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PODisabling stdout buffering of a forked process
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copied!<p>I wrote a code in C/C++ which forks a child process, duplicates the stdin/stdout into a pipe ends and calls execvp.</p> <p>Everything is working fine (i.e. the output from stdin/err/out is captured by the parent process)</p> <p>The problem is that the child stdout is buffered.</p> <p>so if the child code looks like this:</p> <pre><code>printf("Enter any key and hit ENTER:\n"); fgets(line); printf("read: %s\n", line); exit(0); </code></pre> <p>In the parent process I don't see the line 'Enter any key:' - it will be "flushed" only after the program calls exit (which auto flushes the stdout buffer) or an explicit call to 'flush(stdout)' is added</p> <p>I did some research and tried adding a call to disable the stdout buffering by adding a call to:</p> <p>setvbuf(stdout, NULL, _IONBF, 0); just before calling execvp(...) in the parent process</p> <p>so the relevant code looks now like this:</p> <pre><code>int rc = fork(); if ( rc == 0 ) { // Child process if(workingDirectory.IsEmpty() == false) { wxSetWorkingDirectory( workingDirectory ); } int stdin_file = fileno( stdin ); int stdout_file = fileno( stdout ); int stderr_file = fileno( stderr ); // Replace stdin/out with our pipe ends dup2 ( stdin_pipe_read, stdin_file ); close( stdin_pipe_write ); dup2 ( stdout_pipe_write, stdout_file); dup2 ( stdout_pipe_write, stderr_file); close( stdout_pipe_read ); setvbuf(stdout, NULL, _IONBF, 0); // execute the process execvp(argv[0], argv); exit(0); } </code></pre> <p>With no luck.</p> <p>Any ideas?</p> <p>EDIT:</p> <p>here is a sample of the parent code, the only thing needs changing is the path to the child executable:</p> <pre><code>#include <unistd.h> #include <signal.h> #include <sys/types.h> #include <sys/select.h> #include <errno.h> #include <sys/wait.h> #include <string> #include <string.h> #include <cstdio> static int read_handle(-1); static pid_t pid; bool read_from_child(std::string& buff) { fd_set rs; timeval timeout; memset(&rs, 0, sizeof(rs)); FD_SET(read_handle, &rs); timeout.tv_sec = 1; // 1 second timeout.tv_usec = 0; int rc = select(read_handle+1, &rs, NULL, NULL, &timeout); if ( rc == 0 ) { // timeout return true; } else if ( rc > 0 ) { // there is something to read char buffer[1024*64]; // our read buffer memset(buffer, 0, sizeof(buffer)); if(read(read_handle, buffer, sizeof(buffer)) > 0) { buff.clear(); buff.append( buffer ); return true; } return false; } else { /* == 0 */ if ( rc == EINTR || rc == EAGAIN ) { return true; } // Process terminated int status(0); waitpid(pid, &status, 0); return false; } } void execute() { char *argv[] = {"/home/eran/devl/TestMain/Debug/TestMain", NULL}; int argc = 1; int filedes[2]; int filedes2[2]; // create a pipe int d; d = pipe(filedes); d = pipe(filedes2); int stdin_pipe_write = filedes[1]; int stdin_pipe_read = filedes[0]; int stdout_pipe_write = filedes2[1]; int stdout_pipe_read = filedes2[0]; int rc = fork(); if ( rc == 0 ) { // Child process int stdin_file = fileno( stdin ); int stdout_file = fileno( stdout ); int stderr_file = fileno( stderr ); // Replace stdin/out with our pipe ends dup2 ( stdin_pipe_read, stdin_file ); close( stdin_pipe_write ); dup2 ( stdout_pipe_write, stdout_file); dup2 ( stdout_pipe_write, stderr_file); close( stdout_pipe_read ); setvbuf(stdout, NULL, _IONBF, 0); // execute the process execvp(argv[0], argv); } else if ( rc < 0 ) { perror("fork"); return; } else { // Parent std::string buf; read_handle = stdout_pipe_read; while(read_from_child(buf)) { if(buf.empty() == false) { printf("Received: %s\n", buf.c_str()); } buf.clear(); } } } int main(int argc, char **argv) { execute(); return 0; } </code></pre>

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