[英]Pipe to a grep program in C
我正在尝试复制此 bash 命令: ps aux | grep bash
ps aux | grep bash
转 C 语言。 我写了一些东西,但它似乎不起作用。 结果实际上什么也没发生,提示在 bash 中再次打印。 这是代码:
int main()
{
int pid;
int pip[2];
char *argexec1[] = {"/bin/grep", "bash", NULL};
char *argexec2[] = {"/bin/ps", "aux", NULL};
if (pipe(pip) == -1){
perror("pipe error \n");
exit(1);
}
if (pid = fork() == -1){
perror("fork error");
exit(1);
}
if (pid == 0)
{
dup2(pip[0], 0);
close(pip[1]);
close(pip[0]);
execvp("grep", argexec1);
perror("exec1 failed\n");
exit(1);
}
else
{
dup2(pip[1], 1);
close(pip[0]);
close(pip[1]);
execvp("ps", argexec2);
perror("exec2 failed\n");
exit(1);
}
在子进程中,如果execvp()失败,不要调用exit()而是 _ exit()否则继承的 I/O 上下文可能会触发重复的 I/O 刷新和其他一些不受欢迎的事情。
. “==”的优先级高于“=”。 这会使您的声明在此处失败:
if (pid = fork() == -1)
添加括号:
if ((pid = fork()) == -1)
这是您的修复程序:
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
int main(void)
{
int pid;
int pip[2];
char *argexec1[] = {"/bin/grep", "bash", NULL};
char *argexec2[] = {"/bin/ps", "aux", NULL};
if (pipe(pip) == -1){
perror("pipe error \n");
exit(1);
}
if ((pid = fork()) == -1){
perror("fork error");
exit(1);
}
if (pid == 0) {
// Child process
dup2(pip[0], 0);
close(pip[1]);
close(pip[0]);
execvp("grep", argexec1);
perror("exec1 failed\n");
_exit(1);
} else {
// Father process
dup2(pip[1], 1);
close(pip[0]);
close(pip[1]);
execvp("ps", argexec2);
perror("exec2 failed\n");
exit(1);
}
return 0;
}
但实际上,如果你想让它像shell一样,你应该在退出程序之前等待命令结束并返回管道中最后一个进程的退出代码。 所以,父亲应该启动两个进程并等待它们的结束:
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>
int main(void)
{
int status1, status2;
int pid1, pid2;
int pip[2];
char *argexec1[] = {"/bin/grep", "bash", NULL};
char *argexec2[] = {"/bin/ps", "aux", NULL};
if (pipe(pip) == -1){
perror("pipe error \n");
exit(1);
}
if ((pid1 = fork()) == -1){
perror("fork error");
exit(1);
}
if (pid1 == 0) {
// Child process#1
dup2(pip[0], 0);
close(pip[1]);
close(pip[0]);
execvp("grep", argexec1);
perror("exec1 failed\n");
_exit(2);
} else {
// Father process
if ((pid2 = fork()) == -1){
perror("fork error");
exit(1);
}
if (pid2 == 0) {
// Child process#2
dup2(pip[1], 1);
close(pip[1]);
close(pip[0]);
execvp("ps", argexec2);
perror("exec2 failed\n");
_exit(2);
} else {
// Father process
close(pip[0]);
close(pip[1]);
// Wait for the end of the programs
if (-1 == waitpid(pid1, &status1, 0)) {
perror("wait1 error");
exit(1);
}
if (-1 == waitpid(pid2, &status2, 0)) {
perror("wait2 error");
exit(1);
}
// Return the exit code of the last program in the pipe
if (WIFEXITED(status1)) {
return WEXITSTATUS(status1);
} else {
// The process may have received a signal, return the whole status...
return status1;
}
}
}
return 0;
}
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