Dup2 redirect stdout to file. When you use dup2() to redirect stdout.

Dup2 redirect stdout to file Modify a common command like grep or cat to take input from a file instead of STDIN by duping the descriptor. Since popen() runs the command via shell, probably one can tell it to redirect stderr to stdout (and send stdout to /dev/null). Commented Sep 23, 2011 at 9:42. The basic problem is >>> sys. More clearly, after the function for one command terminates, at the second call are stdin and stdout on their supposed position? While dealing with redirecting output to a file you may use freopen(). And in the meantime, there's a C source code library that will do all this for you, redirecting stdout or stderr. Some points related to dup/dup2 can be noted please. txt dup2(save_fd, 1); restore stdout close(1); I know I Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog Well, if you want to use printf, then you can actually dup2(file, 1) which makes file descriptor 1 (stdout) write to your newly opened file. Since you're not writing any newlines, it will not flush the buffer automatically in either mode. Just execute it and instead of using system() or whatnot, use CreateProcess(). Once the stdout is redirected to your stream, read from it and send the text to the edit control. same as you can store the output of the program like this suppose, object file name is a, run the following command to save output in a file:. I'm trying to redirect both stdout and stderr to a single file with a C program. My C code running on Linux/FreeBSD/OSX needs to call an external program via the system() function, which will use /bin/sh to interpret the supplied command line. txt, 1);//redirect output to file. I call freopen to redirect the stdout to out. If the file exists then the file open in I would like to redirect stderrto a file AND to stdout. dup2 the write end of the pipe to fd 2. txt. About; Products dup2 to redirect stdout and stderr to another file descriptor. I am using an external, precompiled third-party library that produces a ridiculous amount of output, which I would like to redirect to a log file to keep the console clean. txt", "a+", stdout); Here "a+" for append mode. On the better-way-to-do-this part. No need to flush the stream manually because the std In a program i want all the printfs to be written to syslog. Most of the functions like fprintf or cout are directly output to A pipe is one possible file. stdout is associated with file descriptor 1 and stderr to 2. ; consuming the data in the file descriptor, then into File, then into ReadBuf Insert a sys. Then make a pipe with pipe(). And be careful you don't have other threads running that can steal the os's first file handle after the os. I'm having a strange issue using the dup2 system call to redirect STDOUT to a file. It doesn't appear in the output file? You can't find the output file? You don't see any output in the terminal window? Your compiled program is probably named program if the source code is program. However, the problem is that any execvp() out after dup2 keeps getting redirected back to the file. 3>&2 you duplicate (dup2) filedescritor 2 onto filedescriptor 3, possibly closing filedescriptor 3 (dup2(2,3)) if it's already open (which won't do a thing to your parent process, because this happens in a forked off child (if it does not (redirections on shell The job of init_redirect(&file_desc, &copy_out) is to:. Capture some output. flush() # <--- important when redirecting to files # Duplicate stdout (file descriptor 1) # to a different file descriptor number newstdout = os. I'm posting a portable solution in answer form so it can be accepted. You should read the man pages of pipe and dup2. Blank string on redirection with dup2. This is done by dup2(). As this class modifies global file descriptors, I adapted it to a mutex-guarded static class that protects against multiple Having issues trying to redirect file to stdin using dup2() and open() and execv() Ask Question Asked 3 years, 3 months ago. The code works fine, and redirects only the C++ stdout while leaving unchanged the Python print() calls, however, # flushes logfile_err os. man freopen says: The freopen() function opens the file whose name is the string pointed to by path and associates the stream pointed to by stream with it. // Preserve original file descriptor for All I see is redirection of stdout to a file using dup2 and freopen which is not what I need. stdout. txt is binary file, I int file = open( "logfile", O_CREAT | O_RDWR, 0644 ); duplicate the logfile file descriptor to use with stdout and stderr. the only two methods i've found so far are: run > filename tty filename @Anders, this is close, but for general compatibility you need to first get the file descriptors and streams in a working state. Stderr. h&gt; #include &lt;string. txt The output of this command will be stored into the file. (Edit: Ah, and the misplaced ) as Sean Pedersen shows. The 'stdout' file pointer will continue to work, but anything not informed of the change might not -- Redirect stdout/stderr using dup2, then resinstate later. /input. stdin - 0 stdout - 1 stderr - 2 If you want to redirect stdout and stderr to log. How do I redirect stderr (or stdout+stderr) to a file if I don't know which shell (bash, csh, dash) is interpreting my command?. fileno() == 1 and default_stdout == 1 but after your dup2 call 1 just points to the test. h> #include <fcntl. I have to dup the predefined message printed in stdout to a file with dup2. 1. You can 'hijack' stdout and stderr by replacing the pointers; stdout and stderr are nothing more than FILE *. . What I expected is for "A" to be written to the file and "B" to be written to the screen. While dup2() is the canonical way to redirect descriptors in Linux, there are some alternatives worth mentioning: popen() – Opens a pipe and redirects stdout/stderr. So, am I able to redirect Qualcomm's outstream from stdout to another file in the parent process? Here's a program I'm trying to make: #include &lt;stdio. When does this descriptor get closed? If you dup2(a, STDOUT_FILENO), you close stdout and you make stdout's file descriptor point to the same file table entry as a. The write end of the pipe is still open in the grandparent process. The HANDLE you give to CreateProcess() can stdio normally buffers output to stdout-- it's line-buffered when it's connected to a terminal, fully-buffered when connected to a file. Follow answered May 17, 2018 at 19:14. Viewed 456 times Trouble using dup2 to redirect stdout/stderr into the same file. the terminal, if the calling program is an interactive bash session). Is there some magic I can do with dup2 (or fcntl), so that I redirect stdout to a file (i. Here’s a diagram that represents the previous example: A child process runs a bin file, which is provided by Qualcomm. If you're on a POSIX system, just use the dup2() system call to redirect fd's 0 (stdin), 1 (stdout), and 2 (stderr) to the files of your choice. I want to redirect stdout to a file. Basically, you're ripping the floor out from under your program without guaranteeing that the Right Thing has happened to put a new floor back. e. The command sends its output to the standard output stream, which is now the file in C i want to redirect the output of a process from stdout to write to a "shared memory segment" which can be thought of as a char array or a string with a pointer i know that there is dup2 but it takes file discriptors as argument not a pointer to an array. A Little example with the first two commands. The parent also restores its stdout, but not stdin. The issue here is exactly that doing > foo 2> foo does two open() calls leading into two OFDs and two separate seek positions, while > I am trying to start another program from my process. Hi friends I am facing one problem while redirecting the out of the stderr and stdout to a file let example my problem with a simple example I have a file (say test. Any solution by assignment to stdout is wrong, as stdout is not an lvalue (it's a macro that expands to an expression of type FILE *). This is a portion my code: FILE* fdArch=fopen("file. This is a fairly standard practice, and probably the way shells accomplish it right now. How does one redirect data streams originally meant for printing to the terminal, to a file? The main problem is in how to redirect the stdout to store data into memory, which you dup2(fd, 1); // redirect output to the file . I suggest you open a pipe pair first, then used fdopen() to create a new FILE * which is assiocated with the sending end of the pipe, then point stdout to your new FILE. Follow answered Sep 24, 2011 at 5:53. And when you do the first output, in Python 2 it executes the isatty() system call once for the underlying file, and stores the result. Anyway, get same results. If we want to redirect the output, one way is give, for example, fprintf function more arguments indicating in and out. fileno) to solve your problem. My question is: How can i redirect the out back to the stdout with dup2? dup2 the original stderr (typically the terminal) to a new file descriptor to save it. Share. Now your stderr is a pipe. Proces Then use dup2() magic to allow stdin to read that file’s contents as input. Pipe. Could someone tell me what's wrong with this code? #include &lt;stdio. fileno(), 1) Where output_pipe is an instance of multiprocessing. fileno() Doesn't help you, because it just saves the file number. I am trying to do so like this:child process More specifically, you can dup2() the stdin file descriptor to another file descriptor, do other stuff with stdin, and then copy it back when you want. You can use a redirection around any command, including compound commands. This HANDLE can be a file to which you direct the output. To do that, he needs to close the old stdout and make it a copy of the socket, with dup2 (newsock, STDOUT_FILENO); – What I need to do is redirect content of file to stdin, so it can be sent to user called program. txt Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company I would like to use pipes to redirect stdin and stdout of a child process. I'm confused as to why. dup2 (STDOUT_FILENO, newsock); would close the socket according to the excerpt you quoted. Then terminate that redirect and resume output I want to redirect stdin/stdout to some file. More specifically, you can dup2() the stdin file descriptor to another file descriptor, do other stuff with stdin, and then copy it back when you want. txt will NOT work. After the dup2(p, STDIN_FILENO); operation succeeds, /dev/stdin is no help; it yields the file that p was opened for, not the original standard input. I am trying to redirect stdout to file and back. stdout is a FILE * pointer of the standard output stream. opening a file and calling write() Use dup2 to redirect the output to a file of your choice. I've updated the question – mihajlv. Why I need it. Process 1: redirects its STDOUT to a pipe and then prints random nubers. Forums fork, dup2, and redirection of the stdin/stdout Thread starter bobetko; Start date May 1, 2006; Status Not open for further replies. There are two default outputs in Unix systems, stdout and stderr. int save_fd; save_fd=dup(1); //saves the current stdout close(1); //closes stdout dup2(file. g. Program executes a command "sh" and creates pipes between the parent process and the child process You just need to use dup2() to duplicate the socket's file descriptor onto the stderr and stdout file descriptors. How to redirect stdout to a file and then restore stdout back? 1. When you fork the child to run the ls command, you note the redirection, and open the file; you then use dup2() (or close() and dup()) so that the file descriptor is now standard output for the child; you close the duplicated file descriptor - the one returned by open(); then you execute ls as usual; its standard output is I am using multiprocessing package to spawn a second process from which I would like to redirect stdout and stderr into the first process. Method 1: Using contextlib. The os function dup2() should provide what you need (if not references to exactly what you need). out you could simply do the following: I'm working on a simple shell, but right now I am just trying to understand redirection. Unix is famous for modeling pretty much everything i Use dup2() on both ends to redirect stdin and stdout. Webb. After using dup2() before one of my execs() it redirects to the file in dup2() as expected. Basically I replaced cout's streambuf with one that is implemented using c file I/O which does end up being redirected. stdout is sys. Please check the return My main aim is to implement a program in Rust to redirect stdio over pipes locally and is similar to this. stdout" over fd 1 rather than making a new fd 5. You can do this in a variety of ways. The program works as expected and writes the But how exactly can this redirection be achieved effectively within Python? Below are several methods you can implement to redirect stdout to a file, enhancing the reliability of your applications. I'm using 2 functions which i found here: In C how do you redirect stdin/stdout/stderr to files when making an execvp() or similar call? Below is a simple program i wrote to test the functions after i had errors. I'm doing a fairly straightforward fork/exec tool, and I want to redirect the child's output to a file instead of the parents stdout. I'd stick with write() for a simple program. I am trying to develop a simple command-line client to the server. So when dup2() closes the old descriptor 1 it does not flush the buffer and the content could be flushed to a different output. This way is bad because you probably want to restore your standard output after this system call completes. The following code seems to work. Starting with Python 3. However, when I try to read on the other end, it just hangs. So the program should be executed like this, I mean this is how I execute it in shell and it works: /opt/prog < in. see man dup2. int fd[2]; pipe(fd) and then dup2(fd[WRITE],STDOUT_FILENO) is there a way to use the dup call to duplicate both 1 and 2 to fd[WRITE]? Skip to main content. C++ dup2 and execl. Like "copying" one stream-source to Trouble using dup2 to redirect stdout/stderr into the same file. orig_stdout) # close duplicate 1: on dup2(src, dst). A pipe is actually two files, one at each end, so you can use it to tunnel data by writing to one end and reading I'm looking for an example of redirecting stdout to a file using Perl. But just throwing out the data printed by the DLL would be okay, too. dup is short for duplication and when you do e. dup2(fd, STDOUT_FILENO); dup2(STDOUT_FILENO, STDERR_FILENO); close(fd); Your code is equivalent to the POSIX shell syntax (which is available in all shells whose syntax is based on Bourne shell): 2>&1 >filename which makes stderr go to the old stdout while redirecting stdout to the file. Fd())) But then I don't see how can redirect this another time to stdout. Im uaing GCC 4. A number of operating systems track open files through the use of file descriptors. I tried reading using Pipe. After I connect to the server and setup everything correctly I want to redirect STDIN to the socket connected to the server and the same socket to STDOUT. 4, the contextlib module introduces redirect_stdout, a straightforward way to I am unable to redirect STDOUT to a file. They are stdin, stdout, stderr, in integers they are 0,1,2. @MichaelAnderson I want to redirect a FILE * to stdout rather then redirect stdout to a file. Will reopening standart variables do what I want? Check out the API call SetStdHandle. Modified 13 years, 8 months ago. Also, you can use a tempfile. I would like to capture the messages printed by that external program to stderr When you call os. By default, the Android system sends stdout and stderr (System. 3. For instance when I enter a command like sort < hello. close(fd); close(defout); printf("to file\n"); exit(0); dup2(defout, 1); // The best scenario to understand dup and dup2 is redirection. what you should do next is to open a textfile , and do a Since you asked, here is a simple example. c file should be copied in new mentioned file called t. txt" in Notepad++ to monitor the file change (by manually clicking on the file tab to refresh). But doing so is a bit tricky and unusual, because once you do that you can't write to the original stdout anymore (unless you also save it somewhere else). What dup2 does is copy a file descriptor into another. "); fclose (stdout); return 0; } So I'm trying to redirect standard output to a file using dup(). – Alex Robinson I'm trying to redirect the output of a process to the stdin of another process. def redirect_stdout(): print "Redirecting stdout" sys. is there any way to redirect it to a string? You cannot redirect stdout for a single thread, but you can write to a different fd by e. dup2(fd, STDOUT_FILENO) create a file strem associated to the stdout file descriptor FILE* f = _fdopen(STDOUT When you create a process using CreateProcess() you can choose a HANDLE to which stdout and stderr are going to be written. ar Mon May 28 12:45:51 CEST 2007 Previous message: Issue of redirecting the stdout to both file and screen Next message: Formal interfaces with Python Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] En Mon, 28 May 2007 06:17:39 -0300 edit: using fopen to create the capture file. This fact is described on developer. Plus you don't have a name to (re)open it with. realtime redirecting stdout to file in linux c. Is there a way to redirect to a file then effectively remove the redirection such that stdout (Redirecting to a file would be ideal. So far so good, and when we run this code with /SUBSYSTEM:CONSOLE (project properties → Linker → System) everything works fine: I have a problem when I used dup2 direct stdout first to a temporal file, and after to a common file. close(self. First thing we need to know is that the system has 3 default file ids(or variables indicating output or input sources) that deals with the input and output. My question is: do stdin and stdout go back to their place(0,1) after function terminates, or do i have to do something like savestdin = dup(0). OP wants to keep the socket open and redirect output to it. I am running multiple commands using execvp(). It can, however, be used as a file handle in some ways, according to MSDN. dup2(self. Thanks in advance. I have read you can do similar operations with GDB, but I don't know how you would redirect to the SSH session's STDOUT/STDERR instead of to a file. com in following way:. Q1: If dup2(fd3, STDOUT_FILENO), string2 will be in log. The dup and dup2 functions were what I needed. By: Search Advanced search Home. Uri Simchoni Uri dup2 to redirect stdout and stderr to another file descriptor. I know how I can do that using OS functions dup2() and open() but I read some ways to do that using reopen() on stdin or stdout. Then use fd as input to dup2 to redirect stdout to this capture file. I have a process running on an embedded system (linux). None of the answers worked for my particular case where I needed a cross platform way of redirecting the output as soon as it was echo'd out so that I could follow the logs with tail -f log. Is it possible to redirect stdout twice with dup2? 4. Most of the functions like fprintf or cout are directly output to stdout. Dup2(int(fatal_logfile. Meaning, the other terminal will write to the socket, and my program will read it with getchar() and respond using printf(). /a > file. I don't think I need a fork() because I don't need a different process for execute just the print statement in file. If fd2 is associated with an open file at the time of the call, that file is closed. "); fclose (stdout); return 0; } This redirects stdout to myfile. I am a bit confused about dup2. This will let you use the code without recompiling it. I would like to redirect its ouputs (standard and error) to that of an SSH session. Webb Stephen M. The original standard input might be a pipe, a socket, or some other fancy and ephemeral file type, which cannot be reopened. More flexible than dup2() but POSIX-only. __stdout__ with open(os. I'm going through the socket programming and implemented it well. ( If we are forking the descriptor is duplicated by default in the child /* freopen example: redirecting stdout */ #include <stdio. You need to create a pipe with the pipe() function that will go between ls and grep and other pipe between grep and more. ls > file. That is, at any point in time, STDOUT_FILENO is valid and either it's still the old one, or it is the new one and old one has been closed. Note that based on info here dup and dup2 are deprecated in favour or _dup and _dup2. here is a simple linux terminal command. Improve this question. Here is my code: #include <stdlib. Stephen M. So doing this: default_stdout = sys. Don't use pipe when the output is to go to a file. Modified 3 years, 3 months ago. dup2(output_pipe. It works with a fork(). dup(1) # /dev/null is used just to discard what Note that the zsh documentation says: If the user tries to open a file descriptor for writing more than once, the shell opens the file descriptor as a pipe to a process that copies its input to all the specified outputs, similar to tee, (and there's an analogous comment for multiple inputs and cat). I am trying to redirect output of stdout and stderr using a file. Note: freopen DOES NOT redirect standard output! You are only redirecting the C file pointer 'stdout', not the file descriptor it uses. What happend when I call execve() after using dup2() to redirect stdin? 