Question

此代码是关于。

比赛条件：调度和编译器行为在进程或线程同步中发挥重要作用。演示同步需求的最简单方案来自两个线程/进程之间创建的竞争条件，试图修改共享变量的值，这通常会导致数据不一致和错误的结果。以下示例演示了这种情况：

我是C的新手，我遇到了这个警告发生的问题。警告意味着什么，我该如何解决它。我写的代码在这里：

q1.c: In function ‘runner’:
q1.c:13:1: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘long int’ [-Wformat=]
 printf("T tid: %d x before: %d\n", syscall(SYS_gettid),x); int i;
 ^
q1.c:19:1: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘long int’ [-Wformat=]
 printf("T tid: %d x after: %d\n", syscall(SYS_gettid),x);

以下是代码：

// Race condition
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/syscall.h>
int x=0;
void * runner(void *arg)
{
printf("T tid: %d   x before: %d\n", syscall(SYS_gettid),x); int i;
for (i = 0; i < 100000; i++ )
{
x = x + 1;
}
printf("T tid: %d   x after: %d\n", syscall(SYS_gettid),x);
}

int program()
{
pthread_t t1,t2,t3,t4;
printf("Parent pid: %d  x before threads: %d\n", getpid(),x); int i;
if(pthread_create(&t1,NULL, runner, NULL)){ printf("Error creating thread 1\n"); return 1;
}
if(pthread_create(&t2,NULL, runner, NULL)){ printf("Error creating thread 2\n"); return 1;
}
if(pthread_create(&t3,NULL, runner, NULL)){ printf("Error creating thread 1\n"); return 1;
}
if(pthread_create(&t4,NULL, runner, NULL)){ printf("Error creating thread 1\n"); return 1;
}

if(pthread_join(t1,NULL)){ printf("error joining thread 1"); return 1;
}
if(pthread_join(t2,NULL)){ printf("error joining thread 1"); return 1;
}
if(pthread_join(t3,NULL)){ printf("error joining thread 1"); return 1;
}

if(pthread_join(t4,NULL)){ printf("error joining thread 1"); return 1;
}
printf("Parent pid: %d  x after threads: %d\n", getpid(),x); return 0;
}

int main(int argc, char *argv[]) { 
int count=0;
// loop runs the program count times 
while(count<5)
{
// running program program();
count++;
//reset global x for next run of program. x=0;
printf("\n\n");
}
return 0;
}

Answer 1

您必须使用"%d"更改"%ld"，"%d"更改为已签名int，此处l代表long所以{{ 1}}用于签名"%ld"。

Answer 2

代码应始终以人类想要阅读的方式编写。即使是作者在阅读代码时也存在重大问题如许多“复制并粘贴”错误所示

注意：当一行开始'//'注释时，该行的其余部分都是注释

除main之外的任何函数都应该有原型。在调用它们的任何函数之前声明被调用函数的方法将适用于琐碎的程序，但不适用于多文件程序具有交叉呼叫路径的程序 I.E.总是提供原型声明。

在调用某些功能时，请务必知道（I.E.查找）完整的详细信息关于那个功能。在OPs发布代码的情况下，这尤其适用于syscall（）函数。

编写线程时，除了原型之外还有更多要知道的东西一个线程。如在OPs代码中，还需要知道如何退出线程

始终在编译时启用所有警告。警告是问题在继续之前应该修复的代码中。

BTW：使用单个互斥锁可以消除竞争条件问题

// Race condition
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/syscall.h>

void *runner( void * );
int   program( void );

int x=0;


// <-- compiler raises warning about unused parameter
void * runner(void *arg)
{
    // the returned value from the syscall is a long int
    //printf("T tid: %d   x before: %d\n",
    printf("T tid: %ld   x before: %d\n",
            syscall(SYS_gettid),
            x);
    int i;

    for (i = 0; i < 100000; i++ )
    {
        x = x + 1;
    }

    // <-- the returned value from syscall is a long int
    //printf("T tid: %d   x after: %d\n",
    printf("T tid: %ld   x after: %d\n",
            syscall(SYS_gettid),
            x);

    // <-- this line missing, compiler raises warning
    pthread_exit(NULL);
} // end function: runner


int program()
{
    pthread_t t1,t2,t3,t4;

    printf("Parent pid: %d  x before threads: %d\n",
            getpid(),
            x);

    //int i; // <-- this variable not used, results in compiler warning


    if( pthread_create(&t1, NULL, runner, NULL) )
    {
        printf("Error creating thread 1\n");
        return 1;
    }

    if( pthread_create(&t2, NULL, runner, NULL) )
    {
        printf("Error creating thread 2\n");
        return 1;
    }

    if( pthread_create(&t3, NULL, runner, NULL) )
    {
        // <-- copy and paste error
        //printf("Error creating thread 1\n");
        printf("Error creating thread 3\n");
        return 1;
    }

    if( pthread_create(&t4, NULL, runner, NULL) )
    {
        // <-- copy and paste error
        //printf("Error creating thread 1\n");
        printf("Error creating thread 4\n");
        return 1;
    }


    if( pthread_join(t1, NULL) )
    {
        printf("error joining thread 1");
        return 1;
    }

    if( pthread_join(t2, NULL) )
    {
        // <-- copy and paste error
        //printf("error joining thread 1");
        printf("error joining thread 2");
        return 1;
    }

    if( pthread_join(t3, NULL) )
    {
        // <-- copy and paste error
        //printf("error joining thread 1");
        printf("error joining thread 3");
        return 1;
    }

    if( pthread_join(t4, NULL) )
    {
        // <-- copy and paste error
        //printf("error joining thread 1");
        printf("error joining thread 4");
        return 1;
    }

    printf("Parent pid: %d  x after threads: %d\n",
            getpid(),
            x);
    return 0;
} // end function: program


// <-- this line cause compiler to raise two warnings about unused parameters
//int main(int argc, char *argv[])
int main()
{
    int count=0;

    // <-- there is no loop code perhaps you meant to put the 'while' on the next line
    // loop runs the program count times while(count<5)
    while( count < 5 )
    {
        // <-- program is not run, perhaps you meant to put the 'program()' on the next line
        // running program program();
        program();

        count++;
        // <-- x is not being reset, perhaps you menat to put 'x=0;' on the next line
        //reset global x for next run of program. x=0;
        x=0;
        printf("\n\n");
    }

    return 0;
}  // end function: main

Answer 3

syscall()的原型返回long

#include <sys/syscall.h>
long syscall(long number, ...);

调用printf()时，其格式说明符（如"%d"）应与参数类型匹配（在参数通过可变参数的常规促销之后）。改为

                 v---- long --------v 
                              v----------- int ----------v
printf("T tid: %ld x before: %d\n", syscall(SYS_gettid), x);

警告：格式'％d'需要类型为'int'的参数，但参数2的类型为'long int'[-Wformat =]

3 个答案: