[英]Linux 3.0: Executing child process with piped stdin/stdout
在Linux 3.0 / C ++下:
我想要一個執行以下操作的函數:
string f(string s)
{
string r = system("foo < s");
return r;
}
顯然上面的方法不起作用,但你明白了。 我有一個字符串s,我想傳遞作為應用程序“foo”的子進程執行的標准輸入,然后我想將其標准輸出記錄到字符串r然后返回它。
我應該使用linux系統調用或posix函數的組合?
eerpini提供的代碼無法正常工作。 請注意,例如,之后使用在父級中關閉的管道末端。 看着
close(wpipefd[1]);
以及隨后寫入該封閉描述符。 這只是換位,但它表明此代碼從未被使用過。 以下是我測試過的版本。 不幸的是,我改變了代碼風格,所以這不被接受為eerpini代碼的編輯。
唯一的結構變化是我只重定向子進程中的I / O(注意dup2調用僅在子路徑中。)這非常重要,因為否則父進程的I / O會搞亂。 感謝eerpini的初步答案,我用它來開發這個。
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#define PIPE_READ 0
#define PIPE_WRITE 1
int createChild(const char* szCommand, char* const aArguments[], char* const aEnvironment[], const char* szMessage) {
int aStdinPipe[2];
int aStdoutPipe[2];
int nChild;
char nChar;
int nResult;
if (pipe(aStdinPipe) < 0) {
perror("allocating pipe for child input redirect");
return -1;
}
if (pipe(aStdoutPipe) < 0) {
close(aStdinPipe[PIPE_READ]);
close(aStdinPipe[PIPE_WRITE]);
perror("allocating pipe for child output redirect");
return -1;
}
nChild = fork();
if (0 == nChild) {
// child continues here
// redirect stdin
if (dup2(aStdinPipe[PIPE_READ], STDIN_FILENO) == -1) {
exit(errno);
}
// redirect stdout
if (dup2(aStdoutPipe[PIPE_WRITE], STDOUT_FILENO) == -1) {
exit(errno);
}
// redirect stderr
if (dup2(aStdoutPipe[PIPE_WRITE], STDERR_FILENO) == -1) {
exit(errno);
}
// all these are for use by parent only
close(aStdinPipe[PIPE_READ]);
close(aStdinPipe[PIPE_WRITE]);
close(aStdoutPipe[PIPE_READ]);
close(aStdoutPipe[PIPE_WRITE]);
// run child process image
// replace this with any exec* function find easier to use ("man exec")
nResult = execve(szCommand, aArguments, aEnvironment);
// if we get here at all, an error occurred, but we are in the child
// process, so just exit
exit(nResult);
} else if (nChild > 0) {
// parent continues here
// close unused file descriptors, these are for child only
close(aStdinPipe[PIPE_READ]);
close(aStdoutPipe[PIPE_WRITE]);
// Include error check here
if (NULL != szMessage) {
write(aStdinPipe[PIPE_WRITE], szMessage, strlen(szMessage));
}
// Just a char by char read here, you can change it accordingly
while (read(aStdoutPipe[PIPE_READ], &nChar, 1) == 1) {
write(STDOUT_FILENO, &nChar, 1);
}
// done with these in this example program, you would normally keep these
// open of course as long as you want to talk to the child
close(aStdinPipe[PIPE_WRITE]);
close(aStdoutPipe[PIPE_READ]);
} else {
// failed to create child
close(aStdinPipe[PIPE_READ]);
close(aStdinPipe[PIPE_WRITE]);
close(aStdoutPipe[PIPE_READ]);
close(aStdoutPipe[PIPE_WRITE]);
}
return nChild;
}
由於您希望對流程進行雙向訪問,因此您必須使用管道明確地執行popen在幕后的操作。 我不確定這是否會在C ++中發生任何變化,但這是一個純粹的C示例:
void piped(char *str){
int wpipefd[2];
int rpipefd[2];
int defout, defin;
defout = dup(stdout);
defin = dup (stdin);
if(pipe(wpipefd) < 0){
perror("Pipe");
exit(EXIT_FAILURE);
}
if(pipe(rpipefd) < 0){
perror("Pipe");
exit(EXIT_FAILURE);
}
if(dup2(wpipefd[0], 0) == -1){
perror("dup2");
exit(EXIT_FAILURE);
}
if(dup2(rpipefd[1], 1) == -1){
perror("dup2");
exit(EXIT_FAILURE);
}
if(fork() == 0){
close(defout);
close(defin);
close(wpipefd[0]);
close(wpipefd[1]);
close(rpipefd[0]);
close(rpipefd[1]);
//Call exec here. Use the exec* family of functions according to your need
}
else{
if(dup2(defin, 0) == -1){
perror("dup2");
exit(EXIT_FAILURE);
}
if(dup2(defout, 1) == -1){
perror("dup2");
exit(EXIT_FAILURE);
}
close(defout);
close(defin);
close(wpipefd[1]);
close(rpipefd[0]);
//Include error check here
write(wpipefd[1], str, strlen(str));
//Just a char by char read here, you can change it accordingly
while(read(rpipefd[0], &ch, 1) != -1){
write(stdout, &ch, 1);
}
}
}
有效地你這樣做:
dup()用於在文件描述符表中創建重復條目。 而dup2()更改描述符指向的內容。
注意:正如Ammo @在他的解決方案中所提到的,我上面提供的內容或多或少是一個模板,如果你只是試圖執行代碼它將無法運行,因為很明顯有一個exec *(函數族)缺失,所以child會在fork()之后幾乎立即終止。
Ammo的代碼有一些錯誤處理錯誤。 子進程在dup失敗后返回而不是退出。 也許孩子的副本可以替換為:
if (dup2(aStdinPipe[PIPE_READ], STDIN_FILENO) == -1 ||
dup2(aStdoutPipe[PIPE_WRITE], STDOUT_FILENO) == -1 ||
dup2(aStdoutPipe[PIPE_WRITE], STDERR_FILENO) == -1
)
{
exit(errno);
}
// all these are for use by parent only
close(aStdinPipe[PIPE_READ]);
close(aStdinPipe[PIPE_WRITE]);
close(aStdoutPipe[PIPE_READ]);
close(aStdoutPipe[PIPE_WRITE]);
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