1. Redirections are implemented via the dup family of system functions. 7. The function is not aware of the buffer which was filled by fprintf(). txt","a",stdout); printf("Oh no!\n"); dup2(o,fileno(stdout)); close(o); Another edit: if you're using it to redirect output from the child process like your comment elsewhere suggest, you can redirect it after the fork. The buffer is flushed automatically when the program exits, at which time it gets written to the last stream that FD 1 is connected to. flush() before the close(1) statement to make sure the redirect 'file' file gets the output. A working example can be found on MSDN here, but is duplicated below in case the link breaks in the future. – Kerrek SB. c. The program opens file1 and redirects stdout to that file. close file_desc in the child process after the dup2 (and before the exec) Share. And if course, there are disk files. 10. Here we get the name of the output file from the command line as before and set that to be the standard output but now execute a command (ls -al / in this example). __stdout__ True Thus you are using the original sys. txt","w",stdout); printf ("This sentence is redirected to a file. Then fileno to get the fd of the capture file. sh)in which i run 2 command in the background ps -ef & ls & and now i am run this file and redirect the output to In order to provide a thread-safe & cross platform solution, I have adapted rmflow's approach into a similar interface. We can avoid this problem by creating the pipe in the first child, like this: File redirect. // crt_dup. Basically, I want to redirect stdin file to stdout file. Here we get the name of the output file from the command I tried to open file using "I/O low level open" (for writting) and to use its file descriptor to make redirection. err) output to /dev/null. input. stdout and sys. Currently, the ls runs, and the output file is created, but the output still goes to stdout and the file is blank. Use. how to restore stdout after using dup2 to capture stdout to file? 2. dup2(file, 1); instead. Let’s start out with a basic “type something into the keyboard, press enter, and get a result” model of running a single command in the terminal with no input/output redirection. Ask Question Asked 13 years, 8 months ago. redirect_stdout; replacing sys. freopen() – Redirects individual streams to a file. Redirect stdout/stderr using what you have done with dup2 is connecting the parent's stdout to child's stdin, leaving the child's stdout without a redirection. You should open an altogether In general, no. Want to redirect stdin to file in execve(). 2. which redirect log file during execution of an open process, it is used as postscript in logrotate process #!/bin/bash pid=$(cat /var/run/app/app. like this: int n, fd[2]; pipe (fd); switch (fork()) { case -1: //error; Something went wrong with call dup2() then, it should have duplicated the fd for "myprog. Open "C:\temp\stdout. I know how redirect stderr to file using dup2: err := syscall. Follow answered Oct 3, 2018 at 5:58. Much important note here is that the dup2 must use direct number instead of _fileno(stdout) because the documentation says it can return negative number and it does if a standard handle is already closed (parent proces has called CreateProcess w/o standard handles inheritance). mkstemp() file in place of 'file'. /program > file. txt and finally closes stdout. Thanks to everyone for your input. Set fd to your file descriptor and fd2 to 1 (stdout). In the terminal after using exec 1>file command, whatever commands I give to terminal, its output doesn't get shown in terminal. This is how redirection works in C (and thus how shells implement it under-the-hood). dup2 : write() redirected but not fprintf() or You can "redirect" stdout into file using freopen(). I want to achieve behavior similar to the of nc tool. txt then I print ("/tmp/crap. We’ll do this using the open and dup2 functions. C stdout to file using dup. For that I tried the following code int main() The code is following. The more useful example of dup2 is input or output (or both) redirection. Using . as returned by _get_osfhandle(1)), so first _close FDs 0-2 to free them for reuse. I know that STDOUT is getting redirected to the file, the output of those commands gets redirected to file. . I'm having trouble making it work without forking. cannot dup2 write end of a pipe to stdout. Now sys. Using posix pipe() and dup() with C++ to redirect I/O problems I'm attempting redirect the Stderr file descriptor from inside the process, but there seems to be no implementations of it, and I don't see a clear route to anything similar to dup2 from C/C++. I say object because it could be a socket as well as a file. I think you were trying to save the previous fd in current_out, but as Seth Robertson points out, this doesn't currently work, since the wrong file descriptor is being saved. Both _dup and _dup2 accept file descriptors as parameters. h&gt; #include &lt;unistd. , anything written to descriptor 1 would go to a file), but