C Lecture 4

Function Prototypes

• Always tell compiler about identifiers before using them.
  Thus, delcare variables before using them
    and declare functions before calling them
• function declarations aka prototypes
• prototypes often put in header files

return-type func-name(arg-1-type arg-1-name,
	arg-2-type arg-2-name, etc );

Example:

#include <stdio.h>

char *my_fgets(char *b, int bsize, FILE *fp);

int
main()
{
    char buf[1024];
    while (fgets(buf, sizeof(buf), stdin))
	printf("%s\n", buf);
    return 0;
}

char *
my_fgets(char *b, int bsize, FILE *fp)
{
    ...
}

Character classification routines (ctype.h)

• isalpha(char c): is c an alphabetic character?
• isupper(char c): is c an upper case alphabetic?
• islower(char c): is c an lower case alphabetic?
• isspace(char c): white space (space, tab, \n)?
• isdigit(char c): is c a decimial digit?
• isxdigit(char c): is c a hexidecimal digit?
• isalnum(char c): is c an letter or a digit?
• toupper(char c): return upper case version of c
• tolower(char c): return lower case version of c

#include <ctype.h>

int
atoi(char *str)
{
    char *p;
    int num;
    int sign = 1;

    for (p = str; isspace(*p); p++)
	;
    if (*p == '-') {
	sign = -1;
	p++;
    }
    for (num = 0; isdigit(*p); p++)
	num = num * 10 + (*p - '0');
    return sign * num;
}

Command Line Arguments

• main is passed words from command line

      int
      main(int argc, char *argv[])
      {
  	...
      }
  

• first parameter (often called argc) is the number of arguments
• second parameter (often called argv) is an array of the arguments.
• argv[0] is the name of the program
• argv[1] is the first parameter, etc.

#include <stdio.h>

int
main(int argc, char *argv[])
{
    int i;

    for (i = 1; i < argc; i++) {
	if (i > 1)
	    printf(" ");
	printf("%s", argv[i]);
    }
    printf("\n");
    return 0;
}