Embedded FreeBSD Cookbook phần 2 ppt

11 Chapter Two

Systems Programming

#include <sys/types.h>

#include <unistd.h>

pid_t fork(void);

Fork is unique in that the single call to fork returns twice. Fork returns

the process identifier (PID) of the child to the parent process, and it returns

0 to the newly created child process.

When a process is created, the following list of attributes is inherited from the

parent file descriptors: process group ID, access groups, working directory,

root directory, control terminal, resources, interval timers, resource limits,

file mode mask, and signal mask.

The execve System Call

Many times a process is created to run another program. The execve sys￾tem call is used this way and is invoked immediately following a fork sys￾tem. An execve system call will replace the currently executing program

with a new program.

#include <unistd.h>

int execve(const char *path, char *const argv[], char *const envp[]);

The execve system call takes three parameters; path is the path and filename

of the new program to execute; argv contains a pointer to the argument string

to pass to the program; envp contains a list of environment variables.

The execve system call returns –1 on error. The execve system call has

numerous wrappers in the Standard C Library, known as the exec family

of functions.

Process Termination

A process can terminate intentionally by calling exit or unintentionally

by receiving a signal from another process. Regardless of the reason, when

a process terminates, a notification is returned to the parent. If the parent

process does not receive the notification, the child process becomes a

zombie. The child will remain a zombie until the parent retrieves the child’s

exit status.

12 Embedded FreeBSD

Cookbook

The _exit System Call

A process will terminate when the program invokes the _exit system call.

The _exit system call will cause the SIGCHLD signal to be thrown to the

parent process.

#include <unistd.h>

void _exit(int status);

The _exit system call will never return. The _exit system call is more

commonly called by the Standard C library function exit.

The wait System Call

The wait system call allows a parent process to check to see if termination

information is available for a child process. Wait will suspend execution

until the child process returns.

#include <sys/types.h>

#include <sys/wait.h>

pid_t wait(int *status);

On success, the child process identifier (PID) is returned to the parent process,

or –1 is returned if there is an error. The child’s exit status is returned by the

status parameter.

An Example

Listing 2-1 illustrates the usage of the fork, wait, execve, and exit calls.

The parent process makes a fork system call to create a child process.

There are two control paths in the main program, one for the parent and

another for the child.

int main(int argc, char **argv)

{

pid_t pid;

int status;

if ((pid = fork()) > 0)

{

printf(“%d: waiting for the child\n”, pid);

wait(&status);