then if I used some other mechanism, it would I am working on a project that requires me to have output from within a mini shell on my C program to output to a file. So the string that I believe dup2 should be redirecting output into is blank. perl; io-redirection; Share. txt is a file in the current working directory. Something went wrong with call dup2() then, it should have duplicated the fd for "myprog. Where function is called: At first I tried changing the stdout to fdout similar to the general command in the else statement but that was a no go. Similarly, if output redirection is at the end of the pipeline, then redirect stdout to write the contents of the last command to the specified file. ) – Thanatos On Windows, the return value of the Socket API is a SOCKET, which is not a file descriptor (or necessarily a small, non-negative integer). I have two processes. Assuming you are trying to redirect your stdout to a file 'output. c // This program uses the From In C how do you redirect stdin/stdout/stderr to files when making an execvp() or similar call?: The right way to do it is to replace the file descriptors STDIN_FILENO, STDOUT_FILENO and STDERR_FILENO with the opened files using dup2(). When running it: it prints "aaa" immediately, which is correct; then I input "asdf", however, "bbb" is not printed or flushed. Where entries 0, 1 and 2 are reserved for stdin, stdout and stderr respectively. class outbuf : public std::streambuf { public: outbuf() { setp(0, 0); } virtual int_type overflow(int_type c = traits_type::eof()) { return fputc(c, stdout) == EOF +1, 2>&1 redirects file descriptor 2 (stderr) to file descriptor 1 (stdout) – Sjoerd. Fd()), int(os. But I also want to print all logs to Then both processes (the shell and the child) redirect their stdin and stdout to the same files. e the command should execute on the server side and the output should be redirected to the client side. pid Question is, how can I redirect output to go to the some file instead to stdout? int Search titles and first posts only. When the child process is running, it always prints lots pieces of logs in shell command. h&gt; #include &lt;sys/types. Viewed 4k times If stdout is redirected to a file, the buffer is not flushed unless fflush() is called. At the moment I have the following code: void child(int pipeIn[], int pipeOut[]) { char buff[20]; const char ms You also need to close the file descriptor after you dup2 it to the right FD: no reason to keep it open, and env won't close it until it dies. You should also then close the original files in the child process: contextlib. 4. I was under the impression that using 2 distinct file descriptors for redirection should work. txt or another log viewing app. ). Now this two processes have to be seperated by input (f writes to the file, while s writes to stdout) so I am not sure how to implement it while in a switch statement. The following code redirect all thread output to a single file - int fd = open(<filename_threadid. txt>, <flags>) _dup2(fd, 1) How should I restore the original stdout so the next thread can reliably map its stdout to the filename_threadid? Using a simple linux command you can save the output into the file. Otherwise all sequenced calls will fail, which means _close(fd) will close the I'm trying to write a program that connects to a server opened with nc -v -l 1337 on another terminal, and redirects stdin, stdout, stderr to the socket. You can also use 1 instead of 2 so that stdout gets redirected to the 'file' Share. redirect-stdio true redirects only output generated by java code, but not the native code. I am a newbie in shell scripting and I am using Ubuntu-11. open a new file using the function open(), call the filetemp, and then assign its identifier to the integer pointed at by &file_desc When I launch GDB the targeted process prints a lot of data so I want to redirect it to NULL until a certain point in time. Trouble using dup2 to redirect stdout/stderr into the same file. Commented Jun 23, 2012 at 9:25. How to redirect stdout to a file and then restore stdout back? 0. In addition, dup2() is guaranteed to be atomic. Issue of redirecting the stdout to both file and screen Gabriel Genellina gagsl-py2 at yahoo. 3 as the compiler. Here is my code: Hence we can use freopen to reopen stdout or stderr as a file like here, but we cannot specify the same file twice. If dup2(g_ctl[0], STDOUT_FILENO), string2 won't be received by g_ctl[1]. h&gt; # I'm trying to learn dup2 and switch the stdout to a file rather than terminal. For example: some_function { echo "This also $1 to the file" } { echo "This goes to the file" some_function "goes" } >some_file echo "This does not go This function is especially useful for redirecting predefined streams like stdin, stdout and stderr to specific files (see the example below). In Windows, kernel handles can be duplicated using DuplicateHandle, which calls the system function NtDuplicateObject. Closing file descriptors has something to do with it The following code is writing both "A" and "B" to the file "out. The original stream (if it exists) is closed. Pipe works by connecting the input in fd[0] to the output of fd[1]. close original write end fd. Conditions. The child process is invoked by my parent process, which is developed by me. Redirecting stdout to pipe and