P_C_process_race_condition

父子进程避免竞争条件的四种方法
1、fork引发的问题：
#include "apue.h"
static void charatatime(char *);
int main(void)
{
pid_t pid;
if ((pid = fork()) < 0)
err_sys("fork error");
else if (pid == 0) //子进程先执行？
charatatime("output from child\n");
else // 父进程先执行？
charatatime("output from parent\n");
exit(0);
}
static void charatatime(char *str)
{
char *ptr;
int c;
setbuf(stdout, NULL); //设置为无缓冲输出
for (ptr = str; (c = *ptr++) != 0; )
putc(c, stdout);
}
2、方法一：添加父进程中添加waitpid()，让子进程优先
#include "apue.h"
static void charatatime(char *);
int main(void)
{
pid_t pid;
if ((pid = fork()) < 0)
err_sys("fork error");
else if (pid == 0)
charatatime("output from child\n");
else
{
if(waitpid(pid,NULL,0)!=pid)
//子进程优先
error");
err_sys("waitpid
charatatime("output from parent\n");
}
exit(0);
}
static void charatatime(char *str)
{
char *ptr;
int c;
setbuf(stdout, NULL); //设置为无缓冲输出 for (ptr = str; (c = *ptr++) != 0; )
putc(c, stdout);
}
3、方法二：改用vfork ，子进程优先
#include "apue.h"
static void charatatime(char *);
int main(void)
{
pid_t pid;
if ((pid = vfork()) < 0) //子进程优先
err_sys("fork error");
else if (pid == 0)
charatatime("output from child\n");
else
charatatime("output from parent\n");
exit(0);
}
static void charatatime(char *str)
{
char *ptr;
int c;
setbuf(stdout, NULL); //设置为无缓冲输出 for (ptr = str; (c = *ptr++) != 0; )
putc(c, stdout);
}
4、方法三：添加函数，可以灵活选择先执行哪个进程
#include "apue.h"
static void charatatime(char *);
int main(void)
{
pid_t pid;
TELL_WAIT();
if ((pid = fork()) < 0)
err_sys("fork error");
else if (pid == 0)
{
WAIT_PARENT(); // 父进程先运行
charatatime("output from child\n");
}
else
{
charatatime("output from parent\n");
TELL_CHILD(pid);
}
exit(0);
}
static void charatatime(char *str)
{
char *ptr;
int c;
setbuf(stdout, NULL); //设置为无缓冲输出 for (ptr = str; (c = *ptr++) != 0; )
putc(c, stdout);
}
5、方法四：添加sleep函数，可以灵活选择先执行哪个进程
#include "apue.h"
static void charatatime(char *);
int main(void)
{
pid_t pid;
if ((pid = fork()) < 0)
err_sys("fork error");
else if (pid == 0)
{
sleep(5); //保证父进程优先运行
charatatime("output from child\n");
}
else
charatatime("output from parent\n");
exit(0);
}
static void charatatime(char *str)
{
char *ptr;
int c;
setbuf(stdout, NULL); //设置为无缓冲输出 for (ptr = str; (c = *ptr++) != 0; )
putc(c, stdout);
}。