I am working on a program that asks the user to input s,f or 0 as user input. S prints a predefined message to the system and f writes that predefined message to a file given by the user as an argument. 0 terminates the program.
I need to make the program only have one write statement that writes to stdout. I have to dup the predefined message printed in stdout to a file with dup2.
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.
When you input f, it shouldn't print anything to stdout.
Here is my code:
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <sys/wait.h>
#define BUFFER_SIZE 128
#define PERMS 0666
int main(int argc, char *argv[])
{
char outBuffer[BUFFER_SIZE] = "This is a message\n";
int count;
int fd;
char input =0;
int a;
int c;
int k[2];
pipe(k);
if(argc!=2){
printf("Provide an valid file as an argument\n");
exit(1);
}
if((fd = open(argv[1],O_CREAT|O_WRONLY|O_APPEND,PERMS)) == -1)
{
printf("Could not open file\n");
exit(1);
}
printf("0 to terminate, s to write to stdout, f to write to file\n");
do{
scanf(" %c", &input);
switch(input)
{
case 'f':
if((c = fork()) == 0)
{
close(k[0]);
dup2(fd,1);
close(k[1]);
}
else
{
close(k[0]);
close(k[1]);
wait(0);
wait(0);
}
break;
case 's':
write(1,outBuffer,strlen(outBuffer));
break;
default:
printf("Invalid Choice\n");
}
}while(input != '0');
close(fd);
return 0;
}
f right now just directs stdout to the file after pressing s(prints the outBufer message) or after triggering the default switch
my desired output:
f
f
s
This is a message
s
This is a message
f
file contains:
This is a message
This is a message
This is a message
Here's my take on the problem; I removed fork() and pipe() both to compile natively on Windows (gcc -g -std=c11 -Wall -c main.c && gcc main.o -o main.exe) and because I don't think they are required. When I compiled/tested with MSYS2, I had to explicitly flush stdout, or the informational message wasn't displayed until quitting.
I refactored the switch block so that we can control the main loop with break and continue, which I rewrote as a while(1).
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#define BUFFER_SIZE 128
#define PERMS 0666
int main(int argc, char *argv[]) {
char outBuffer[BUFFER_SIZE] = "This is a message\n";
int fd, fd_f, fd_s;
char input = 0;
if(argc != 2){
printf("Provide an valid file as an argument\n");
exit(1);
}
if((fd_f = open(argv[1], O_CREAT|O_WRONLY|O_APPEND, PERMS)) == -1){
printf("Could not open file\n");
exit(1);
}
printf("0 to terminate, s to write to stdout, f to write to file\n");
fflush(stdout); // messages are not printed on MSYS2 without explicitly flushing
fd_s = dup(STDOUT_FILENO); // set fd_s to stdout with dup(1)
fd = dup(STDOUT_FILENO);
// if fd is not initialised, calling dup2 will close(0) [i.e. close(stdin)]!
while(1) {
scanf("%c", &input);
fflush(stdin);
if(input == '0') break;
else if(input == 'f') dup2(fd_f, fd); // close(fd) and alias fd_f to fd
else if(input == 's') dup2(fd_s, fd);
else {
printf("Invalid Choice\n");
fflush(stdout);
continue;
}
write(fd, outBuffer, strlen(outBuffer));
}
close(fd_f);
close(fd);
return 0;
}
Output:
stdout
>main.exe file.txt
0 to terminate, s to write to stdout, f to write to file
f
f
s
This is a message
s
This is a message
f
p
Invalid Choice
0
>
# ./main.exe file.txt
0 to terminate, s to write to stdout, f to write to file
f
f
s
This is a message
f
p
Invalid Choice
0
file.txt
This is a message
This is a message
This is a message