linux是一个典型的多用户多任务操作系统。多用户指多个用户可以同时使用计算机；多任务指linux可同时执行几个任务，可以在还未执行完一个任务时又执行另一个任务。进程即运行中的程序，linux可以同时启动多个进程。

一.进程概念
1.根据操作系统对进程的定义
进程就是操作系统资源管理的最小单位。
2.理解
进程是程序的一次执行过程，它是动态的实体。需要将它和程序/线程对比着更好理解

(1)进程和程序的区别
a.进程是动态的，是运行中的程序。
b.程序是静态的，是保存在硬盘上的可执行代码

(2)进程和线程
a.进程内部又划分出许多线程，线程基本上不拥有系统资源，他们共享该进程拥有的全部资源，目的是为了让计算机同时能执行更多的任务。
b.进程在执行过程中拥有独立的内存单元，其内部的线程共享这些内存。
c.一个线程可以创建和撤销另一个线程，同一个进程中的多个线程可以并行执行。
二.进程标识
linux中，每个进程都用唯一的进程ID（非负数）来标识每个进程除过ID还有其他信息来标识，可以通过unistd.h中声明的函数获得。下表为这些函数的声明。

用户ID和组ID的相关概念
1.实际用户ID(uid)：标识运行该进程的用户
2.有效用户ID(euid)：Unix系统通过进程的有效用户ID和有效用户组ID来决定进程对系统资源的访问权限。
3.实际组ID(gid)：实际用户所属组的组ID
4.有效组ID(egid)：见2.
进程用来决定我们对资源的访问权限。一般情况下，有效用户ID等于实际用户ID，有效用户组ID等于实际用户组ID。当设置-用户-ID（SUID）位设置，则有效用户ID等于文件的所有者的uid，而不是实际用户ID；同样，如果设置了设置-用户组-ID（SGID）位，则有效用户组ID等于文件所有者的gid，而不是实际用户组ID。如下：

#include<stdio.h>
#include<unistd.h>

int main()
{
    pid_t pid;
    pid = fork();
    if(pid == 0)
    {printf("%s\n","create child process ok.");}
    else if(pid == -1)
    {printf("%s\n","create child process false.");}
    else
    {
        printf("%s %d\n","this is parent process, PPid is", getppid());
    }
    printf("%s %d\n","uid is", getuid());
    printf("%s %d\n","euid is", geteuid());
    printf("%s %d\n","gid is", getgid());
    printf("%s %d\n","egid is", getegid());
}

编译之后，funtest的属性为：

结果为

通过下列命令改变funtest的所属用户，设置-用户-ID（SUID）位设置

chown kiosk funtest 
chmod u+s funtest

结果为


三.进程的结构
进程是一个结构体/include/linux/sched.h下的struct task_struct对其进行了定义

struct task_struct {
    volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */
    void *stack;
    atomic_t usage;
    unsigned int flags; /* per process flags, defined below */
    unsigned int ptrace;

#ifdef CONFIG_SMP
    struct llist_node wake_entry;
    int on_cpu;
#endif
    int on_rq;

    int prio, static_prio, normal_prio;
    unsigned int rt_priority;
    const struct sched_class *sched_class;
    struct sched_entity se;
    struct sched_rt_entity rt;

#ifdef CONFIG_PREEMPT_NOTIFIERS
    /* list of struct preempt_notifier: */
    struct hlist_head preempt_notifiers;
#endif

    /*
     * fpu_counter contains the number of consecutive context switches
     * that the FPU is used. If this is over a threshold, the lazy fpu
     * saving becomes unlazy to save the trap. This is an unsigned char
     * so that after 256 times the counter wraps and the behavior turns
     * lazy again; this to deal with bursty apps that only use FPU for
     * a short time
     */
    unsigned char fpu_counter;
#ifdef CONFIG_BLK_DEV_IO_TRACE
    unsigned int btrace_seq;
#endif

    unsigned int policy;
    int nr_cpus_allowed;
    cpumask_t cpus_allowed;

#ifdef CONFIG_PREEMPT_RCU
    int rcu_read_lock_nesting;
    char rcu_read_unlock_special;
    struct list_head rcu_node_entry;
#endif /* #ifdef CONFIG_PREEMPT_RCU */
#ifdef CONFIG_TREE_PREEMPT_RCU
    struct rcu_node *rcu_blocked_node;
#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
#ifdef CONFIG_RCU_BOOST
    struct rt_mutex *rcu_boost_mutex;
#endif /* #ifdef CONFIG_RCU_BOOST */

