• Create source files with the extension .java.
• Each source file corresponds to a Java class and has the same name as the class.
• Compile each source into byte code in machine-independent class files with the extension .class.
• Byte code is an abstract machine language, the same for all versions of Java, whatever machine they run on.
• When a Java program is executed, the Java Virtual Machine executes (interprets) the byte code, first finding the class files needed to run the program.

• Create source files with the extension .c.
• Compile them into object files with the extension .o. Each object file is a machine-language version of the original file, written in the language of the particular target machine. These file also contain the names of external references.
• Link together the .o files into a single executable file.
• The linker takes care of references to library functions (such as printf), by including code for these in the executable also. Thus the executable can be quite large. On Unix systems, a C executable typically has no extension name.
• Finally, the executable file is loaded into memory and put into execution. This file will only run on the particular machine it was compiled for.

Consider executing the command:

% gcc -o myhello myhello.c

This is a two-step process: first the compiler creates the object file myhello.o and then the linker creates the executable, myhello. Check this by using an ls command to see that the object file is present. You can do these two steps explicitly as follows:

% gcc -c myhello.c
% gcc -o myhello myhello.o

The first line creates myhello.o and the second line creates myhello from myhello.o.

If you have several functions, you can put some of them in another file and compile them separately.

• Create an array of size 2000. We will use an array of ints, but only single bits are needed.
• Fill the array with 1's.
• Change a 1 entry to 0 in case the index of the entry is not a prime.
• Start with entries at position 0, 1, and 2, of 0, 0, and 1 (since 2 is a prime).
• Begin sieving by looking at the 1 in index 2.
• Having found a 1 set all indexes that are multiples of this index to 0.
• Proceed to the next entry of 1.
• Keep going up to the square root of 2000.  (why?)

Here are the prototypes of some functions we might use for this program:

• A prototype of a function gives the return value, name, and parameter types of a function.
• A prototype is a duplicate of the function's heading placed at the top of program when only one file is being used, or it's placed in a file with extension .h, when the program is written with separate files.
• This is similar to what you put in an interface in Java.

void initializesieve(int primes[], int length);
/* fill the array primes of size length with 1's */

void makesieveone(int primes[], int length, int prime);
/* set the multiples of prime to 0. */

void makesieve(int primes[], int length);
/* set the multiples of all primes to 0. */

void printsieve(const int primes[], int length);
/* print out the array (for debugging)*/

void printprimes(const int primes[], int length);
/* print out the primes assuming the array has been made,
  that is, for each 1 entry, print the index value. */

Below, we will put the function definitions into a file named sieve.c and the prototypes into a file named sieve.h. Note that the first two functions will be called by the function makeSieve and do not need to be accessible outside the file. (In Java, we would declare these "helper" functions private.)

In C, we declaring such functions static, meaning that the functions are not accessible outside the file, and their names do not appear in the sieve.h file. (This use of the word static is confusing and counter-intuitive.)

Then create a separate file called sievemain.c which contains the main program:

File: sievemain.c
#include <stdio.h> #include "sieve.h" #define SIZE 2000 int main() { int list[SIZE]; int listsize = SIZE; makesieve(list, listsize); printsieve(list, listsize); printprimes(list, listsize); return 0; }

• Note the two #include statements.
• The first one contains prototypes for necessary systems functions.
• The sieve.h file contains prototypes of the necessary external functions -- in this case the three given above.
• The #define defines a constant in this case the integer 2000, named SIZE.
• The lines beginning with # are handled by the C preprocessor.
• Note that you need to pass the size of the array as a separate parameter. Unlike Java, in C you cannot determine the size of an array from the array.
• What is the difference between include in C and import in Java?

Here is the sieve.h file:

File: sieve.h
void makesieve(int p[], int len); void printsieve(const int p[], int len); void printp(const int p[], int len);

Here is the file with the other external functions, the sieve.c file. Notice that it also includes the header file sieve.h to insure that the two source files are compatible.

