Chapter 5 - Process Management

chp5/atexit.c

@@ -0,0 +1,15 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+void out(void) {
+    printf("atexit() succeeded!\n");
+}
+
+
+int main(void) {
+    if (atexit(out)) {
+        fprintf(stderr, "atexit() failed\n");
+        return 1;
+    }
+    return 0;
+}

chp5/daemon.c

@@ -0,0 +1,43 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+#define NR_OPEN 1048576
+
+int main(void) {
+    // create new process
+    pid_t pid = fork();
+    if (pid == -1) {
+        return -1;
+    } else if (pid != 0) {
+        printf("Running daemon with pid %d\n", pid);
+        exit(EXIT_SUCCESS);
+    }
+
+    // create a new session and process group
+    if (setsid() == -1) return -1;
+
+    // set the working directory to the root directory
+    if (chdir("/") == -1) return -1;
+
+    // close all open files--NR_OPEN is overkill, but works
+    for (int i = 0; i < NR_OPEN; i++) { 
+        close(i);
+    }
+
+    // redirect fd's 0,1,2 to /dev/null
+    open("/dev/null", O_RDWR); // stdin
+    dup(0);                    // stdout
+    dup(0);                    // stderr
+
+    // do its daemon thing
+    for (int i = 0; i < 10; i++) {
+        printf("Hello from daemon %d\n", getpid());
+        sleep(1000);
+    }
+
+    return 0;
+}

chp5/session.c

@@ -0,0 +1,9 @@
+#include <stdio.h>
+#define _XOPEN_SOURCE 500
+#include <unistd.h>
+
+int main(void) {
+    printf("My session id=%d\n", getsid(0));
+    printf("My process group id=%d\n", getpgid(0));
+    return 0;
+}

chp5/system.c

@@ -0,0 +1,63 @@
+#include <stdio.h>
+#define _XOPEN_SOURCE
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+/* my_system - synchronously spawns and waits for the command
+ * "/bin/sh -c <cmd>".
+ * 
+ * Returns -1 on error of any sort, or the exit code from the
+ * launched process. Does not block or ignore any signals.
+ */
+int my_system(const char *);
+
+int main(int argc, char *argv[]) {
+    if (argc < 2) {
+        fprintf(stderr, "usage: %s <command>", argv[0]);
+        return EXIT_FAILURE;
+    }
+    
+    const char *command = argv[1];
+    pid_t pid = fork();
+    if (pid == -1) {
+        perror("fork");
+        return EXIT_FAILURE;
+    } else if (pid > 0) {
+        int ret = my_system(command);
+        if (ret == -1) {
+            fprintf(stderr, "command: \"%s\" error", command);
+            return EXIT_FAILURE;
+        }
+        return ret;
+    }
+    
+    int status;
+    wait(&status);
+    return status;
+}
+
+int my_system(const char *cmd) {
+    pid_t pid = fork();
+    if (pid == -1) {
+        return -1;
+    } else if (pid == 0) {
+        const char *argv[4];
+        argv[0] = "sh";
+        argv[1] = "-c";
+        argv[2] = cmd;
+        argv[3] = NULL;
+
+        execv("/bin/sh", argv);
+        exit(-1);
+    }
+    
+    int status;
+    if (waitpid(pid, &status, 0) == -1) {
+        return -1;
+    } else if (WIFEXITED(status)) {
+        return WEXITSTATUS(status);
+    }
+    return -1;
+}

chp5/wait.c

@@ -0,0 +1,31 @@
+#include <unistd.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+int main(void) {
+    if (!fork())
+        return 1;
+
+    int status;
+    pid_t pid = wait(&status);
+    if (pid == -1) {
+        perror("wait");
+    }
+
+    printf("pid=%d\n", pid);
+    
+    if (WIFEXITED(status))
+        printf("Normal termination with exit status=%d\n", WEXITSTATUS(status));
+
+    if (WIFSIGNALED(status))
+        printf("Killed by signal=%d%s\n", WTERMSIG(status), WCOREDUMP(status) ? " (dumped core)" : "");
+
+    if (WIFSTOPPED(status))
+        printf("Stopped by signal=%d\n", WSTOPSIG(status));
+
+    if (WIFCONTINUED(status))
+        printf("Continued\n");
+
+    return 0;
+}