redirect stdout to file - in C - c

Question

i've been trying to redirect the output to a file and reading from file instead of stdin by using a function, but it doesnt seem to be working as when i redirect into a file and check to see if a file has been created with the output there is nothing there. what could be wrong with this function.
/* Redirect a standard I/O file descriptor to a file
* Arguments: filename the file to/from which the standard I/O file
* descriptor should be redirected
* flags indicates whether the file should be opened for reading
* or writing
* destfd the standard I/O file descriptor which shall be
* redirected
* Returns: -1 on error, else destfd
*/
int redirect(char *filename, int flags, int destfd){
int ret;
if(flags == 0){
destfd = open(filename,O_RDONLY);
if (destfd < 0){
return -1;
}
ret = dup2(0,destfd);
if(ret < 0){
return -1;
}
close(destfd);
}
if(flags == 1){
destfd = open(filename,O_APPEND|O_WRONLY);
if (destfd < 0){
return -1;
}
ret = dup2(destfd,1);
if(ret < 0){
return -1;
}
close(destfd);
}
return destfd;
}

Answer 1

There are several problems with your code, not least of which the really awful formatting which make it very hard to read.
For example this call to dup2 is backwards - it's replacing the recently opened destfd with a copy of stdin.
ret = dup2(0,destfd);
a then a few lines later you close destfd.
Your if statements could benefit from you learning about else and else if
if(flags == 0) {
// code
} else if(flags == 1) {
// code
}
Really though you could simplify the whole function by treating the flags parameter as the same flags you'd pass to open and have destfd as the file descriptor you want to replace.
int redirect(char *filename,int flags, int destfd)
{
int fd;
fd=open(filename,flags,S_IRUSR|S_IWUSR);
if(fd!=-1)
{
if(dup2(fd,destfd)==-1)
{
close(fd);
fd=-1;
}
else
{
close(fd);
}
}
return fd;
}
Then you could call it like
redirect("output.txt",O_CREAT|O_WRONLY,1); // Redirect stdout
redirect("input.txt",O_RDONLY,0); // Redirect stdin

Answer 2

The os function dup2() should provide what you need (if not references to exactly what you need).
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.
The dup() function duplicates an open file descriptor. Specifically, it provides an alternate interface to the service provided by the fcntl() function using the F_DUPFD constant command value, with 0 for its third argument. The duplicated file descriptor shares any locks with the original.
On success, dup() returns a new file descriptor that has the following in common with the original:
Same open file (or pipe)
Same file pointer (both file descriptors share one file pointer)
Same access mode (read, write, or read/write)
Everything I said can be found on the manpage of dup