reading from it from a single process. I am creating a shell code. txt' then you can write-freopen("output. Example: cat < file \\user wants run cat and redirect content of that file to it by typing this to my program prompt. dup2( file, STDOUT_FILENO); dup2( file, STDERR_FILENO); I hope above code help you. _dup2 forces fd2 to refer to the same file as fd1. txt","w"); char nameBuff[100]; m The way we can redirect the output is by closing the current file descriptor and then reopening it, pointing to the new output. h&gt; int main(int argc, char* a The dup2() version on the other hand ensures the file descriptor will be the correct one, by specifiying it explicitly. One can also use mktemp() (man 3 mktemp) to create a temp file name, compose command for system() to redirect stderr of the command to the temp file and after system() returns read the temp file. txt file. _dup returns the next available file descriptor for the given file. ) Finally, I suspect that the DLL is writing to stdout through the CRT POSIX-like API, because it is a cross-platform DLL. I have a mini shell program that runs small commands, and I would like to make it so when someone has a > filename at the end of a command, it will redirect all text from printf() to a file and instead of the Trouble using dup2 to redirect stdout/stderr into the same file. Use dup2 to swap stdout with file descriptor and back again. close(1) but before the 'file' is opened to use the handle. pid = fork(); // create child if (!pid) // if pid==0 then its a child process . dup2, the file numbers don't change. Hopefully the extra copy of the file being open won't interfere, but don't try call close(5) on the spare copy, I just tested that out and the program immedeately Redirection: executing a process after dup2 . The mode argument is used just as in the fopen() function. When you dup2 a pipe into "stdout", all you do is, in effect, is ask the io library to do an assignment: fds[1] = fds[pipe]. Simply open(2) two files fd1 and fd2 and then use dup2(fd1, 1) for stdout and dup2(fd2, 2) for stderr. The _fileno value of the standard streams is also -2, so you need to reopen stdin, @WilliamPursell There really isn't a difference in this case. Compatibility is not a problem, the code will only run on Unix systems. c > t. Pipe object:. 5. A console is another possibility. txt Below is the code I wrote, but it doesn't seem to work. These should run for 5 seconds and then be killed. I've tried: directly implementing Read (impl Read for Stderr), but it'd take an entire library of code to cover. It's pretty much the same thing as redirecting to pipes. Use the receiving end of the pipe to extract what was written to the 'old' stdout. What your first statement is doing is making every write to STDOUT_FILENO to go to the object represented by new_fd. fflush does not flush and save the captured output. dup/dup2 - Technically the purpose is to share one File table Entry inside a single process by different handles. This would involve the following steps: Create a copy of stdout (new) Create a temp file (tmp) Redirect stdout into tmp; Tell python to use new as stdout; Redirect tmp into the "real" stdout; Tell python to use the "real Join Date Jul 2007 Location Farncombe, Surrey, England Posts 15,677 Any solution using freopen is wrong, as it does not allow you to restore the original stdout. I would like to be able to temporarily redirect the stdout into a temp file, while python still is able to print to stdout. When you use dup2() to redirect stdout. so the childr would print sorted strings to stdout. You should still be able to fix this if you redo gdb and call dup2(4,1) and call dup2(6,2). I want to know if is it even possible to redirect it to a variable without redirecting it first to a file? Im using ubuntu and C to create the code. Stack Overflow. h> #include <stdio. I got out of the habit of using C-arrays when std::array came out, because std::array stores the size of the array with no run time cost. orig_stderr, 2) # stderr file descriptor points to original stdout os. May 1, 2006 #1 bobetko The way open works, it searches for free entry in file descriptor table. dup2 expects file descriptor, also you've messed up the parameters order. com. Q2: The third library have some stdout/stderr log, if using dup2(socket_fd, STDOUT_FILENO), all logs will be collected by socket. Etc. But the cool part is that it lets you assign as many callback functions as @WilliamPursell I'm not sure your clarification improves things :-) How about this: OP is asking if it's possible to direct the called program's stdout to both a file and the calling program's stdout (the latter being the stdout that the called program would inherit if nothing special were done; i. redirect stdin from file descriptor in linux c. txt //output goes to file. android. 