Inter-Process Communication (IPC) using Shared Files: Producer Consumer Problem

Source Code of producer.c

#include <stdio.h> #include <stdlib.h> #include <fcntl.h> #include <unistd.h> #include <string.h> #define FileName "data.dat" #define DataString "Now is the winter of our discontent\nMade glorious summer by this sun of York\n" void report_and_exit(const char* msg) { perror(msg); exit(-1); /* EXIT_FAILURE */ } int main() { struct flock lock; lock.l_type = F_WRLCK; /* read/write (exclusive versus shared) lock */ lock.l_whence = SEEK_SET; /* base for seek offsets */ lock.l_start = 0; /* 1st byte in file */ lock.l_len = 0; /* 0 here means 'until EOF' */ lock.l_pid = getpid(); /* process id */ int fd; /* file descriptor to identify a file within a process */ if ((fd = open(FileName, O_RDWR | O_CREAT, 0666)) < 0) /* -1 signals an error */ report_and_exit("open failed..."); if (fcntl(fd, F_SETLK, &lock) < 0) /** F_SETLK doesn't block, F_SETLKW does **/ report_and_exit("fcntl failed to get lock..."); else { write(fd, DataString, strlen(DataString)); /* populate data file */ fprintf(stderr, "Process %d has written to data file...\n", lock.l_pid); } /* Now release the lock explicitly. */ lock.l_type = F_UNLCK; if (fcntl(fd, F_SETLK, &lock) < 0) report_and_exit("explicit unlocking failed..."); close(fd); /* close the file: would unlock if needed */ return 0; /* terminating the process would unlock as well */ }

Source Code of consumer.c

#include <stdio.h> #include <stdlib.h> #include <fcntl.h> #include <unistd.h> #include <string.h> #define FileName "data.dat" void report_and_exit(const char* msg) { perror(msg); exit(-1); /* EXIT_FAILURE */ } int main() { struct flock lock; lock.l_type = F_WRLCK; /* read/write (exclusive) lock */ lock.l_whence = SEEK_SET; /* base for seek offsets */ lock.l_start = 0; /* 1st byte in file */ lock.l_len = 0; /* 0 here means 'until EOF' */ lock.l_pid = getpid(); /* process id */ int fd; /* file descriptor to identify a file within a process */ if ((fd = open(FileName, O_RDONLY)) < 0) /* -1 signals an error */ report_and_exit("open to read failed..."); /* If the file is write-locked, we can't continue. */ fcntl(fd, F_GETLK, &lock); /* sets lock.l_type to F_UNLCK if no write lock */ if (lock.l_type != F_UNLCK) report_and_exit("file is still write locked..."); lock.l_type = F_RDLCK; /* prevents any writing during the reading */ if (fcntl(fd, F_SETLK, &lock) < 0) report_and_exit("can't get a read-only lock..."); /* Read the bytes (they happen to be ASCII codes) one at a time. */ int c; /* buffer for read bytes */ while (read(fd, &c, 1) > 0) /* 0 signals EOF */ write(STDOUT_FILENO, &c, 1); /* write one byte to the standard output */ /* Release the lock explicitly. */ lock.l_type = F_UNLCK; if (fcntl(fd, F_SETLK, &lock) < 0) report_and_exit("explicit unlocking failed..."); close(fd); return 0; }

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