number,2);         return 0;     }      printf("invalid input arguments\n");     return -1; }