File I/O System Calls Reference

January 7, 2003 Serguei Mokhov, [email protected]

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File I/O System Calls Reference

COMP 229, 444, 5201

Revision 1.2

Date: July 21, 2004


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Contents

• Calls Overview– open()– read()– write()– close()– lseek()

• References


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open()

• Opens/creates files (whatever is abstracted by a file).

SYNOPSIS #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h>

int open(const char *pathname, int flags); int open(const char *pathname, int flags, mode_t mode); int creat(const char *pathname, mode_t mode);


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open(): flags• O_RDONLY or O_WRONLY or O_RDWR + zero or more of the

following, concatenated by “|” (some are omitted, see man page for details):– O_CREAT, doesn’t fail if file exists– O_EXCL, in conjunction with O_CREAT will cause open() to fail if file

exists– O_TRUNC, if file exists and permissions allow, make it of 0 length– O_APPEND, all stuff written to an open file will be appended regardless lseeks() you do.

– O_NONBLOCK, non-blocking I/O– O_SYNC, synchronous I/O, thus blocking– O_NOFOLLOW, don’t follow symlinks, i.e. fail– O_DIRECTORY, fail if a directory


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open(): modes• Permissions on new files:

– S_IRWXU – User: RWX– S_IRUSR (S_IREAD) – User: R– S_IWUSR (S_IWRITE) – User: W– S_IXUSR (S_IEXEC) – User: X– S_IRWXG – Group: RWX– S_IRGRP – Group: R– S_IWGRP – Group: W– S_IXGRP – Group: X– S_IROTH – Others: R– S_IWOTH – Others: W– S_IXOTH – Others: X


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open()• Performs file path to file descriptor translation, to

use with read(), write(), etc.• Returns -1 on error.• File descriptor – next available integer starting

from 0.– 0 – STDIN_FILENO– 1 – STDOUT_FILENO– 2 – STDERR_FILENO– 3 – probably your file

• File descriptors are not shared by opening; however, a forked process among other things inherits parent’s open files.


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Raw Device Read

• /dev/hda0 – first partition of the first hard disk

• To read you need superuser privileges.• open() just like filename, advice: do it

read-only :-), i.e. O_RDONLY


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read()

• Well, read our bytes from somewhere, where our file descriptor points to, to a pre-allocated buffer.

SYNOPSIS #include <unistd.h>

ssize_t read(int fd, void *buf, size_t count);


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read()

• Returns– number of bytes read on success– 0 - EOF– (-1) on failure

• Make sure you read() as many bytes as you intended to before proceeding to processing of what you read().


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write()

• Writes out data to a file descriptor.


ssize_t write(int fd, const void *buf, size_t count);


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write()• Returns

– (-1) on error– Number of bytes written on success– 0 if count is 0– The rest is more or less specific

• Double check that the # of bytes written is indeed as requested in count before you proceed.

• Note, neither having successful write() nor successful close() after it mean your data physically has reached the disk; use fsync() to have more guarantee that this will happen since kernels usually defer writes for performance reasons.


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close()

• “Release” file descriptor.


int close(int fd);


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close()

• Descriptor ready for re-use.

• All locks removed.

• Returns– 0 on success– (-1) on error– ALWAYS CHECK THE RETURN VALUES!


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lseek()

• Navigate within the file using file pointer.

SYNOPSIS #include <sys/types.h> #include <unistd.h>

off_t lseek(int fildes, off_t offset, int whence);


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lseek()

• whence:– SEEK_SET offset is set to offset bytes– SEEK_CUR current + offset– SEEK_END file size + offset

• If file pointer is beyond file boundaries, zeros will be returned by read() from the gap between current position and actual file end until the gap is filled in with smth.


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lseek()

• Returns– Resulting offset on success– ((off_t)-1) on error


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References

• Rusling• Chapter 8, Vahalia• Chapters 3-5, Stevens• Manual pages:

– man 2 open

– man 2 read

– man 2 write

– man 2 close

– man 2 lseek

Documents

File I/O System Calls Reference