In C how do you redirect stdin/stdout/stderr to files when making an execvp() or similar call?
96,100
Solution 1
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()
. You should also then close the original files in the child process:
else if (pid == 0)
{
dup2(fileno(someopenfile), STDIN_FILENO);
dup2(fileno(someotherfile), STDOUT_FILENO);
dup2(fileno(somethirdopenfile), STDERR_FILENO);
fclose(someopenfile);
fclose(someotheropenfile);
fclose(somethirdopenfile);
execvp(args[0], args);
// handle error ...
}
Solution 2
Take a look at freopen
function.
I had to do something similar with stdout
and wrote two functions that do the work for me:
static int fd;
static fpos_t pos;
void switchStdout(const char *newStream)
{
fflush(stdout);
fgetpos(stdout, &pos);
fd = dup(fileno(stdout));
freopen(newStream, "w", stdout);
}
void revertStdout()
{
fflush(stdout);
dup2(fd, fileno(stdout));
close(fd);
clearerr(stdout);
fsetpos(stdout, &pos);
}
Solution 3
You can use this when stdin , stdout , stderr are terminal-
//change stdin,stdout,stderr
freopen("new_stdin","r",stdin);
freopen("new_stdout","r",stdout);
freopen("new_stderr","r",stderr);
//----do something;
//reset stdin,stdout,stderr
freopen("/dev/tty","r",stdin);
freopen("/dev/tty","r",stdout);
freopen("/dev/tty","r",stderr);
Author by
Matt
If I'm giving you a hard time, it's because I'm trying to learn as much as I can from you.
Updated on July 09, 2022Comments
-
Matt almost 2 years
I have the following code:
pid_t pid = fork(); if (pid == -1) { // ... } else if (pid == 0) { stdin = someopenfile; stdout = someotherfile; stderr = somethirdopenfile; execvp(args[0], args); // handle error ... } else { // ... }
The problem is, the input/output of the
execvp()
call is still the console, rather than the files. Clearly I am doing something wrong, what is the right way to do this?