0. I replace all printf to syslog so i thought of redirecting stdout and stderr to syslog. C redirecting stdout to file with predefined message. in. Unix associates input with the terminal keyboard and output with the terminal display by default. /* freopen example: redirecting stdout */ #include <stdio. Make a Redirection: executing a process after dup2 . out and System. h I like to redirect output from stdout for each thread to a file. txt", with the first call to open returning 3 and second call returning 4. Then I read in the handout that the out redirection in case of piping should be handled in program 2 so I tried it there too but was confused because program 2 handles the STDIN from the read end of the pipe. In C how do you redirect stdin/stdout/stderr to files when making an execvp() or similar call? 3 how to write the output of a process run using execl to a file in c redirect stdout and stderr to one or more files from inside c++. I'm just hard coding an ls command and trying to write it to a file for now. Here is my code, I am able to redirect output of other commands, when I type ls > t. You should still be able to fix this if you redo gdb and I'm writing my own custom shell, and as a part of my project, when the program reads either "<" or ">" character from user input, it needs to redirect stdin and stdout to a The way we can redirect the output is by closing the current file descriptor and then reopening it, pointing to the new output. I came across something strange - everything works fine if I comment out the first use of printf (before When I handle redirection of IO to a file, a new file must be opened and duped to STDOUT (I do not support input redirection). To still use the same file for both we can redirect stderr to stdout using dup2 like here. This is the example that works everywhere but not sure why it is not working for me. In particular, you can use it with ReadFile() and WriteFile(). :. setprop log. devnull, 'w') You can use dup2() to copy a file descriptor over another FD that already exists-- so you can copy something new/different over stdout or stderr to replace it. I've tried a couple of different methods that I've found here but haven't been able to get it to work. It makes it easier to write safe code. Search titles only. It's an interesting design, having the shell implicitly create extra processes. I know the easiest way to redirect the STDOUT/STDERR is to DUP2 their file descriptors BEFORE forking. I am using multiprocessing. string1 and ls -al output will be received, Why ?. stdout object all the time. I am using freopen and it creates the file in the correct directory but the file is blank. to redirect the console output to a file. its STDOUT/STDERR is the console which is on a serial port. h> int main () { freopen ("myfile. In a UCRT GUI app the standard FDs are initialized to the File handle -2 (i. How do i collect the output of With thanks to the articles in the comments above I found the information I needed. It's just that you now have 2 numbers that both point to the same file. USE2FILES macro is supposed to switch between using either 2 file descriptors (to the same file) which get duped to stdout and stderr respectivly or 1 file descriptor which gets duplicated both to stdout and stderr. Now i want to implement a system call ls in it when the input is given as ls on the client machine, the output of the server should be printed on client machine (i. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company I'm trying to use redirects in C to redirect input to one file and then set standard output back to print to the screen. Is there an equivilant of dup2() I should use? I can't seem to find it. c: /* redirect. Improve this answer. The most simple solution would be use select() and use read()/write() to pass data from STDIN to The C runtime in Windows provides a "low I/O" layer that wraps file handles with 'file descriptors' for use with POSIX functions such as dup2-- but this is just a with-process compatibility layer to facilitate cross-platform C code. c --- function to create a temporary redirect of a * file descriptor in order to execute a function (passed as a * parameter) with that descriptor redirected so it will output * to the redirected desriptor, instead of to the one it used to. redirect_stdout. The relevant difference is that dup() (or dup2() or the shell's n>&m) produces a new file descriptor pointing to the same open file description, while another open() creates a new open file description (along with a new fd pointing to it). txt is a file in the root directory which is probably wrong. h> #include <unistd. txt, then the hello. The OS (libc, loader or kernel, not sure which) sets up 3 open file descriptors On Posix, you might be able to use dup and dup2 on the underlying file descriptors (cf. orig_stdout, 1) # stdout file descriptor points to original stdout os. h&gt; #include &lt;stdlib. You could attempt to use the MS library function, _open_osfhandle(), to turn an OS level file I've written below a few additional comments that should make clearer what it's going on inside the redirect_stdout function:. I need to make the program only have one write statement that writes to stdout. recv_bytes with a I could confirm a related problem with Python 2 in Linux, but not with Python 3. Some shortcuts have been taken for brevity but hopefully it gives you some idea. [Edit] Take a look at using dup2. Limited compared to dup2(). I don't have access to its code but I know it uses stdin and stdout for user interaction. The program is being written for Unix OS but I think using standart C/C++ library instead of OS functions is good. dup2 internally duplicates a file descriptor from the src to dst, closing dst if its already open. Never tried that. cyafbr amkobp enxxb fblxa igir wygmbk vtnyeejn rpmxei wjdxe ujwq