Topics

• Commandline Arguments & GNU getopt
• Getting Current Month & Year
• printf Family of Functions
• String Manipulation Functions
• Initialization of Arrays
• 2D Arrays vs Array of Arrays

Commandline Arguments & GNU getopt

• Section 5.10 of K&R explains commandline arguments and explains how to write C code to process them. This method works well for simple cases. For more complex commandline processing, use a library function specially written for the purpose.

• GNU’s getopt is a special library function very useful for commandline input processing. Its manual page has an example program fragment that you can modify to suit your need.

Getting Current Month & Year

#include <time.h>
    .
    .
    .
/*
   Set 'month' (in the caller) to current month, and
   set 'year' (in the caller) to current year.
 */
void getMonthYear(int *month, int *year) {
    time_t timenow;
    struct tm *current;
    time(&timenow);
    current = gmtime(&timenow);
    *month = current->tm_mon+1;
    *year = current->tm_year+1900;
}
    .
    .
    .
int month, int year;
getMonthYear(&month, &year);
/* At this point month equals 9 and year equals 2016 */
    .
    .
    .

printf Family of Functions

• fprintf and sprintf belong to the stdio library. You have to have a line

  #include <stdio.h>

  in order to use them. They are similar to printf but have one extra paramenter as the first parameter.

• For fprintf, the first parameter is a file descriptor (e.g., stderr) and the effect of calling fprintf is to print like printf does but direct the output to the stream given as the first argument instead. E.g. the call

      fprintf(stderr, "%3d%3d\n", 2, 3)

  will print the NULL-terminated string " 2 3" (without the quotes) to the standard error output stream.

• For sprintf, the first parameter is a string variable and the effect of calling sprintf is to print like printf does but direct the output to the string given as the first argument instead. E.g. the call

      char s[80];
      sprintf(s, "%3d%3d\n", 2, 3)

  will create the NULL-terminated string " 2 3" at the (first) location of s. Make sure the char array s has enough space to hold the NULL-terminated string before calling sprintf.

String Manipulation Functions

• strlen, strcmp, strcpy, strcat, and strncat are string library functions. You need to include the line

  #include <string.h>

  in order to use them.

• strlen(s) returns the length (as an unsigned) of the NULL-terminated string s

• strcpy(t, cs) copies the NULL-terminated constant string cs to string t; return t

• strncpy(t, cs, n) copies at most n characters of the NULL-terminated constant string cs to string t; return t. Pad with ‘\0’ if t has fewer than n characters.

• strcat(t, cs) appends the NULL-terminated constant string s to the end of string t; return t

• strncat(t, cs, n) appends at most n characters from the NULL-terminated constant string s to string t; return t

• strcmp(cs, ct) compare constant string cs to constant string ct; return a negative integer if cs < ct, 0 if cs = ct, or positive integer of cs > ct

• strcmp(cs, ct, n) compare at most n characters of constant string cs to constant string ct; return a negative integer if cs < ct, 0 if cs = ct, or positive integer of cs > ct

• memset(s, c, n) places character c as the first n characters of s; return s

Initialization of Arrays

• Declaring an array variable does not initialize its content with any special values. E.g. in the code

  int main() {
      char s[10];
      ...
  }

  at the point … you cannot assume elements of array s contain any special value, i.e., they contain garbage.

• Similarly, memory allocated dynamically contains no special value. E.g. in the code

  int main() {
      char *s = (char *) malloc(10);
      ...
  }

  at the point … you cannot assume the ten bytes starting at s contain any special value, i.e., they contain garbage.

• To initialize a chunk of consecutive memory to a specific value, you can use the library function memset in string.h. E.g. in the above codes, at point … you can write

      memset(s, ' ', 10*sizeof(char));

  to initialize all ten bytes of array s to blanks.

2D Arrays vs Array of Arrays

• See K&R Chapter 5, Sections 6, 7, 8, 9