man sigprocmask:

NAMEsigprocmask - examine and change blocked signalsSYNOPSIS#include <signal.h>int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);Feature Test Macro Requirements for glibc (see feature_test_macros(7)):sigprocmask(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCEDESCRIPTIONsigprocmask()  is used to fetch and/or change the signal mask of the calling thread.  The signal mask is the set of signals whose delivery is currently blocked forthe caller (see also signal(7) for more details).The behavior of the call is dependent on the value of how, as follows.SIG_BLOCKThe set of blocked signals is the union of the current set and the set argument.SIG_UNBLOCKThe signals in set are removed from the current set of blocked signals.  It is permissible to attempt to unblock a signal which is not blocked.SIG_SETMASKThe set of blocked signals is set to the argument set.If oldset is non-null, the previous value of the signal mask is stored in oldset.If set is NULL, then the signal mask is unchanged (i.e., how is ignored), but the current value of the signal mask is nevertheless returned in oldset (if it is notNULL).The use of sigprocmask() is unspecified in a multithreaded process; see pthread_sigmask(3).RETURN VALUEsigprocmask() returns 0 on success and -1 on error.ERRORSEINVAL The value specified in how was invalid.CONFORMING TOPOSIX.1-2001.NOTESIt is not possible to block SIGKILL or SIGSTOP.  Attempts to do so are silently ignored.Each of the threads in a process has its own signal mask.A child created via fork(2) inherits a copy of its parent’s signal mask; the signal mask is preserved across execve(2).If  SIGBUS,  SIGFPE, SIGILL, or SIGSEGV are generated while they are blocked, the result is undefined, unless the signal was generated by the kill(2), sigqueue(2),or raise(3).See sigsetops(3) for details on manipulating signal sets.SEE ALSOkill(2), pause(2), sigaction(2), signal(2), sigpending(2), sigqueue(2), sigsuspend(2), pthread_sigmask(3), sigsetops(3), signal(7)COLOPHONThis page is part of release 3.22 of the Linux man-pages project.  A  description  of  the  project,  and  information  about  reporting  bugs,  can  be  found  athttp://www.kernel.org/doc/man-pages/.Linux                             2008-10-17                    SIGPROCMASK(2)

sigprocmask设定对信号屏蔽集内的信号的处理方式(阻塞或不阻塞)。

参数：

how：用于指定信号修改的方式，可能选择有三种

SIG_BLOCK//将set所指向的信号集中包含的信号加到当前的信号掩码中。即信号掩码和set信号集进行或操作。

SIG_UNBLOCK//将set所指向的信号集中包含的信号从当前的信号掩码中删除。即信号掩码和set进行与操作。

SIG_SETMASK //将set的值设定为新的进程信号掩码。即set对信号掩码进行了赋值操作。

set：为指向信号集的指针，在此专指新设的信号集，如果仅想读取现在的屏蔽值，可将其置为NULL。

oldset：也是指向信号集的指针，在此存放原来的信号集。可用来检测信号掩码中存在什么信号。

返回说明：

成功执行时，返回0。失败返回-1，errno被设为EINVAL。

sigprocmask示例(演示添加信号掩码)：

代码示例：

#include <stdio.h>
#include <signal.h>
void checkset(int i);    void main()
{sigset_t blockset;    sigemptyset(&blockset);    sigaddset(&blockset,SIGINT);    sigaddset(&blockset,SIGTSTP);    if(sigismember(&blockset,SIGINT))    printf("sig int in main\n");    checkset(0);    sigprocmask(SIG_SETMASK,&blockset,NULL);    if(sigismember(&blockset,SIGINT))    printf("sig int in main\n");    checkset(1);    sigaddset(&blockset,SIGTERM);    sigprocmask(SIG_BLOCK,&blockset,NULL);    checkset(2);    //sigdelset(&blockset,SIGTERM);     sigprocmask(SIG_UNBLOCK,&blockset,NULL);    if(sigismember(&blockset,SIGTERM))    printf("sig term in main\n");    if(sigismember(&blockset,SIGINT))    printf("sig int in main\n");    if(sigismember(&blockset,SIGTSTP))    printf("sig tstp in main\n");    checkset(3);if(sigismember(&blockset,SIGTERM))    printf("sig term in main\n");    if(sigismember(&blockset,SIGINT))    printf("sig int in main\n");    if(sigismember(&blockset,SIGTSTP))    printf("sig tstp in main\n");
}void checkset(int i)
{sigset_t set;printf("check set start:%d\n", i);if(sigprocmask(SIG_BLOCK,NULL,&set)<0){printf("check set sig procmask error!!\n");exit(0);}if(sigismember(&set,SIGINT))printf("sig int\n");if(sigismember(&set,SIGTSTP))printf("sig tstp\n");if(sigismember(&set,SIGTERM))printf("sig term\n");printf("check set end\n\n");
}

运行结果：

sig int in main
check set start:0
check set endsig int in main
check set start:1
sig int
sig tstp
check set endcheck set start:2
sig int
sig tstp
sig term
check set endsig term in main
sig int in main
sig tstp in main
check set start:3
check set endsig term in main
sig int in main
sig tstp in main

sigdelset()位于sigprocmask()之前的效果：

#include <stdio.h>
#include <signal.h>
void checkset(int i);    void main()
{sigset_t blockset;    sigemptyset(&blockset);    sigaddset(&blockset,SIGINT);    sigaddset(&blockset,SIGTSTP);    if(sigismember(&blockset,SIGINT))    printf("sig int in main\n");    checkset(0);    sigprocmask(SIG_SETMASK,&blockset,NULL);    if(sigismember(&blockset,SIGINT))    printf("sig int in main\n");    checkset(1);    sigaddset(&blockset,SIGTERM);    sigprocmask(SIG_BLOCK,&blockset,NULL);    checkset(2);    sigdelset(&blockset,SIGTERM);     sigprocmask(SIG_UNBLOCK,&blockset,NULL);    if(sigismember(&blockset,SIGTERM))    printf("sig term in main\n");    if(sigismember(&blockset,SIGINT))    printf("sig int in main\n");    if(sigismember(&blockset,SIGTSTP))    printf("sig tstp in main\n");    checkset(3);if(sigismember(&blockset,SIGTERM))    printf("sig term in main\n");    if(sigismember(&blockset,SIGINT))    printf("sig int in main\n");    if(sigismember(&blockset,SIGTSTP))    printf("sig tstp in main\n");
}void checkset(int i)
{sigset_t set;printf("check set start:%d\n", i);if(sigprocmask(SIG_BLOCK,NULL,&set)<0){printf("check set sig procmask error!!\n");exit(0);}if(sigismember(&set,SIGINT))printf("sig int\n");if(sigismember(&set,SIGTSTP))printf("sig tstp\n");if(sigismember(&set,SIGTERM))printf("sig term\n");printf("check set end\n\n");
}

运行结果：

sig int in main
check set start:0
check set endsig int in main
check set start:1
sig int
sig tstp
check set endcheck set start:2
sig int
sig tstp
sig term
check set endsig int in main
sig tstp in main
check set start:3
sig term
check set endsig int in main
sig tstp in main

当 sigdelset(&blockset,SIGTERM);     位于     sigprocmask(SIG_UNBLOCK,&blockset,NULL);之前，那么从打印信息看，if(sigismember(&blockset,SIGTERM))的条件为false，但是if(sigprocmask(SIG_BLOCK,NULL,&set)<0)的set有值。