c - C 中动态分配线程

Question

我正在用 C 创建一个小程序，它计算用户输入的数字的能力，直到用户输入负数。它使用线程来完成此操作。

我在运行时遇到段错误，所以我做错了什么，但我不知道到底是什么。

这是我的代码:

/* 
 * File:   main.c
 * Author: thomasvanhelden
 *
 * Created on June 15, 2014, 3:17 AM
 */

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

/**
 * Calculates faculty of given number
 *      Can run as a thread
 * @param param The given number
 * @return Doesn't return anything
 */
void *fac(void *param) {
    int* number = (int*) param;
    int faculty = 1;
    int i; // counter

    printf("De faculteit van %i is: ", number);

    for (i = 2; i <= number; i++) {
        faculty *= i;
    }

    printf("%i\n", faculty);
}

/*
 * 
 */
int main(int argc, char** argv) {
    pthread_t **threads; 
    int getal, numThreads = 0, counter, stop = 0;

    // ask for numbers until user enters negative number
    while (stop == 0) {
        scanf("%i", getal);

        if (getal >= 0) {
            numThreads++;
            threads = realloc(threads, sizeof(pthread_t*) * (numThreads+1));
            threads[numThreads - 1] = malloc(sizeof(pthread_t));

            // Create the thread
            if (pthread_create(&threads[numThreads - 1], NULL, fac, &getal)) {
                // something went wrong
                printf("Error creating thread %i!\n", numThreads);
                return 1;
            }

        } else {
            // User entered negative number and wants to stop
            stop = 1;
        }
    }

    // join all the threads
    for (counter = 0; counter < numThreads; counter++) {
        if (pthread_join(threads[counter], NULL)) {
            printf("Something went wrong joining thread %i\n", counter + 1);
        }
    }


    return (EXIT_SUCCESS);
}

Answer 1

我想你的意思是“阶乘”？无论如何，您的代码存在多个问题。即使没有启用严格标志，我也会收到以下警告:

fac.c:23:40: warning: format specifies type 'int' but the argument has type 'int *' [-Wformat]
    printf("De faculteit van %i is: ", number);
                             ~~        ^~~~~~
fac.c:25:19: warning: ordered comparison between pointer and integer ('int' and 'int *')
    for (i = 2; i <= number; i++) {
                ~ ^  ~~~~~~
fac.c:30:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
fac.c:41:21: warning: format specifies type 'int *' but the argument has type 'int' [-Wformat]
        scanf("%i", getal);
               ~~   ^~~~~
fac.c:49:32: warning: incompatible pointer types passing 'pthread_t **' (aka 'struct _opaque_pthread_t ***') to
      parameter of type 'pthread_t *' (aka 'struct _opaque_pthread_t **'); remove &
      [-Wincompatible-pointer-types]
            if (pthread_create(&threads[numThreads - 1], NULL, fac, &getal)) {
                               ^~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/pthread.h:310:42: note: passing argument to parameter here
int pthread_create(pthread_t * __restrict, const pthread_attr_t * __restrict,
                                         ^
fac.c:63:26: warning: incompatible pointer types passing 'pthread_t *' (aka 'struct _opaque_pthread_t **') to
      parameter of type 'pthread_t' (aka 'struct _opaque_pthread_t *'); dereference with *
      [-Wincompatible-pointer-types]
        if (pthread_join(threads[counter], NULL)) {
                         ^~~~~~~~~~~~~~~~
                         *
/usr/include/pthread.h:333:28: note: passing argument to parameter here
int pthread_join(pthread_t , void **) __DARWIN_ALIAS_C(pthread_join);
                           ^
6 warnings generated.

实际上，如果您在 gdb 下运行该程序，则在输入数字后它会崩溃:

...
4

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000
0x00007fff92c1401a in __svfscanf_l ()
(gdb) bt
#0  0x00007fff92c1401a in __svfscanf_l ()
#1  0x00007fff92c0c0eb in scanf ()
#2  0x0000000100000d72 in main (argc=1, argv=0x7fff5fbffa18) at fac.c:41

查看 bt 的堆栈跟踪，我们立即发现它在调用 scanf() 的第 41 行崩溃。如果您回顾一下编译器警告，它会告诉您它需要一个 int*，但您只传递一个 int。这是因为 scanf 需要一个指针才能使用用户输入修改变量。

如果您修复了所有剩余的警告，您的代码就可以工作。

• 当您需要其值时，需要在 fac 函数中取消引用 number
• 您需要将threads初始化为NULL，以便realloc能够工作(否则它是未定义的，可能非零，因此缓冲区将永远不会被(重新)分配
• pthread_create 和 pthread_join 调用需要更新，以便值/指针与预期签名匹配

进行上述更改后，我可以按预期运行它:

$ ./fac_fixed
3
De faculteit van 3 is: 6
4
De faculteit van 4 is: 24
5
De faculteit van 5 is: 120
6
De faculteit van 6 is: 720
7
De faculteit van 7 is: 5040
8
De faculteit van 8 is: 40320
9
De faculteit van 9 is: 362880
12
De faculteit van 12 is: 479001600
-1

至于代码的结构，我不明白为什么你要创建一个线程数组，因为你一次只按顺序计算一个线程。您实际上最多只需要一个工作线程，但由于 main 除了阻塞联接之外什么也不做，所以它实际上不需要位于线程中。但至少它不会崩溃!