Concepts:
Command line arguments.

Text:
Implement a C program that receives through the command line a string as input. The program must transform the content of the string in uppercase letters and print it on the screen. The option -c <number> between the name of the program and the string is used to convert and print only the first <number> characters of the string.

Examples:

C:> prog_name.exe a_string_123
A_STRING_123
C:> prog_name.exe -c 4 a_string_123
A_ST

Soluzions:
Two solutions will be provided.

Solution 1:
In this first solution, less complex, no controls have been performed on command line parameters. The hypotesis is that the user of the program use it in a correct way.

argc_argv_1a.c

/*
Implement a C program that receives through the command line a string as input.
The program must transform the content of the string in uppercase letters and print it on the screen.
The option '-c <number>' between the name of the program and the string is used to convert 
and print only the first <number> characters of the string.
*/
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
 
#define LEN 50
 
 
int main(int argc, char *argv[]) {
    int i;
    char s[LEN+1];
    int string_length;
 
    /* Running the program as follows:
       prog_name.exe -c 4 sTriNg
       the variables *argv[] and argc contains the following values:
 
    argv[0]="prog_name.exe"
    argv[1]="-c"
    argv[2]="4"
    argv[3]="sTriNg"
    argc=4
 
    And the output of the program will be STRI
    */
 
 
    if(argc==4){ /* If the user has specified the option -c */
 
      string_length = atoi(argv[2]);
      strncpy(s, argv[3], string_length);
      s[string_length] = '\0';
 
    }else{ /* If the only argument is <string> */
 
      strcpy(s, argv[1]);
      string_length = strlen(s);
 
    }
 
    for(i=0; i<string_length; i++){
      s[i] = toupper(s[i]);
    }
 
    printf("The resulting string is: %s\n", s);
 
    return 0;
}

Solution 2:
In this second solution, all the needed controls on command line arguments have been performed. Unfortunately, at the expense of the program readability.

argc_argv_1b.c

/*
Implement a C program that receives through the command line a string as input.
The program must transform the content of the string in uppercase letters and print it on the screen.
The option '-c <number>' between the name of the program and the string is used to convert 
and print only the first <number> characters of the string.
*/
 
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
 
 
#define LEN 50
 
 
int main(int argc, char *argv[])
{
    int i;
    char s[LEN+1];
    int string_length;
 
    /* Running the program as follows:
       prog_name.exe -c 4 sTriNg
       the variables *argv[] and argc contains the following values:
 
    argv[0]="prog_name.exe"
    argv[1]="-c"
    argv[2]="4"
    argv[3]="sTriNg"
    argc=4
 
    And the output of the program will be STRI
    */
 
 
    /* Check on the number of command line arguments */
    if(argc!=2 && argc!=4) {
        printf("Error: the program must be executed with the following arguments\n");
        printf("%s (-c <number>)? <string>\n", argv[0]);
        exit(-1) ; /* As the return 1; command, but with exit() the program is killed also 
                   if executed inside a function */
    }
 
    if(argc==4) { /* If the user has specified the option -c */
 
      /* Control if argv[1] contains -c */
      if(strcmp(argv[1], "-c")!=0){
        printf("Error, the only available option is -c\n");
        exit(-2);
      }
 
      /* Control if the second arguments is a number */
      for(i=0; i<strlen(argv[2]); i++){
        if(isdigit(argv[2][i])==0){
          printf("Error: the argument of the -c option must be a number\n");
          exit(-3);
        }
      }
      string_length = atoi(argv[2]);
      /* string_length = atoi(argv[2]); can be substituted with the command */
      /* sscanf(argv[2], "%d", &string_length); */
      printf("STRING LENGTH: %d\n", string_length);
 
      /* Control if the number (i.e., the variable string_length) is positive */
      if(string_length < 0){
        printf("The number of the -c option must be positive or equal to 0\n");
        exit(-4);
      }
      /* If the actual string is shorter then string_length, set the value
         of string_length of the actual length of the string */
      if (string_length > strlen(argv[3])) string_length = strlen(argv[3]);
 
      /* Control if the string contained in argv[3] can be stored in s */
      if(strlen(argv[3])>LEN){
        printf("Error: <string> too long\n") ;
        exit(-5);
      }
      /* The string has been stored inside the array s (a better solution can exploit directly argv[3]) */
      strncpy(s, argv[3], string_length);
      s[string_length] = '\0';
 
    }else{ /* If the only argument is <string> */
 
      /* Control if the string contained in argv[3] can be stored in s */
      if(strlen(argv[1])>LEN){
        printf("Error: <string> too long\n") ;
        exit(-5) ;
      }
 
      /* Store the string */
      strcpy(s, argv[1]);
      string_length = strlen(s);
    }
 
    for(i=0; i<string_length; i++){
      if(!isprint(s[i])){
        printf("The string is not valid. Some characters are not printable\n");
        exit(-6);
      }
 
      s[i] = toupper(s[i]);
    }
 
    printf("The resulting string is: %s\n", s);
 
    return 0;
}