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1
External and static variables External and static variables
• External variable: declared outside the
body of a function
• File scope: visible from the point of the
declaration to the end of the file.
• Static storage duration: through the
duration of the program.
• External/global variables have file scope
and static storage duration.
2
static variables static variables
static int i;
void f(void)
static int j;
- static used outside a block means that the variable is only visible in the file in which it is declared
- static used in a block means that the variable lives beyond the duration of the block, and is initialized only once.
ExampleExample
#include <stdio.h>
int nextvalue() { static int i = 0; i++; return i; }
int main() { int i; for(i = 10; i > 0; i--) { printf("%d\n", nextvalue()); } return 0; }
output: 1 2 3 4 5 6 7 8 9
extern extern
• informs the compiler that i is an int
variable, but doesn't cause it to allocate
space.
filea.c fileb.c
extern int i;
void f(void) { i++; }
int i = 0; extern void f(void); void g(void) { f(); printf("%d\n", i); }
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typedef typedef
• You can define new types using typedef.
• You have already seen the effects of
typedef
typedef unsigned int size_t;
• Example
struct personrec {
char name[20];
int age;
};
typedef struct personrec Person;
Person *p = malloc(sizeof(Person)); 6
typedef typedef
• You can define new types using typedef.
• You have already seen the effects of
typedef
typedef unsigned int size_t;
• Example
typedef struct {
char name[20];
int age;
} Person;
Person *p = malloc(sizeof(Person));
Header files Header files
• When you begin to split up your C
program into multiple files, you need
header files to store function and type
declarations.
void add(int); int isEmpty(); extern List *head; int main() { add(10); isEmpty(); head = NULL; }
main.c
List *head = NULL; int isEmpty() {...} void add(int v) {...} void remove(int v) {...}
list.c
list.h
struct node { int value; struct node * next; } ; typedef struct node List; extern List *head; int isEmpty(int); void add(int); void remove(int)
#include "list.h"
int main() { add(10); isEmpty(); head = NULL; }
main.c
#include "list.h" List *head = NULL; int isEmpty() {...} void add(int v) {...} void remove(int v) {...}
list.c
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Makefiles Makefiles
• Makefiles were originally designed to
support separate compilation of C files.
CFLAGS= -g -Wall
all : switch1 switch2 switch
switch1 : reverse.o switch1.o
gcc ${CFLAGS} -o switch1 $^
switch2 : reverse.o switch2.o
gcc ${CFLAGS} -o switch2 $^
switch3 : reverse.o switch3.o
gcc ${CFLAGS} -o switch3 $^
%.o : %.c
gcc ${CFLAGS} -c $<
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mystrcmpmystrcmp.c.c
int main(int argc, char **argv) { char *result; if (argc != 3) { fprintf(stderr, "Usage: %s string1 string2\n", argv[0]); exit(1); } switch (mystrcmp(argv[1], argv[2])) { case -1: result = "less than"; break; case 0: result = "equal to"; break; case 1: result = "greater than"; break; default: result = "causing a problem comparing to"; break; } printf("%s is %s %s\n", argv[1], result, argv[2]); return 0; }
mystrcmp mystrcmp.c (continued).c (continued)
int mystrcmp (const char *a, const char b) { while (a && *b && *a == b) { a++; b++; } return (a - *b); }
May return other than -1, 0 and 1
mystrcmpmystrcmp.c (return -1,0, or 1).c (return -1,0, or 1)
int mystrcmp (const char *a, const char b) { while (a && *b && *a == b) { a++; b++; } if (a < b) return -1; else if (a > b) return 1; else / a same as b */ return 0; }
returns sign of a -b
