---

1. p1.c fork() example

after fork(), parent or child could be scheduled next

$ make
gcc -o p1 p1.c -Wall
gcc -o p2 p2.c -Wall
gcc -o p3 p3.c -Wall
gcc -o p4 p4.c -Wall
$ cat -n ostep/code/cpu-api/p1.c
     1	#include <stdio.h>
     2	#include <stdlib.h>
     3	#include <unistd.h>
     4
     5	int
     6	main(int argc, char *argv[])
     7	{
     8	    printf("hello world (pid:%d)\n", (int) getpid());
     9	    int rc = fork();
    10	    if (rc < 0) {
    11	        // fork failed; exit
    12	        fprintf(stderr, "fork failed\n");
    13	        exit(1);
    14	    } else if (rc == 0) {
    15	        // child (new process)
    16	        printf("hello, I am child (pid:%d)\n", (int) getpid());
    17	    } else {
    18	        // parent goes down this path (original process)
    19	        printf("hello, I am parent of %d (pid:%d)\n",
    20		       rc, (int) getpid());
    21	    }
    22	    return 0;
    23	}
$ ./p1
hello world (pid:21918)
hello, I am parent of 21919 (pid:21918)
hello, I am child (pid:21919)
$ ./p1
hello world (pid:21920)
hello, I am child (pid:21921)
hello, I am parent of 21921 (pid:21920)
$

---

2. p2.c fork() and wait() example

parent waits for child to finish first

$ cat -n ostep/code/cpu-api/p2.c
     1	#include <stdio.h>
     2	#include <stdlib.h>
     3	#include <unistd.h>
     4	#include <sys/wait.h>
     5
     6	int
     7	main(int argc, char *argv[])
     8	{
     9	    printf("hello world (pid:%d)\n", (int) getpid());
    10	    int rc = fork();
    11	    if (rc < 0) {
    12	        // fork failed; exit
    13	        fprintf(stderr, "fork failed\n");
    14	        exit(1);
    15	    } else if (rc == 0) {
    16	        // child (new process)
    17	        printf("hello, I am child (pid:%d)\n", (int) getpid());
    18		sleep(1);
    19	    } else {
    20	        // parent goes down this path (original process)
    21	        int wc = wait(NULL);
    22	        printf("hello, I am parent of %d (wc:%d) (pid:%d)\n",
    23		       rc, wc, (int) getpid());
    24	    }
    25	    return 0;
    26	}
$ ./p2
hello world (pid:21937)
hello, I am child (pid:21938)
hello, I am parent of 21938 (wc:21938) (pid:21937)
$ ./p2
hello world (pid:21939)
hello, I am child (pid:21940)
hello, I am parent of 21940 (wc:21940) (pid:21939)
$

--> try checking the child exit status

---

3. p3.c fork() and execvp() example

child runs wc

$ cat -n ostep/code/cpu-api/p3.c
     1	#include <stdio.h>
     2	#include <stdlib.h>
     3	#include <unistd.h>
     4	#include <string.h>
     5	#include <sys/wait.h>
     6
     7	int
     8	main(int argc, char *argv[])
     9	{
    10	    printf("hello world (pid:%d)\n", (int) getpid());
    11	    int rc = fork();
    12	    if (rc < 0) {
    13	        // fork failed; exit
    14	        fprintf(stderr, "fork failed\n");
    15	        exit(1);
    16	    } else if (rc == 0) {
    17	        // child (new process)
    18	        printf("hello, I am child (pid:%d)\n", (int) getpid());
    19	        char *myargs[3];
    20	        myargs[0] = strdup("wc");   // program: "wc" (word count)
    21	        myargs[1] = strdup("p3.c"); // argument: file to count
    22	        myargs[2] = NULL;           // marks end of array
    23	        execvp(myargs[0], myargs);  // runs word count
    24	        printf("this shouldn't print out");
    25	    } else {
    26	        // parent goes down this path (original process)
    27	        int wc = wait(NULL);
    28	        printf("hello, I am parent of %d (wc:%d) (pid:%d)\n",
    29		       rc, wc, (int) getpid());
    30	    }
    31	    return 0;
    32	}
$ ./p3
hello world (pid:22206)
hello, I am child (pid:22207)
 32 123 966 p3.c
hello, I am parent of 22207 (wc:22207) (pid:22206)
$

---

4. p4.c fork() and I/O redirect

child redirects stdout to file p4.output then runs wc

$ cat -n ostep/code/cpu-api/p4.c
     1	#include <stdio.h>
     2	#include <stdlib.h>
     3	#include <unistd.h>
     4	#include <string.h>
     5	#include <fcntl.h>
     6	#include <assert.h>
     7	#include <sys/wait.h>
     8
     9	int
    10	main(int argc, char *argv[])
    11	{
    12	    int rc = fork();
    13	    if (rc < 0) {
    14	        // fork failed; exit
    15	        fprintf(stderr, "fork failed\n");
    16	        exit(1);
    17	    } else if (rc == 0) {
    18		// child: redirect standard output to a file
    19		close(STDOUT_FILENO);
    20		open("./p4.output", O_CREAT|O_WRONLY|O_TRUNC, S_IRWXU);
    21
    22		// now exec "wc"...
    23	        char *myargs[3];
    24	        myargs[0] = strdup("wc");   // program: "wc" (word count)
    25	        myargs[1] = strdup("p4.c"); // argument: file to count
    26	        myargs[2] = NULL;           // marks end of array
    27	        execvp(myargs[0], myargs);  // runs word count
    28	    } else {
    29	        // parent goes down this path (original process)
    30	        int wc = wait(NULL);
    31		assert(wc >= 0);
    32	    }
    33	    return 0;
    34	}
$ ./p4
$ cat p4.output
 34 114 884 p4.c
$