#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
    struct sched_info sched_info;
#endif

    struct list_head tasks;
#ifdef CONFIG_SMP
    struct plist_node pushable_tasks;
#endif

    struct mm_struct *mm, *active_mm;
#ifdef CONFIG_COMPAT_BRK
    unsigned brk_randomized:1;
#endif
#if defined(SPLIT_RSS_COUNTING)
    struct task_rss_stat    rss_stat;
#endif
/* task state */
    int exit_state;
    int exit_code, exit_signal;
    int pdeath_signal;  /*  The signal sent when the parent dies  */
    unsigned int jobctl;    /* JOBCTL_*, siglock protected */
    /* ??? */
    unsigned int personality;
    unsigned did_exec:1;
    unsigned in_execve:1;   /* Tell the LSMs that the process is doing an
                 * execve */
    unsigned in_iowait:1;

    /* task may not gain privileges */
    unsigned no_new_privs:1;

    /* Revert to default priority/policy when forking */
    unsigned sched_reset_on_fork:1;
    unsigned sched_contributes_to_load:1;

    pid_t pid;
    pid_t tgid;

#ifdef CONFIG_CC_STACKPROTECTOR
    /* Canary value for the -fstack-protector gcc feature */
    unsigned long stack_canary;
#endif
    /*
     * pointers to (original) parent process, youngest child, younger sibling,
     * older sibling, respectively.  (p->father can be replaced with
     * p->real_parent->pid)
     */
    struct task_struct __rcu *real_parent; /* real parent process */
    struct task_struct __rcu *parent; /* recipient of SIGCHLD, wait4() reports */
    /*
     * children/sibling forms the list of my natural children
     */
    struct list_head children;  /* list of my children */
    struct list_head sibling;   /* linkage in my parent's children list */
    struct task_struct *group_leader;   /* threadgroup leader */

    /*
     * ptraced is the list of tasks this task is using ptrace on.
     * This includes both natural children and PTRACE_ATTACH targets.
     * p->ptrace_entry is p's link on the p->parent->ptraced list.
     */
    struct list_head ptraced;
    struct list_head ptrace_entry;

    /* PID/PID hash table linkage. */
    struct pid_link pids[PIDTYPE_MAX];
    struct list_head thread_group;

    struct completion *vfork_done;      /* for vfork() */
    int __user *set_child_tid;      /* CLONE_CHILD_SETTID */
    int __user *clear_child_tid;        /* CLONE_CHILD_CLEARTID */

    cputime_t utime, stime, utimescaled, stimescaled;
    cputime_t gtime;
#ifndef CONFIG_VIRT_CPU_ACCOUNTING
    cputime_t prev_utime, prev_stime;
#endif
    unsigned long nvcsw, nivcsw; /* context switch counts */
    struct timespec start_time;         /* monotonic time */
    struct timespec real_start_time;    /* boot based time */
/* mm fault and swap info: this can arguably be seen as either mm-specific or thread-specific */
    unsigned long min_flt, maj_flt;

    struct task_cputime cputime_expires;
    struct list_head cpu_timers[3];

/* process credentials */
    const struct cred __rcu *real_cred; /* objective and real subjective task
                     * credentials (COW) */
    const struct cred __rcu *cred;  /* effective (overridable) subjective task
                     * credentials (COW) */
    char comm[TASK_COMM_LEN]; /* executable name excluding path
                     - access with [gs]et_task_comm (which lock
                       it with task_lock())
                     - initialized normally by setup_new_exec */
/* file system info */
    int link_count, total_link_count;
#ifdef CONFIG_SYSVIPC
/* ipc stuff */
    struct sysv_sem sysvsem;
#endif
#ifdef CONFIG_DETECT_HUNG_TASK
/* hung task detection */
    unsigned long last_switch_count;
#endif
/* CPU-specific state of this task */
    struct thread_struct thread;
/* filesystem information */
    struct fs_struct *fs;
/* open file information */
    struct files_struct *files;
/* namespaces */
    struct nsproxy *nsproxy;
/* signal handlers */
    struct signal_struct *signal;
    struct sighand_struct *sighand;

    sigset_t blocked, real_blocked;
    sigset_t saved_sigmask; /* restored if set_restore_sigmask() was used */
    struct sigpending pending;

    unsigned long sas_ss_sp;
    size_t sas_ss_size;
    int (*notifier)(void *priv);
    void *notifier_data;
    sigset_t *notifier_mask;
    struct hlist_head task_works;

    struct audit_context *audit_context;
#ifdef CONFIG_AUDITSYSCALL
    uid_t loginuid;
    unsigned int sessionid;
#endif
    struct seccomp seccomp;

