我先发布我的代码，然后解释我遇到的问题：
#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>
#define MAX_ARGS 20
#define BUFSIZE 1024
int get_args(char* cmdline, char* args[])
{
int i = 0;
/* if no args */
if((args[0] = strtok(cmdline, "\n\t ")) == NULL)
return 0;
while((args[++i] = strtok(NULL, "\n\t ")) != NULL) {
if(i >= MAX_ARGS) {
printf("Too many arguments!\n");
exit(1);
}
}
/* the last one is always NULL */
return i;
}
void execute(char* cmdline)
{
int pid, async, oneapp;
char* args[MAX_ARGS];
char* args2[] = {"-l", NULL};
int nargs = get_args(cmdline, args);
if(nargs <= 0) return;
if(!strcmp(args[0], "quit") || !strcmp(args[0], "exit")) {
exit(0);
}
printf("before the if\n");
printf("%s\n",args[nargs - 2]);
int i = 0;
// EDIT: THIS IS WHAT WAS SUPPOSED TO BE COMMENTED OUT
/*
while (args[i] != ">" && i < nargs - 1) {
printf("%s\n",args[i]);
i++;
}
*/
// Presence of ">" token in args
// causes errors in execvp() because ">" is not
// a built-in Unix command, so remove it from args
args[i - 1] = NULL;
printf("Escaped the while\n");
// File descriptor array for the pipe
int fd[2];
// PID for the forked process
pid_t fpid1;
// Open the pipe
pipe(fd);
// Here we fork
fpid1 = fork();
if (fpid1 < 0)
{
// The case where the fork fails
perror("Fork failed!\n");
exit(-1);
}
else if (fpid1 == 0)
{
//dup2(fd[1], STDOUT_FILENO);
close(fd[1]);
//close(fd[0]);
// File pointer for the file that'll be written to
FILE * file;
// freopen() redirects stdin to args[nargs - 1],
// which contains the name of the file we're writing to
file = freopen(args[nargs - 1], "w+", stdin);
// If we include this line, the functionality works
//execvp(args[0],args);
// We're done writing to the file, so close it
fclose(file);
// We're done using the pipe, so close it (unnecessary?)
//close(fd[1]);
}
else
{
// Wait for the child process to terminate
wait(0);
printf("This is the parent\n");
// Connect write end of pipe (fd[1]) to standard output
dup2(fd[1], STDOUT_FILENO);
// We don't need the read end, so close it
close(fd[0]);
// args[0] contains the command "ls", which is
// what we want to execute
execvp(args[0], args);
// This is just a test line I was using before to check
// whether anything was being written to stdout at all
printf("Exec was here\n");
}
// This is here to make sure program execution
// doesn't continue into the original code, which
// currently causes errors due to incomplete functionality
exit(0);
/* check if async call */
printf("Async call part\n");
if(!strcmp(args[nargs-1], "&")) { async = 1; args[--nargs] = 0; }
else async = 0;
pid = fork();
if(pid == 0) { /* child process */
execvp(args[0], args);
/* return only when exec fails */
perror("exec failed");
exit(-1);
} else if(pid > 0) { /* parent process */
if(!async) waitpid(pid, NULL, 0);
else printf("this is an async call\n");
} else { /* error occurred */
perror("fork failed");
exit(1);
}
}
int main (int argc, char* argv [])
{
char cmdline[BUFSIZE];
for(;;) {
printf("COP4338$ ");
if(fgets(cmdline, BUFSIZE, stdin) == NULL) {
perror("fgets failed");
exit(1);
}
execute(cmdline) ;
}
return 0;
}
那么，问题是什么？简单：上面的代码创建了一个具有预期名称的文件，即命令行中提供的名称，该名称放在args [nargs - 1]中。例如，运行程序然后键入
ls＆gt;的test.txt
创建一个名为test.txt的文件...但它实际上并没有向它写任何东西。我确实设法让程序将垃圾字符打印到文件中多次，但这只发生在绝望的冰雹玛丽编码中，我基本上只是试图让程序将SOMETHING写入文件。
我确实认为我已经设法将问题的原因缩小到代码的这个区域：
else if (fpid1 == 0)
{
printf("This is the child.\n");
//dup2(fd[1], STDOUT_FILENO);
close(fd[1]);
//close(fd[0]);
// File pointer for the file that'll be written to
FILE * file;
// freopen() redirects stdin to args[nargs - 1],
// which contains the name of the file we're writing to
file = freopen(args[nargs - 1], "w+", stdout);
// If we include this line, the functionality works
//execvp(args[0],args);
// We're done writing to the file, so close it
fclose(file);
// We're done using the pipe, so close it (unnecessary?)
//close(fd[1]);
}
else
{
// Wait for the child process to terminate
wait(0);
printf("This is the parent\n");
// Connect write end of pipe (fd[1]) to standard output
dup2(fd[1], STDOUT_FILENO);
// We don't need the read end, so close it
close(fd[0]);
// args[0] contains the command "ls", which is
// what we want to execute
execvp(args[0], args);
// This is just a test line I was using before to check
// whether anything was being written to stdout at all
printf("Exec was here\n");
}
更具体地说，我认为问题在于我使用（或尝试使用）dup2（）和管道功能的方式。我基本上是通过消除过程找到的。我花了几个小时评论事情，移动代码，添加和删除测试代码，我发现了以下内容：
1。）删除对dup2（）的调用并使用execvp（args [0]，args）将ls命令的结果输出到控制台。父进程和子进程正确地开始和结束。因此，对execvp（）的调用正常工作。
2。）行
file = freopen(args[nargs - 1], "w+", stdout)
成功创建一个具有正确名称的文件，因此对freopen（）的调用不会失败。虽然现在没有立即证明此函数正常工作，但请考虑事实＃3：
3.）在子进程块中，如果我们将freopen从stdin（而不是stdout）重定向到输出文件并取消对execvp（args [0]，args）的调用，如下所示：
// freopen() redirects stdin to args[nargs - 1],
// which contains the name of the file we're writing to
file = freopen(args[nargs - 1], "w+", stdin);
// If we include this line, the functionality works
execvp(args[0],args);
并运行程序，然后它工作，并且ls命令的结果成功写入输出文件。知道了这一点，似乎可以说freopen（）也不是问题。
换句话说，我唯一能够成功做的就是将父进程中执行的execvp（）调用的输出传递给stdout，然后从stdout传递到文件使用freopen（）。
感谢任何帮助。我昨天上午10点以来一直在这，我完全没有想法。我只是不知道自己做错了什么。为什么这不起作用？