Scanf in while loop not able to input anything - c

Question

I'm using a Linux 2.0.26 VM and never have this problem.
The while loop works because I added a printf inside it in order to test it.
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
void main()
{
int i;
mknod("pipe.txt", S_IFIFO | 0666, 0);
for (i = 0; i < 2; i++) {
if (fork() == 0) {
if (i == 0)
to_pipe();
//else
// pipe_a_archivo();
}
}
wait(NULL);
unlink("pipe.txt");
}
void to_pipe()
{
int num, fdini;
fdini = open("pipe.txt", O_WRONLY);
do {
//printf("Test");
scanf("%d", &num);
write(fdini, &num, sizeof(int));
} while (num != 0);
close(fdini);
unlink("pipe.txt");
exit();
}
This program should get numbers from keyboard and write them in a file. My problem is that whenever I execute the program from the command line, nothing happens, scanf() doesn't work because it doesn't let me input any number. I know for sure that the loop works because if uncomment printf() it prints on the screen. Any help on solving this problem?

Answer 1

I think that most of your problem is due to you not implementing the pipe_a_archivo() function to read from the FIFO and write the data to a file. Certainly, the code in the question is not a good MCVE (Minimal, Complete, Verifiable Example). Amongst other problems, there is no action for the second iteration of the loop.
This code error checks function calls, and includes a plausible implementation of pipe_a_archivo(), and then works sensibly:
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <unistd.h>
static void to_pipe(void);
static void pipe_a_archivo(void);
#define FIFO_NAME "pipe.txt"
#define FILE_NAME "archive.txt"
int main(void)
{
if (mkfifo(FIFO_NAME, 0666) != 0)
{
fprintf(stderr, "failed to create FIFO '%s'\n", FIFO_NAME);
exit(EXIT_FAILURE);
}
if (fork() == 0)
to_pipe();
if (fork() == 0)
pipe_a_archivo();
int corpse;
int status;
while ((corpse = wait(&status)) > 0)
printf("PID %d exited with status 0x%.4X\n", corpse, status);
unlink("pipe.txt");
}
static void to_pipe(void)
{
int num, fdini;
fdini = open(FIFO_NAME, O_WRONLY);
do
{
printf("Enter a number: ");
fflush(stdout);
scanf("%d", &num);
write(fdini, &num, sizeof(int));
} while (num != 0);
close(fdini);
exit(0);
}
static void pipe_a_archivo(void)
{
int fd_in = open(FIFO_NAME, O_RDONLY);
if (fd_in < 0)
{
fprintf(stderr, "Failed to open FIFO '%s' for reading\n", FIFO_NAME);
exit(EXIT_FAILURE);
}
FILE *fp_out = fopen(FILE_NAME, "w");
if (fp_out == NULL)
{
fprintf(stderr, "Failed to open file '%s' for writing\n", FILE_NAME);
exit(EXIT_FAILURE);
}
int num;
while (read(fd_in, &num, sizeof(num)) == sizeof(num))
{
fprintf(fp_out, "%d\n", num);
}
close(fd_in);
fclose(fp_out);
exit(0);
}
I removed the loop in main() because a loop that tests which iteration it is on and then calls an appropriate function is really not a good design. This code also only deletes the FIFO in the main program, and only after both child processes have exited.
Sample run:
$ ./fifo29
Enter a number: 23
Enter a number: 34
Enter a number: 12931344
Enter a number: 0
PID 10939 exited with status 0x0000
PID 10940 exited with status 0x0000
$ cat archive.txt
23
34
12931344
0
$