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

Question

这段代码是关于。

比赛条件： 调度和编译器行为在进程或线程同步中起着重要作用。演示同步需求的最简单场景来自试图修改共享变量值的两个线程/进程之间创建的竞争条件，这通常会导致数据不一致和错误结果。下面的例子演示了这种情况：

我是 C 的新手，我对这个警告所发生的事情感到困惑。警告是什么意思，我该如何解决。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);

代码如下：

// 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

您必须将"%d" 更改为"%ld"，"%d" 用于签名int，这里l 代表long，因此"%ld" 用于签名long int。

Answer 2

syscall() 的原型返回 long

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

当调用printf() 时，它的格式说明符，如"%d"，应该与参数的类型相匹配（在参数通过可变参数的通常提升之后）。改为

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

Answer 3

编写代码时应始终考虑到人类会阅读它。 甚至作者在阅读代码时也遇到了重大问题 如许多“复制和粘贴”错误所示

注意：当'//'注释在一行上开始时，该行的所有其余部分都是注释

除 main 之外的任何函数都应该有一个原型。 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，也不适用于 具有交叉调用路径的程序 IE。始终提供原型语句。

当调用某个函数时，始终知道（即查找）完整的细节 关于那个功能。对于 OP 发布的代码， 这尤其适用于 syscall() 函数。

编写线程时，需要了解的不仅仅是原型 为一个线程。就像在OPs代码中一样，也需要知道如何退出线程

总是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。

顺便说一句：使用单个互斥体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