Question

我在C中编写了一个代码来计算文件中的单词和字符。像wc命令一样。代码如下，工作正常。

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>

char* concat(char *s1, char *s2)
{
char *result = malloc(strlen(s1)+strlen(s2)+1);
strcpy(result, s1);
strcat(result, s2);
return result;
} 

int countLines(char *f){
 FILE *file = fopen(f, "r");
int count = 0;
char ch;
while ((ch = fgetc(file)) != EOF){
   if (ch == '\n')
       count++;
}
return count;
}

int countWords(char *f){
char buffer[1];
FILE *file = fopen(f, "r");
  int countW = 0;
  enum states { WSP, WRD };
  int state = WSP;
char last = ' '; 
 while (read(fileno(file),buffer,1) == 1 )
 {
 if ( buffer[0]== ' ' || buffer[0] == '\t' || buffer[0]=='\n' )
 {
    state = WSP;
 }
 else 
 {
    if ( state == WSP )
    {
       countW++;
    }
    state = WRD;
 }
  last = buffer[0];
}
return countW;
}

int countChars(char *f){
 FILE *file = fopen(f, "r");
int chars = 0;
char ch;
while ((ch = fgetc(file))){
     if (ch == EOF) break;
  chars++;
}
return chars;
}

int main(int argc, char** argv)
{
 int lineCount = 0;
 int wordCount = 0;
 int charCount = 0;
 int n = 3;
 int i,status;
 int pids[3];
 char *theprogram = argv[0];
 char *thefile = argv[1];
 if ( argc !=2 )
 {
     printf( "Help: %s filename\n", argv[0]);
 }
 else{
     FILE *file = fopen( argv[1], "r");

   if(file == 0){
         char *sub = concat(theprogram, ": ");
         char *middle = concat(sub, thefile); 
         perror(middle);
   }
   else{
         for (i = 0; i < n; i++) {
             pids[i] = fork();
             if ( pids[i] < 0) { 
                perror("fork"); 
                exit(-1); 
            } else if (pids[i] == 0) { 
                if (i==0){
                        lineCount = countLines(argv[1]);
                        printf("This is child proccess %d, and the number of lines is %d\n", i+1, lineCount);
                        exit(0);
                    }
                    else if (i==1){ 
                        wordCount = countWords(argv[1]);
                        printf("This is child proccess %d, and the number of words is %d\n", i+1, wordCount);
                        exit(0);
                    }
                    else if (i==2){
                        charCount += countChars(argv[1]);
                        printf("This is child proccess %d, and the number of characters is %d\n", i+1, charCount);
                        exit(0);
                    }
            } 
          }
            for(i = 0; i < n; i++){
                wait(NULL);
            }
            return 0;      
            }
 } 
}

我遇到的问题是：

输出并不总是有序，如下所示：

//GOOD
This is child proccess 1, and the number of lines is 5
This is child proccess 2, and the number of words is 5
This is child proccess 3, and the number of characters is 159

//BAD
This is child proccess 1, and the number of lines is 5
This is child proccess 3, and the number of characters is 159
This is child proccess 2, and the number of words is 5

第三个过程有时会在第二个过程结束。我该如何防止这种情况？

Answer 1

快速而肮脏的方法：

        } else if (pids[i] == 0) { 
            usleep(i*1000);  /* wait 1ms for child 1, 2ms for child 2,... */
            if (i==0){
                    lineCount = countLines(argv[1]);
                    printf("This is child proccess %d, and the number of lines is %d\n", i+1, lineCount);
                    exit(0);
                }
                else if (i==1){ 
                    wordCount = countWords(argv[1]);
                    printf("This is child proccess %d, and the number of words is %d\n", i+1, wordCount);
                    exit(0);
                }
                else if (i==2){
                    charCount += countChars(argv[1]);
                    printf("This is child proccess %d, and the number of characters is %d\n", i+1, charCount);
                    exit(0);
                }
        }

另一种方法是让父进程创建三个UNIX套接字，并使用select()或类似内容来监听所有这些套接字。 Childs将连接并写入套接字，退出，然后，父进程将以正确的顺序打印收到的信息。

Answer 2

分叉进程将与主线代码异步执行，因此如果您希望它们按顺序发生，则需要使用某种标志。这可以是信号量，互斥量，或者在if条件中设置一个布尔值。

您需要使用信号量或互斥锁来保护易受感染的数据免受异步写入的影响，但在此上下文中这不是问题。

Answer 3

我认为你可以使用线程而不是进程，初始化一些全局变量并在子线程中分配变量值。控制父线程中的输出。

如何在C中按顺序执行子进程

3 个答案: