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Library Functions. The UNIX system provides a large number of C functions as libraries. Some of these implement frequently used operations, while others are very specialized in their application.

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library functions
Library Functions
  • The UNIX system provides a large number of C functions as libraries. Some of these implement frequently used operations, while others are very specialized in their application.
  • Wise programmers will check whether a library function is available to perform a task before writing their own version. This will reduce program development time.

The standard C library functions are declared in a set of header files that are commonly placed in the /usr/include directory on UNIX systems.

  • The archive and shared libraries that contain the object code of these library functions are the libc.a (static library)and library), respectively. These libraries are commonly placed in the /usr/lib directory on UNIX system
finding information about library functions
Finding Information about Library Functions
  • The UNIX manual has an entry for all available functions. Function documentation is stored in section 3 of the manual, and there are many other useful system calls in section 2. If you already know the name of the function you want, you can read the page by typing (to find about strcat).
  • $man 3 strcat
  • If you don't know the name of the function, a full list is included in the introductory page for section 3 of the manual. To read this, type

$man 3 intro


printf(“a"); write(STDOUT_FILENO, “b", 1); printf(“c\n");



write is a system call -- it is implemented by the interface between user mode (where programs like yours run) and the operating system kernel (which handlesthe actual writing to disk when bytes are written to a file).

printf is a C standard library function -- it is implemented by library code loaded into your user mode program.

The C standard library output functions buffer their output, by default until end-of-line is reached. When the buffer is full or terminated with a newline, it is written to the file via a call to write from the library implementation.

Therefore, the output via printf is not sent to the operating system write immediately. In your example, you buffer the letter 'u', then immediately write the letter 'm', then append "d\n" to the buffer and the standard library makes the call write(STDOUT_FILENO, "ud\n");

use of library functions
Use of Library Functions
  • To use a function, ensure that you have made the required #includes in your C file. Then the function can be called as though you had defined it yourself.
  • Some libraries require extra options before the compiler can support their use. For example, to compile a program including functions from the math.h library the command might be
  •   cc mathprog.c -o mathprog -lm
  • The final -lm is an instruction to link the maths library with the program. The manual page for each function will usually inform you if any special compiler flags are required.
static libraries vs dynamic libraries
Static Libraries Vs Dynamic libraries:
  • Static libraries are linked into your executable at link time so are fixed at that point.
  • For dynamic linking, only a reference to the library is linked in at link time. The dynamic library (the actual code) is linked to your executable at load time, when you run your program.That means you can change the dynamic library at any time and you'll use the new one the next time you load the program
library naming conventions
Library naming conventions:
  • Libraries are typically names with the prefix "lib". This is true for all the C standard libraries. When linking, the command line reference to the library will not contain the library prefix or suffix.
  • Thus the following link command:
  • gccsrc-file.c -lm -lpthreadThe libraries referenced in this example for inclusion during linking are the math library and the thread library. They are found in /usr/lib/libm.a and /usr/lib/libpthread.a.
standard i o and formatted i o in c
Standard I/O and Formatted I/O in C
  • The standard I/O library (stdio) and its header file, stdio.h, provide a versatile interface to low-level I/O system calls.
  • The library, now part of ANSI standard C, provides many sophisticated functions for formatting output and scanning input. It also takes care of the buffering requirements for devices.
  • whereas the system calls are not part of ANSI C
c stream pointers
C Stream Pointers
  • You need to open a file to establish an access path. This returns a value that is used as a parameter to other I/O library functions.
  • This return value is called a stream and is implemented as a pointer to a structure, a FILE *.
  • Three file streams are automatically opened when a program is started. They are stdin, stdout, and stderr. These are declared in stdio.h and represent the standard input, output, and error output, respectively, which correspond to the low-level file descriptors 0, 1, and 2.
what is file pointer in c programming language
What is FILE pointer in c programming language?
  • C communicates with files using a new data type called a file pointer.
  • FILE pointer is struct data type which has been defined in standard library stdio.h. This data type points to a stream or a null value. It has been defined in stdio.h as


    short           level;

    unsigned        flags;

    char           fd;

    unsigned char  hold;

    short          bsize;

    unsigned char  *buffer, *curp;

    unsigned       istemp;

    short          token;


what is file pointer and its working method
What is file pointer and its working method? 
  • For files you want to read or write, you need a file pointer, e.g.:FILE *fp;
  • when a pointer is pointing to the address of a file then it is called as a file pointer.
  • File pointer is the pointer which holds the address of the file which is opened either in read or write or append or binary modes(r,w,a,r+,w+,a+,b).FILE *fp;This is the declaration of the file pointer.

here in the above statement we are declaring a file pointer.there are many modes of opening a file. they are read ,write,append ..

