In C how do you redirect stdin/stdout/stderr to files when making an execvp() or similar call?
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?
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 ...
}
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);
}