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In addition, the following secondary flags may be bitwise-OR ed with the secondary flags to extend the functionality of the open() call: O_CREAT O_EXCL Creates the file on the file system if it does not already exist. The reverse of O_CREAT: if a file already exists, the call will fail.
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O_NOCTTY Prevents the process from being overtaken by a terminal (tty) device that is specified by pathname. O_TRUNC O_APPEND Allows a file to be truncated. Allows data to be appended to the end of a file.
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O_NONBLOCK O_SYNC O_NOFOLLOW O_DIRECTORY
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Prevents waiting. Prevents the opening of a file if it is a symbolic link. Fails if the named file is not a directory.
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Enforces synchronous I/O.
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O_LARGEFILE Allows large files whose sizes cannot be addressed (in 32-bit systems) to be opened. The open() function always returns an integer, which is the file descriptor (if positive), or an error (if negative). The errors associated with open(), which are set by errno, include EEXIST, EISDIR, EACCESS, ENAMETOOLONG, ENOENT, ENOTDIR, ENODEV, EROFS, ETXTBSY, EFAULT, ELOOP, ENOSPC, ENOMEM, EMFILE, and ENFILE. Two operations are supported by low-level I/O: reading (with the read() function) and writing (with the write() function). The main difference between high- and low-level reading and writing functions is that the latter require you to specify your own buffer size, and the type of data being read and written is not assumed. The read() call has the form
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ssize_t read(int fd, void *buf, size_t count)
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where fd is a file descriptor, buf is a pointer to a (variable-sized) buffer, and count is the number of bytes to be read from the file. If the call is successful, the number of bytes read successfully is returned. If the call fails, one of the following codes is returned by errno: EINTR, EAGAIN, EIO, EISDIR, EBADF, EINVAL, or EFAULT. These are defined and described at the end of this chapter. The write() call has the form
ssize_t write(int fd, void *buf, size_t count)
where fd is a file descriptor, buf is a pointer to a (variable-sized) buffer, and count is the number of bytes to be written to the file. If the call is successful, the number of bytes written successfully is returned. If the call fails, one of the following codes is returned by errno: EINTR, EAGAIN, EIO, EISDIR, EBADF, EINVAL, EPIPE, or EFAULT. These are defined and described at the end of this chapter. A file opened with open() can be closed with close(int fd), where fd is the file descriptor. Let s examine how these low-level calls can be used in practice. We revisit the user database application and modify the file operations to use low-level rather than highlevel routines.
#include #include #include #include #include <stdio.h> <sys/types.h> <sys/stat.h> <unistd.h> <fcntl.h>
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Application Development and Debugging
void printMenu(); char getInput(); void enterData(int fd); struct dbRecord { int uid; int gid; char username[8]; char homeDirectory[64]; char shell[64]; char comment[64]; }; main(int argc, char *argv[]) { int fd; char menuChoice; if ((fd=open(argv[1],O_RDWR|O_CREAT|O_APPEND))<0) { fprintf(stderr, "Cannot open database file %s\n", argv[1]); exit(1); } do { printMenu(); menuChoice=getInput(); switch (menuChoice) { case 'e': enterData(fd); break; case 'q': printf("Session terminated\n"); exit(1); break; } } while (menuChoice!='q'); close(fd); } void printMenu() { printf("Database Main Menu\n"); printf("------------------\n"); printf("(e)nter new dbRecord\n");
Part VI:
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printf("(q)uit\n"); } char getInput() { char answer; printf("\nYour Choice: "); answer=getchar(); return answer; } void enterData(int fd) { struct dbRecord user; printf("Data Entry\n"); printf("----------\n\n"); printf("Enter UID: "); scanf("%i",&user.uid); printf("Enter GID: "); scanf("%i",&user.gid); printf("Enter username: "); scanf("%s",user.username); printf("Enter full name: "); scanf("%s",user.comment); printf("Enter shell: "); scanf("%s",user.shell); printf("Enter home directory: "); scanf("%s",user.homeDirectory); write(fd, (char *)&user, sizeof(struct dbRecord)); }
The first thing to notice is that we ve added in several different header files, including sys/types.h, sys/stat.h, unistd.h, and fcntl.h. These are all necessary to support low-level I/O. Next, we ve changed the declaration of the enterData() function from a pointer to type FILE, to a single integer. This is the integer that contains the file descriptor. This means we must also change the fopen() request to an open() call. This specifies the name of the file to be opened, along with three OR ed flags: O_RDWR, O_CREAT, and O_APPEND. This ensures that the database file will be opened read/write, will be created if it doesn t already exist, and will be opened for appending. In addition, note that the error-checking condition has now changed: instead of checking to see whether the return value of fopen() is NULL, we now simply check to see whether the returned integer value from open() is positive (success) or negative (failure). Finally, the write() call is similar to the original: a file descriptor is passed, using the instantiation of dbRecord (user), where each record is written individually (i.e., the size of the buffer being written is defined by the record size).
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