  • they are as follows..fp=fopen("filename.c",'r');above statement is opening a file in read mode..
standard i o functions in c
Standard I/O Functions in C
  • fopen( )
  • fread( ), fgets( ),gets( ),fgetc( ) , getc( ) , getchar(),
  • fwrite(), fputs(), puts(),fputc(),putc(),putchar(),
  • ftell(),fseek(),fgetpos(),fsetpos(),rewind()
  • fflush(), fclose,
formatted i o in c
Formatted I/O in c
  • Scanf(),fscanf(),sscanf()
  • Printf(),fprintf(),sprintf()
kernel support for files
  • The kernel represents open files using three related data structures:
        • File Descriptor table
        • File table
        • i-node table
file descriptor table
File Descriptor table
  • Each process has its own separate file descriptor table whose entries are indexed by the process’s open file descriptors.
  • Each open descriptor entry points to an entry in the file table.
  • File descriptors
  • Each UNIX process has 20 file descriptors at it disposal, numbered 0 through 19.
  • The first three are already opened when the process begins

0: The standard input

1: The standard output

2: The standard error output

  • When the parent process forks a process, the child process inherits the file descriptors of the parent
file table
File table
  • The set of open files is represented by a file table that is shared by all processes.
  • Each file table entry consists of (for our purposes) the current file position, a reference count of the number of descriptor entries that currently point to it, and a pointer to an entry in the i-node table.
  • Closing a descriptor decrements the reference count in the associated file table entry. The kernel will not delete the file table entry until its reference count is zero.
i node table
i-node table
  • Like the file table, the i-node table is shared by all processes. Each entry contains most of the information in the stat structure, including the st mode and st size members.
system calls
System Calls
  • System call is a request to the kernel to provide specific service.
  • System calls are interface to the kernel.
unix file i o low level file access unix file api
UNIX FILE I/O -Low Level File Access(UNIX FILE API)
  • open()
  • creat()
  • read()
  • write()
  • close()
  • lseek()
  • stat()
  • fstat()
  • ioctl()
  • umask()
  • dup()
  • dup2()
  • link)()
  • unlink()
  • fcntl()
  • chmod()
  • chown()
  • chgrp()
  • To open or create a file
  • Prototype is :

#include <fcntl.h>

#include <sys/types.h>

#include <sys/stat.h>

int open(const char *path, intoflags);

int open(const char *path, int oflags, mode_t mode);

  • Returns: new file descriptor if OK, −1 on error
open 1 st parameter path
Open1st parameter :path
  • The name of the file or device to be opened is passed as a parameter, path;
open 2 nd parameter
Open2nd parameter
  • the oflags parameter is used to specify actions to be taken on opening the file.
  • The oflags are specified as a combination of a mandatory file access mode and other optional modes. The open call must specify one of the file access modes shown in the following table:

Mode Description

O_RDONLY Open for read-only

O_WRONLY Open for write-only

O_RDWR Open for reading and writing

  • The call may also include a combination (using a bitwise OR) of the following optional modes in the oflags parameter:
  • O_APPEND: Place written data at the end of the file.
  • O_TRUNC: Set the length of the file to zero, discarding existing contents.
  • O_CREAT: Creates the file, if necessary, with permissions given in mode.
  • O_EXCL: Used with O_CREAT, ensures that the caller creates the file. The open is atomic; that is, it’s performed with just one function call. This protects against two programs creating the file at

the same time. If the file already exists, open will fail.

open 3 rd parameter mode
Open3rd parameter :mode
  • When you create a file using the O_CREAT flag with open, you must use the three-parameter form. mode, the third parameter, is made from a bitwise OR of the flags defined in the header file sys/stat.h. These are:
  • S_IRUSR: Read permission, owner
  • S_IWUSR: Write permission, owner
  • S_IXUSR: Execute permission, owner
  • S_IRGRP: Read permission, group
  • S_IWGRP: Write permission, group
  • S_IXGRP: Execute permission, group
  • S_IROTH: Read permission, others
  • S_IWOTH: Write permission, others
  • S_IXOTH: Execute permission, others
open examples
  • fd=open (“myfile”, O_CREAT, S_IRUSR|S_IXOTH)
  • fd = Open("foo.txt", O_RDONLY, 0);
  • fd = Open("foo.txt", O_WRONLY|O_APPEND, 0);
creat system call
creat( ) system call
  • To create new file
  • The prototype for the creat() system call is:

#include <fcntl.h>

#include <sys/types.h>

#include <sys/stat.h>

Int creat(const char *path, mode_t mode);

  • Returns: new file descriptor if OK, −1 on error
  • Here path is the filename
  • Mode is same as in open system call
read system call
read() system call
  • The read system call reads up to n bytes of data from the file associated with the file descriptor fd and places them in the data area buf.