/* Thread group tracking */
    u32 parent_exec_id;
    u32 self_exec_id;
/* Protection of (de-)allocation: mm, files, fs, tty, keyrings, mems_allowed,
 * mempolicy */
    spinlock_t alloc_lock;

    /* Protection of the PI data structures: */
    raw_spinlock_t pi_lock;

#ifdef CONFIG_RT_MUTEXES
    /* PI waiters blocked on a rt_mutex held by this task */
    struct plist_head pi_waiters;
    /* Deadlock detection and priority inheritance handling */
    struct rt_mutex_waiter *pi_blocked_on;
#endif

#ifdef CONFIG_DEBUG_MUTEXES
    /* mutex deadlock detection */
    struct mutex_waiter *blocked_on;
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
    unsigned int irq_events;
    unsigned long hardirq_enable_ip;
    unsigned long hardirq_disable_ip;
    unsigned int hardirq_enable_event;
    unsigned int hardirq_disable_event;
    int hardirqs_enabled;
    int hardirq_context;
    unsigned long softirq_disable_ip;
    unsigned long softirq_enable_ip;
    unsigned int softirq_disable_event;
    unsigned int softirq_enable_event;
    int softirqs_enabled;
    int softirq_context;
#endif
#ifdef CONFIG_LOCKDEP
# define MAX_LOCK_DEPTH 48UL
    u64 curr_chain_key;
    int lockdep_depth;
    unsigned int lockdep_recursion;
    struct held_lock held_locks[MAX_LOCK_DEPTH];
    gfp_t lockdep_reclaim_gfp;
#endif

/* journalling filesystem info */
    void *journal_info;

/* stacked block device info */
    struct bio_list *bio_list;

#ifdef CONFIG_BLOCK
/* stack plugging */
    struct blk_plug *plug;
#endif

/* VM state */
    struct reclaim_state *reclaim_state;

    struct backing_dev_info *backing_dev_info;

    struct io_context *io_context;

    unsigned long ptrace_message;
    siginfo_t *last_siginfo; /* For ptrace use.  */
    struct task_io_accounting ioac;
#if defined(CONFIG_TASK_XACCT)
    u64 acct_rss_mem1;  /* accumulated rss usage */
    u64 acct_vm_mem1;   /* accumulated virtual memory usage */
    cputime_t acct_timexpd; /* stime + utime since last update */
#endif
#ifdef CONFIG_CPUSETS
    nodemask_t mems_allowed;    /* Protected by alloc_lock */
    seqcount_t mems_allowed_seq;    /* Seqence no to catch updates */
    int cpuset_mem_spread_rotor;
    int cpuset_slab_spread_rotor;
#endif
#ifdef CONFIG_CGROUPS
    /* Control Group info protected by css_set_lock */
    struct css_set __rcu *cgroups;
    /* cg_list protected by css_set_lock and tsk->alloc_lock */
    struct list_head cg_list;
#endif
#ifdef CONFIG_FUTEX
    struct robust_list_head __user *robust_list;
#ifdef CONFIG_COMPAT
    struct compat_robust_list_head __user *compat_robust_list;
#endif
    struct list_head pi_state_list;
    struct futex_pi_state *pi_state_cache;
#endif
#ifdef CONFIG_PERF_EVENTS
    struct perf_event_context *perf_event_ctxp[perf_nr_task_contexts];
    struct mutex perf_event_mutex;
    struct list_head perf_event_list;
#endif
#ifdef CONFIG_NUMA
    struct mempolicy *mempolicy;    /* Protected by alloc_lock */
    short il_next;
    short pref_node_fork;
#endif
    struct rcu_head rcu;

    /*
     * cache last used pipe for splice
     */
    struct pipe_inode_info *splice_pipe;
#ifdef  CONFIG_TASK_DELAY_ACCT
    struct task_delay_info *delays;
#endif
#ifdef CONFIG_FAULT_INJECTION
    int make_it_fail;
#endif
    /*
     * when (nr_dirtied >= nr_dirtied_pause), it's time to call
     * balance_dirty_pages() for some dirty throttling pause
     */
    int nr_dirtied;
    int nr_dirtied_pause;
    unsigned long dirty_paused_when; /* start of a write-and-pause period */

