dup2

问题 I have a question regarding file descriptors in Unix and C programming. Let's say I use pipe(fd) to get file descriptor 3 and 4 for the pipe ends, 3 connects to the read end and 4 to the write end. Now I use dup2(fd[write_end],1) to copy the descriptor of the write end (which was 4) to file descriptor 1 in my process. If I now do close(fd[write_end]) will it close descriptor 1 or descriptor 4? 回答1: After a successful call to dup2 , both file descriptors are valid. When you then call close(fd

问题 i have a code like this... c = fork(); if(c==0) { close(fd[READ]); if (dup2(fd[WRITE],STDOUT_FILENO) != -1) execlp("ssh", "ssh", host, "ls" , NULL); _exit(1); } close(fd[WRITE]); fd[READ] and fd[WRITE] are pipe file descriptors. when i run it continuously, there are a lot of zombie processes when i use ps ax. How to rectify this? Is this because i am not using the parent to wait for the exit status of the child process... 回答1: If you have no intention to wait for your child processes, set the

问题 Is there some magic I can do with dup2 (or fcntl ), so that I redirect stdout to a file (i.e., anything written to descriptor 1 would go to a file), but then if I used some other mechanism, it would go to the terminal output? So loosely: int original_stdout; // some magic to save the original stdout int fd; open(fd, ...); dup2(fd, 1); write(1, ...); // goes to the file open on fd write(original_stdout, ...); // still goes to the terminal 回答1: A simple call to dup will perform the saving. Here

问题 Using fork I created a child and in the child I'm executing the ls command using execl . To send the output to parent,I used pipe and dup . The parent then prints the output. The code gives the expected output, but when I tried to restore back the stdout which I saved initially in stdout_holder , nothing is printed on terminal (when I used printf("hello") or the execl statement below it). However after few observations, it is observed that hello is printed only when nothing is done after

问题 Using fork I created a child and in the child I'm executing the ls command using execl . To send the output to parent,I used pipe and dup . The parent then prints the output. The code gives the expected output, but when I tried to restore back the stdout which I saved initially in stdout_holder , nothing is printed on terminal (when I used printf("hello") or the execl statement below it). However after few observations, it is observed that hello is printed only when nothing is done after

问题 Using fork I created a child and in the child I'm executing the ls command using execl . To send the output to parent,I used pipe and dup . The parent then prints the output. The code gives the expected output, but when I tried to restore back the stdout which I saved initially in stdout_holder , nothing is printed on terminal (when I used printf("hello") or the execl statement below it). However after few observations, it is observed that hello is printed only when nothing is done after

问题 I'm creating a shell in C, and I need help implementing input and output redirection. When I try to create a file using ">" I get an error message saying the file does not exist. When I try to do something like ls > test.txt; it won't create a new file. I updated the code with the suggestions provided to me, but now I got different errors. However, a new file is still not created for the output redirection. This is my full code: #include <stdio.h> #include <stdlib.h> #include <string.h>

问题 This is a follow up question from here specifically concerning its answer. From a python module I am calling a Hello World executable that simply prints Hello World to the stdout. I am interested in redirecting that output to a python StringIO and ran into this answer which almost brings me all the way to the solution. The critical part of this answer is this code segment: 1. def redirect_stdout(): 2. print "Redirecting stdout" 3. sys.stdout.flush() # <--- important when redirecting to files

问题 I am trying to redirect the output from ls to a file, in a shell I created in C. I type in: ls > junk and what I get out is: ls: cannot access >: No such file or directory Then if I use CTRL-D to exit the shell it prints the results of the ls command to the screen before exiting. I tried to use print statements to figure out where it is happening and no print statements get printed after: dup2(f, STDOUT_FILENO); Also tried dup2(f, 1); Code: pid = fork(); if(pid == 0) { // Get the arguments

问题 I am trying to redirect the output from ls to a file, in a shell I created in C. I type in: ls > junk and what I get out is: ls: cannot access >: No such file or directory Then if I use CTRL-D to exit the shell it prints the results of the ls command to the screen before exiting. I tried to use print statements to figure out where it is happening and no print statements get printed after: dup2(f, STDOUT_FILENO); Also tried dup2(f, 1); Code: pid = fork(); if(pid == 0) { // Get the arguments