Question about writing to a file in Producer-Consumer program - c

Question

I have to write two separate programs, which one is a Producer and second is a Consumer (both running in separate terminals). I provide an argument to the Producer which can be a text or a single character. Then, producer creates a .txt file, puts single character into it then closes it. Consumer opens that file, reads that character and prints it on a terminal, then closes the file and deletes it.The whole process repeats itself. If provided argument includes *, for example * or text* it finishes the both programs, printing * before ending. I can only use functions: open(), close(), read(), write(), unlink(). The expected result looks like this:
I have written both codes, this is the Producer code:
(I am aware of the fact that i have unnecessarily defined SIZE and used it, please don't mind it)
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#define SIZE 1
int main(int argc, char *argv[]){
char buff;
do{
int fdi=-1;
while(fdi<0){
fdi=open("test.txt",O_WRONLY | O_CREAT | O_EXCL, 0666);
}
read(STDIN_FILENO,&buff,SIZE);
write(fdi,&buff,SIZE);
close(fdi);
}while(buff!='*');
return 0;
}
and this is the Consumer code:
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#define SIZE 1
int main(int argc, char *argv[]){
char buff;
do{
int fdi=-1;
while(fdi<0){
fdi=open("test.txt",O_RDONLY | O_EXCL);
}
int rdin=read(fdi,&buff,SIZE);
if(rdin>0){
write(STDOUT_FILENO,&buff,SIZE);
close(fdi);
unlink("test.txt");
}
else{
close(fdi);
}
}while(buff!='*');
return 0;
}
My question is: how does the Producer program not insert more than a one character into the file? I mean, if I for example run the Producer program only, and provide argument text, it will insert in the file the letter t only, the rest will be inserted to other files. Shouldn't it loop and add the whole text word to one file? There is no statement that guarantees that file will contain one character, yet it contains only one character and I don't know why.

Answer 1

In the producer, when you use the O_CREAT and O_EXCL flags when opening a file, the call will fail if the file already exists. So on the first iteration of the outer loop (assuming the file doesn't exist) the file is created and the first character is written. On the next iteration the open call fails because the file exists, so it sits in the inner loop for as long as the file exists.
In the consumer, the open call is done in a loop until it succeeds. This will happen when the producer writes and closes the file. The consumer then reads the character from the file and deletes it. When the consumer deletes the file, the open call in the producer will succeed and write the second character to the file.
This process then repeats until a * character is read by the producer and written to the file, after which the producer exits. Then when the consumer reads the * it also exits.

Answer 2

I will limit my answer to your only specific question:
how does the Producer program not insert more than a one character into the file?
You're doing the following in a loop:
do{
int fdi = -1;
while (fdi < 0){
// Open the file only if it does not exist, creating it.
fdi = open("test.txt", O_WRONLY | O_CREAT | O_EXCL, 0666);
}
// Read 1 char from stdin.
read(STDIN_FILENO, &buff, SIZE);
// Write that char to the beginning of the file.
write(fdi, &buff, SIZE);
// Close and truncate the file.
close(fdi);
} while(buff != '*');
The first time you open the file, it is also created. From the second time onwards, the combination of flags O_CREAT | O_EXCL will make open() fail with error EEXIST, since this combination of flags will open the file only if it doesn't already exist. After writing the first character, your program will run in an endless loop (while (fdi < 0)) trying to open the file a second time.
From the manual page for open():
O_EXCL: Ensure that this call creates the file: if this flag is specified in conjunction with O_CREAT, and pathname
already exists, then open() will fail.
So, first of all, you don't need the O_EXCL flag. Other than that, if you want to append data to the file instead of overwriting its content each time, you should add the O_APPEND flag when you open() the file. From the manual page:
O_APPEND: The file is opened in append mode. Before each write(2), the file offset is positioned at the end of the file,
as if with lseek(2). The modification of the file offset and the write operation are performed as a single
atomic step.