#ifdef CONFIG_LATENCYTOP
    int latency_record_count;
    struct latency_record latency_record[LT_SAVECOUNT];
#endif
    /*
     * time slack values; these are used to round up poll() and
     * select() etc timeout values. These are in nanoseconds.
     */
    unsigned long timer_slack_ns;
    unsigned long default_timer_slack_ns;

    struct list_head    *scm_work_list;
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
    /* Index of current stored address in ret_stack */
    int curr_ret_stack;
    /* Stack of return addresses for return function tracing */
    struct ftrace_ret_stack *ret_stack;
    /* time stamp for last schedule */
    unsigned long long ftrace_timestamp;
    /*
     * Number of functions that haven't been traced
     * because of depth overrun.
     */
    atomic_t trace_overrun;
    /* Pause for the tracing */
    atomic_t tracing_graph_pause;
#endif
#ifdef CONFIG_TRACING
    /* state flags for use by tracers */
    unsigned long trace;
    /* bitmask and counter of trace recursion */
    unsigned long trace_recursion;
#endif /* CONFIG_TRACING */
#ifdef CONFIG_CGROUP_MEM_RES_CTLR /* memcg uses this to do batch job */
    struct memcg_batch_info {
        int do_batch;   /* incremented when batch uncharge started */
        struct mem_cgroup *memcg; /* target memcg of uncharge */
        unsigned long nr_pages; /* uncharged usage */
        unsigned long memsw_nr_pages; /* uncharged mem+swap usage */
    } memcg_batch;
#endif
#ifdef CONFIG_HAVE_HW_BREAKPOINT
    atomic_t ptrace_bp_refcnt;
#endif
#ifdef CONFIG_UPROBES
    struct uprobe_task *utask;
    int uprobe_srcu_id;
#endif
};

linux中一个进程由3部分组成：代码段/数据段/堆栈段。代码段存放程序的可执行代码；数据段存放程序的全局变量/常量/静态变量；堆栈段中的对用于存放动态分配的内存变量，栈用于存放函数的参数/函数内部定义的局部变量。
四.进程状态
1.进程的三态模型
在多道程序系统中，进程在处理器上交替运行，状态也不断地发生变化。进程一般有3种基本状态：运行、就绪和阻塞。
（1）运行：当一个进程在处理机上运行时，则称该进程处于运行状态。处于此状态的进程的数目小于等于处理器的数目，对于单处理机系统，处于运行状态的进程只有一个。在没有其他进程可以执行时（如所有进程都在阻塞状态），通常会自动执行系统的空闲进程。
（2）就绪：当一个进程获得了除处理机以外的一切所需资源，一旦得到处理机即可运行，则称此进程处于就绪状态。就绪进程可以按多个优先级来划分队列。例如，当一个进程由于时间片用完而进入就绪状态时，排入低优先级队列；当进程由I／O操作完成而进入就绪状态时，排入高优先级队列。
（3）阻塞：也称为等待或睡眠状态，一个进程正在等待某一事件发生（例如请求I/O而等待I/O完成等）而暂时停止运行，这时即使把处理机分配给进程也无法运行，故称该进程处于阻塞状态。

2.进程的五态模型
五态模型：对于一个实际的系统，进程的状态及其转换更为复杂。引入新建态和终止态构成了进程的五态模型。
新建态： 对应于进程刚刚被创建时没有被提交的状态，并等待系统完成创建进程的所有必要信息。 进程正在创建过程中，还不能运行。操作系统在创建状态要进行的工作包括分配和建立进程控制块表项、建立资源表格（如打开文件表）并分配资源、加载程序并建立地址空间表等。创建进程时分为两个阶段，第一个阶段为一个新进程创建必要的管理信息，第二个阶段让该进程进入就绪状态。由于有了新建态，操作系统往往可以根据系统的性能和主存容量的限制推迟新建态进程的提交。
终止态：进程已结束运行，回收除进程控制块之外的其他资源，并让其他进程从进程控制块中收集有关信息（如记帐和将退出代码传递给父进程）。类似的，进程的终止也可分为两个阶段，第一个阶段等待操作系统进行善后处理，第二个阶段释放主存。

3.细分进程状态及其转换
由于进程的不断创建，系统资源特别是主存资源已不能满足所有进程运行的要求。这时，就必须将某些进程挂起，放到磁盘对换区，暂时不参加调度，以平衡系统负载；进程挂起的原因可能是系统故障，或者是用户调试程序，也可能是需要检查问题。
活跃就绪：是指进程在主存并且可被调度的状态。
静止就绪（挂起就绪）：是指进程被对换到辅存时的就绪状态，是不能被直接调度的状态，只有当主存中没有活跃就绪态进程，或者是挂起就绪态进程具有更高的优先级，系统将把挂起就绪态进程调回主存并转换为活跃就绪。
活跃阻塞：是指进程已在主存，一旦等待的事件产生便进入活跃就绪状态。
静止阻塞：是指进程对换到辅存时的阻塞状态，一旦等待的事件产生便进入静止就绪状态。