I am trying read and write from a shared file between processes. The file has 6 lines that are meant to be read and then a new line should be inserted by one process. To be more precise i want my reading process to be done simultaneously but the writing process to be one by one process at a time. For that reason i am using a single semaphore to synchronise my processes. First, i am forking to create 7 children. Then, using a for loop i am calling a function which reads a line from the file and then prints it. If the last reading process is done reading it makes an UP operation which is caught by the last child which writes the last line in the file.
This is my file:
Hello! This is a shared file between processes!
Hello! This is a shared file between processes!
Hello! This is a shared file between processes!
Hello! This is a shared file between processes!
Hello! This is a shared file between processes!
Hello! This is a shared file between processes!
and this is my code:
#include <stdio.h> /* printf() */
#include <stdlib.h> /* exit(), malloc(), free() */
#include <sys/types.h> /* key_t, sem_t, pid_t */
#include <sys/shm.h> /* shmat(), IPC_RMID */
#include <errno.h> /* errno, ECHILD */
#include <semaphore.h> /* sem_open(), sem_destroy(), sem_wait().. */
#include <fcntl.h> /* O_CREAT, O_EXEC */
#include <sys/mman.h>
#include <unistd.h>
#include <sys/wait.h>
sem_t wrt;
void read_print(FILE *file)
{
char c[1000];
fscanf(file, "%[^\n]", c); //reading a single line from the file
printf("%s\n", c); //and printing it.
}
int main()
{
FILE *fp;
int i, counter = 0;
pid_t pid[7];
fp = fopen("shared_file.txt", "r+");
sem_init(&wrt, 1, 1); //initialiing the semaphore to be shared between$
if(fp == NULL)
{
printf("Error opening file!");
exit(1);
}
/* creating the child processes */
for(i = 0; i <7; i++)
{
pid[i] = fork();
if(pid[i] == 0)
break;
}
for(i = 0; i<6; i++)
if(pid[i] == 0) //if the processes is a child process
{
read_print(fp); //call the function
counter++; //increment the process counter by 1
if(counter == 6) //if we are at the final process
sem_post(&wrt); //UP operation
exit(0); //and then exits
}
if(pid[6] == 0) //if the final process is a child
{
sem_wait(&wrt); //DOWN operation
fprintf(fp, "...and this is the final line."); //writes a new $
exit(0);
}
fclose(fp); //closes the file.
return 0;
}
