Named Pipe, Communication between 2 children - linux

Question

I have a problem with my code. I want to make communication between 2 children process. One of them is a server, which opens a file and sends each letter to the second process. The second process is counting letters and it should make a new file and save results. I have problems with the last step because the first process gonna finish faster than the second, what causes the end of the program. I have no idea how fix it. Looking for some tips :).
Here you got result.
My code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <string.h>
//stale
#define FIFO "my_fifo"
#define SIZE 26
//zmienne globalne
int desk; //deskryptor pliku
int tab[SIZE];
//prototypy funkcji
void parentKillAll();
void server(FILE * file);
void client();
void cleanUp(FILE * file);
int checkEntryData(int argc, char *argv);
void replaceTabWithZero(int * tab);
void countLetters(int * tab, char ch);
void saveResults(int * tab, char *title);
void showTab(int * tab);
int main(int argc, char *argv[]) {
if (!checkEntryData(argc, argv[1]))
return 1;
replaceTabWithZero(tab);
FILE *file = fopen(argv[1], "r");
umask(0);
mkfifo(FIFO, 0666);
if (file) {
if (fork() == 0) {
server(file);
exit(0);
} else if (fork() == 0) {
client();
saveResults(tab, strcat(argv[1], "Result"));
showTab(tab);
exit(0);
} else {
cleanUp(file);
parentKillAll();
}
} else {
perror("Error");
}
return 0;
}
void parentKillAll() {
sleep(1);
kill(0, SIGKILL);
exit(0);
}
void server(FILE * file) {
char ch;
while ((ch = fgetc(file)) != EOF) {
desk = open(FIFO, O_WRONLY);
write(desk, &ch, 1);
}
}
void client() {
char ch;
while (1) {
desk = open(FIFO, O_RDONLY);
read(desk, &ch, 1);
countLetters(tab, ch);
printf("%c", ch);
}
}
void cleanUp(FILE *file) {
wait(0);
fclose(file);
close(desk);
}
int checkEntryData(int argc, char *argv) {
if (argc < 2) {
fprintf(stderr, "Nie poprawna ilosc argumentow\n");
return 0;
}
if (access(argv, F_OK)) {
fprintf(stderr, "Podany plik \'%s\' nie istnieje\n", argv);
return 0;
}
if (access(argv, R_OK)) {
fprintf(stderr, "Brak uprawnien do odczytu pliku \'%s\'\n", argv);
return 0;
}
return 1;
}
void replaceTabWithZero(int * tab) {
for (int i = 0; i < SIZE; i++)
tab[i] = 0;
}
void countLetters(int *tab, char ch) {
int chVal = ch;
if (chVal > 92)
chVal -= 32;
if (chVal > 64 && chVal < 91)
tab[chVal-65] += 1;
}
void saveResults(int *tab, char * title) {
FILE *plik = fopen(title, "w");
if (plik) {
for (int i = 0; i < SIZE; i++)
fprintf(plik, "%c - %d\n", (i+97), tab[i]);
} else {
perror("Error");
}
fclose(plik);
}
void showTab(int * tab) {
for (int i = 0; i < SIZE; i++)
printf("\n%d", tab[i]);
}

Answer 1

The real problem is that the client process can never finish, because it runs an infinite while(1) loop without any exit conditions.
You should rewrite it so that it exits after reading all available data:
void client() {
char ch;
// Open the fifo only once, instead of once per character
desk = open(FIFO, O_RDONLY);
// Loop until there is no more data to read
while(read(desk, &ch, 1) > 0) {
countLetters(tab, ch);
printf("%c", ch);
}
}
This is technically sufficient to make it work, but you should also look into a series of other issues:
You should have two wait(0) calls so that you wait for both processes, and you shouldn't try to kill anything.
The server process should only be opening the fifo once, not once per character.
You should be comparing fgetc output to EOF before forcing the value into a char. Since you do it after, running your program on a ISO-8859-1 terminal will cause it to confuse EOF and the letter ÿ
You are using strcat on argv[1], even though you don't know how much space that array has. You should use your own buffer of a known length.
You should check the return value of all your system calls to ensure they succeed. Checking with access and then assuming it'll be fine is not as good since calls can fail for other reasons.
Canonical Unix behavior is to exit with 0 for success, and >= 1 for error.
It's good practice to use a larger buffer (e.g. 65536 bytes instead of 1) when using read/write directly. stdio functions like fgetc already uses a larger buffer behind the scenes.
Using a named pipe obviously works, but since you spawn both processes it would be more natural to use an unnamed one.