File: sieve.c

#include <stdio.h> #include "sieve.h" static void initializesieve(int p[], int len) { int i; p[0] = 0; p[1] = 0; for (i = 2; i < len; i++) p[i] = 1; } static void one(int p[], int len, int prime) { int i; for (i = 2*prime; i < len; i = i + prime) p[i] = 0; } void makesieve(int p[], int len) { int i; initializesieve(p, len); for (i = 2; i*i < len; i++) if (p[i] == 1) one(p, len, i); } void printsieve(const int p[], int len) { int i; printf("Sieve follows:"); for (i = 0; i < len; i++) { if (i%50 == 0) printf("\n"); printf("%d", p[i]); } printf("\n"); } void printprimes(const int p[], int len) { int i; int count = 0; printf("Primes follow:"); for (i = 0; i < len; i++) { if (p[i] == 1) { if (count%10 == 0) printf("\n"); printf("%d\t",i); count++; } } printf("\n"); }

Finally, here is a makefile for this program:

File: Makefile
all: sieve.o sievemain sieve.o: sieve.c gcc -c sieve.c sievemain: sieve.h sieve.o sievemain.c gcc -o sievemain sievemain.c sieve.o lint: lint sievemain.c sieve.c run: sievemain clean: rm -f sievemain *.o

• The first target is all so by default make will try to make sieve.o and sievemain.
• We lint both of these together so that lint can check that everything is consistent.
• To erase all of the compiled code we can use make clean.
• As has been mentioned before, you must begin the action lines with a TAB character.

The output of this program is the following:

Program output

Sieve follows: 00110101000101000101000100000101000001000101000100 00010000010100000100010100000100010000010000000100 01010001010001000000000000010001000001010000000001 01000001000001000100000100000101000000000101000101 00000000000100000000000100010100010000010100000000 01000001000001000001010000010001010000000001000000 00000001000101000100000000000001000001000000000101 00010000010000000100000100000100010000010000000100 01000000010000000001010000000001010000010001000001 00000001000101000100000000000100000001000100000001 00010000010000000000010100000000000000000100000100 00000001000001000001010000010000000001000001000001 01000001000001000101000000000001000000000101000100 00010000010100000000000100010000010000000100000000 01000000010000000001000000010000010000010001000000 01000001000100000001000100000000000001000000000100 00000000010100000000010100010100000000010000000000 00010001010001000000000000010001010001000000000000 00000001000100000001000000000100000001000100000100 00010000000000000100010000010000010000000100000100 00000000010001000001010000000001010000010000000001 01000000000101000001000000000000000001000101000100 00010000010000000100000100000100000000000000000000 01010000000001000000010000000001000001000001000000 01000000000001000100000100000101000001000000000001 00000000010000000000000000010100010000010100000100 01010001000000000001010000010000000000000000000000 00000000000100000100000100000001000000000000000001 00000000010000000000000100010100010000010000000100 01010000010000000000010000000001010001010001000001 00000000000100000000000100000001000000000001000001 00010000010000000100010000000100010000000000000100 01000001010001000001010000010000000001000000000000 00000001000001000101000000000000000000000001000101 00000000010000000000010100000000010000000100000100 00010000010000000000000000010000010001010000000000 01000000000100000000000100000001000000000000000100 00000000000100000100010100010100000000010000000000 01000001000001000000000000000001010000000000000001 01000000000000000000000100000100000001000001000101 Primes follow: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 211 223 227 229 233 239 241 251 257 263 269 271 277 281 283 293 307 311 313 317 331 337 347 349 353 359 367 373 379 383 389 397 401 409 419 421 431 433 439 443 449 457 461 463 467 479 487 491 499 503 509 521 523 541 547 557 563 569 571 577 587 593 599 601 607 613 617 619 631 641 643 647 653 659 661 673 677 683 691 701 709 719 727 733 739 743 751 757 761 769 773 787 797 809 811 821 823 827 829 839 853 857 859 863 877 881 883 887 907 911 919 929 937 941 947 953 967 971 977 983 991 997 1009 1013 1019 1021 1031 1033 1039 1049 1051 1061 1063 1069 1087 1091 1093 1097 1103 1109 1117 1123 1129 1151 1153 1163 1171 1181 1187 1193 1201 1213 1217 1223 1229 1231 1237 1249 1259 1277 1279 1283 1289 1291 1297 1301 1303 1307 1319 1321 1327 1361 1367 1373 1381 1399 1409 1423 1427 1429 1433 1439 1447 1451 1453 1459 1471 1481 1483 1487 1489 1493 1499 1511 1523 1531 1543 1549 1553 1559 1567 1571 1579 1583 1597 1601 1607 1609 1613 1619 1621 1627 1637 1657 1663 1667 1669 1693 1697 1699 1709 1721 1723 1733 1741 1747 1753 1759 1777 1783 1787 1789 1801 1811 1823 1831 1847 1861 1867 1871 1873 1877 1879 1889 1901 1907 1913 1931 1933 1949 1951 1973 1979 1987 1993 1997 1999

For a great poem about sieves, see The Jumblies, by Edgar Lear.