C programming question
hi,
i am currently trying to modify my program to be able to allow a user to edit the amount of seats available, at the moment i can get my program to decrement the amount of seats, but i am trying to get it to allow a user to modify the amount of seats. i have found it quite difficult to do this, any help would be appreciated.
thank you
/* Sell seat program -- no record locking */
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
main(int argc, char *argv[])
{
int train, seats, fd;
/* open the seat file in read/write mode */
fd = open(argv[1], O_RDWR);
if (fd<0) {
perror("cannot open seat file");
exit(2);
}
/* check a flight number entered */
if (argc != 2) {
printf("usage: %s train_number\n", argv[0]);
exit(1);
}
/* convert flight number to an integer */
train = atoi(argv[1]);
/* use lseek to move the pointer to the position in the file */
/* containing the data for the corresponding flight */
/* lseek takes as args the file descriptor of the file, an */
/* offset value, which is here calculated by the number of */
/* elements multiplied by the size of an element */
/* and an origin point, here defined using SEEK_SET, ie the */
/* current file position (the start in this case because it */
/* has just been opened */
/* NB lseek doesn't do any reading or writing */
lseek(fd, train*sizeof(int), SEEK_SET);
/* now read an integer from the file */
read (fd, &seats, sizeof(int));
/* decrement it to simulate a sale of a seat */
seats -- ;
sleep(1); /* To increase likelihood of a problem */
printf("%d seats remaining on train %d\n",
seats, train);
/* use lseek to move back one place, because the write */
/* moved it on, and we want to rewrite the element we */
/* just read */
lseek(fd, (off_t)(train*sizeof(int)), SEEK_SET);
/* now write the decremented value back to the file */
write(fd, &seats, sizeof(int));
/* done, close the file */
close(fd);
}
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