#include <unistd.h>

size_t read( intfd, void *buf, size_tnbytes );

  • On successful completion the function returns the number of bytes actually read. It returns 0 if end of file is reached and -1 on error.
write system call
Write() system call
  • Here’s the syntax:

#include <unistd.h>

size_t write(intfildes, const void *buf, size_tnbytes);

  • The write() function writes the number of bytes specified by ‘nbytes’ from the buffer ‘buf’ into the file pointed by ‘fd’
close system call
Close() system call
  • To terminate the association between a file descriptor, fd, and its file. The file descriptor becomes available for reuse.
  • It returns 0 if successful and –1 on error.

#include <unistd.h>

int close(intfd);

example programs
Example programs
  • This program, simple_read.c, copies the first 128 bytes of the standard input to the standard output. It copies all of the input if there are fewer than 128 bytes.

#include <unistd.h>

#include <stdlib.h>

int main()


char buffer[128];


nread = read(0, buffer, 128);

if (nread == -1)

write(2, “A read error has occurred\n”, 26);

if ((write(1,buffer,nread)) != nread)

write(2, “A write error has occurred\n”,27);



c program to implement cp command using system calls
C program to implement cp command using system calls






void main()


int fd1,fd2,n;

char c,src[20],dst[20];

printf("Read source file : ");

scanf("%s", src);

printf("Read destination file : ");





if(fd1==-1 || fd2==-1)

printf("Files cannt be opened");









s tat fstat lstat system calls
stat(),fstat(),lstat() system calls
  • stat ,fstatfunctions retrieve file attributes of a given file.
  • lstat return the symbolic link file attributes, not the file it refers to .
  • the syntax:

#include <unistd.h>

#include <sys/types.h>

intfstat(intfildes, struct stat *buf);

int stat(const char *path, struct stat *buf);

intlstat(const char *path, struct stat *buf);

  • These functions return 0 if they succeed or -1 if they fail.
arguments of stat fstat lstat
Arguments of stat,fstat,lstat
  • The first argument of stat is a file path name, whereas the first argument of fstat is a file descriptor.
  • The second argument to stat,fstat and lstat is the address of a struct stat- typeed variable
  • The struct stat data type is defined in the <sys/stat.h> header
  • The <sys/stat.h> header shall define the structure of the data returned by the functions fstat(), lstat(), and stat().
declaration of stat structure
Declaration of stat structure

struct stat


dev_tst_dev; /* file system ID * /

ino_tst_ino; /*File inode number*/

mode_tst_mode; /* contains file type and access flags*/

nlink_tst_nlink: /* Hard Link count*/

uid_tst_uid /* File user Id*/

gid_tst_gid /* File group ID*/

dev_tst_rdev; /* Contains major and minor device numbers*/

off_tst_size /* File size in nuber of bytes*/

time_tst_atime; /* Last access time*/

time_tst_mtime; /* Last modification time time*/

time_tst_ctime; /* Last status change time time*/




The following POSIX macros are defined to check the file type using the st_mode field:

  • S_ISREG(m) is it a regular file?
  • S_ISDIR(m) directory?
  • S_ISCHR(m) character device?
  • S_ISBLK(m) block device?
  • S_ISFIFO(m) FIFO (named pipe)?
  • S_ISLNK(m) symbolic link?
  • S_ISSOCK(m) socket?

The following flags are defined for the st_mode field:

  • S_IFMT 0170000 bit mask for the file type bit fields
  • S_IFSOCK 0140000 socket
  • S_IFLNK 0120000 symbolic link
  • S_IFREG 0100000 regular file
  • S_IFBLK 0060000 block device
  • S_IFDIR 0040000 directory
  • S_IFCHR 0020000 character device
  • S_IFIFO 0010000 FIFO
  • S_ISUID 0004000 set UID bit
  • S_ISGID 0002000 set-group-ID bit (see below)
  • S_ISVTX 0001000 sticky bit (see below)
  • S_IRWXU 00700 mask for file owner permissions
  • S_IRUSR 00400 owner has read permission
  • S_IWUSR 0200 owner has write permission
  • S_IXUSR 0100 owner has execute permission
  • S_IRWXG 00070 mask for group permissions
  • S_IRGRP 00040 group has read permission
  • S_IWGRP 00020 group has write permission
  • SIXGRP 0010 group has execute permission
  • S_IRWXO 00007 mask for permissions for others (not in group)
  • S_IROTH 00004 others have read permission
  • S_IWOTH 00002 others have write permission
  • S_IXOTH 00001 others have execute permission
lseek system call
lseek() system call
  • The UNIX system file system treats an ordinary file as a sequence of bytes.
  • Generally, a file is read or written sequentially

-- that is from beginning to the end of the file.

  • Sometimes sequential reading and writing is

not appropriate.

  • Random access I/O is achieved by changing the value of this file pointer using the lseek() syystem call.



long lseek(intfile_descriptor, long offset, int whence)

whence new position

0 (SEEK_SET) offset bytes into the file

1(SEEK_CUR) current position in the file

plus offset

2 (SEEK_END) current end-of-file position plus offset