Reading Material for
File Operations
As Part of
Operating Systems [CS F372] Course
Semester II, 2024 – 2025
BIRLA INSTITUTE OF TECHNOLOGY AND
SCIENCE, PILANI –K K BIRLA GOA CAMPUS
INDEX
Sec. Page
Section Title
No. No.
1. Command Line Arguments 3
2. File Operations 4
3. I/O Redirections 9
File Operations Page 2
Command Line Arguments -
Command-line arguments are the values given after the name of the program in the
command-line shell of Operating Systems. Command-line arguments are handled
by the main() function of a C program.
Syntax -
int main(int argc, char *argv[]) { /* ... */ }
● argc (ARGument Count) is an integer variable that stores the number of
command-line arguments passed by the user including the name of the
program. So if we pass a value to a program, the value of argc would be 2
(one for argument and one for program name)
● The value of argc should be non-negative.
● argv (ARGument Vector) is an array of character pointers listing all the
arguments.
● If argc is greater than zero, the array elements from argv[0] to argv[argc-1]
will contain pointers to strings.
● argv[0] is the name of the program , After that till argv[argc-1] every
element is command -line arguments.
Example:
./hello.out check me out 3
Here,
./hello.out is the name of the program and can be accessed in the program
using argv[0].
check, me, out, 3 will be accessible using argv[1], argv[2], argv[3] and argv[4]
respectively.
File Operations Page 3
File Operations:
File handling in C is the process in which we create, open, read, write, and close
operations on a file. C language provides different functions such as fopen(),
fwrite(), fread(), fseek(), fprintf(), etc. to perform input, output, and many different
C file operations in our program.
1. fopen()
Syntax - FILE *fopen ( const char *pathname, const char *mode );
The fopen() function opens the file whose name is the string pointed to by pathname
and associates a stream with it.
2. fclose()
Syntax - int fclose ( FILE *stream );
The fclose() function flushes the stream pointed to by stream and closes the
underlying file descriptor.
3. fprintf()
Syntax - int fprintf ( FILE *fptr, const char *str, … );
fprintf is used to print content in file instead of stdout console.
Example -
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
FILE* demo;
demo = fopen("os_demo.txt", "w+");
fprintf(demo, "%s %s %s", "Welcome", "to", "Operating
Systems");
fclose(demo);
return 0;
}
4. fscanf()
Syntax - int fscanf ( FILE *ptr, const char *format, … )
fscanf reads from a file pointed by the FILE pointer (ptr), instead of reading from
the input stream.
File Operations Page 4
Example 2 - Sample C code which reads data from file using fscanf.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
FILE *demo;
FILE *copiedDemo;
demo = fopen("os_demo.txt", "r");
copiedDemo = fopen("copied_demo.txt", "w");
if (demo == NULL || copiedDemo == NULL) {
perror("Error opening files");
return 1;
}
char buffer[100];
while (fscanf(demo, "%s", buffer) == 1) {
fprintf(copiedDemo, "%s ", buffer);
}
fclose(demo);
fclose(copiedDemo);
return 0;
}
5. fseek()
Syntax - int fseek ( FILE *pointer, long int offset, int position );
fseek() is used to move the file pointer associated with a given file to a specific
position.
6. fread()
Syntax - size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
The function fread() reads nmemb items of data, each size bytes long, from the
stream pointed to by stream, storing them at the location given by ptr.
7. fwrite()
Syntax - size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
The function fwrite() writes nmemb items of data, each size bytes long, to the stream
pointed to by stream, obtaining them from the location given by ptr.
File Operations Page 5
8. ftell()
Syntax - long ftell(FILE *stream);
The ftell() function obtains the current value of the file position indicator for the
stream pointed to by stream.
Example 3 - C Program to copy the contents of cone file to another file using
fread, fwrite, ftell, and fseek.
#include <stdio.h>
#include <stdlib.h>
int main() {
FILE *demo;
FILE *copiedDemo;
demo = fopen("os_demo.txt", "r");
copiedDemo = fopen("copied_demo.txt", "w");
if (demo == NULL || copiedDemo == NULL) {
perror("Error opening files");
return 1;
}
fseek(demo, 0, SEEK_END);
long fileSize = ftell(demo);
fseek(demo, 0, SEEK_SET);
char *content = (char *)malloc(fileSize + 1);
if (content == NULL) {
perror("Memory allocation error");
return 1;
}
fread(content, 1, fileSize, demo);
content[fileSize] = '\0';
fwrite(content, 1, fileSize, copiedDemo);
fclose(demo);
fclose(copiedDemo);
free(content);
return 0;
}
File Operations Page 6
9. fgets()
Syntax - char *fgets (char *str, int n, FILE *stream);
The fgets() reads a line from the specified stream and stores it into the string pointed
to by str. It stops when either (n-1) characters are read, the newline character is read,
or the end-of-file is reached, whichever comes first.
10.fgetpos()
Syntax - int fgetpos(FILE *stream, fpos_t *pos);
The C function fgetpos gets the current file position of the stream and writes it to
pos.
11. fsetpos()
Syntax - int fsetpos(FILE *stream, const fpos_t *pos);
The fsetpos() function moves the file position indicator to the location specified by
the object pointed to by position. When fsetpos() is executed, the end-of-file
indicator is reset.
Example 4 - C code that demonstrates the use of fgets(), fgetpos(), and fsetpos()
to read lines from a text file, get and set file positions
#include <stdio.h>
#include <stdlib.h>
int main() {
FILE *file;
file = fopen("sample.txt", "r");
if (file == NULL) {
perror("Error opening file");
return 1; // Exit with an error code
}
char buffer[100];
fpos_t position;
// Read and print the first three lines
for (int i = 0; i < 3; i++) {
if (fgets(buffer, sizeof(buffer), file) != NULL) {
printf("Line %d: %s", i + 1, buffer);
}
File Operations Page 7
else {
perror("Error reading file");
break;
}
}
// Get the current file position
if (fgetpos(file, &position) != 0) {
perror("Error getting file position");
return 1;
}
// Move back to the beginning of the file
rewind(file);
// Skip the first three lines
for (int i = 0; i < 3; i++) {
if (fgets(buffer, sizeof(buffer), file) == NULL) {
perror("Error reading file");
break;
}
}
// Set the file position back to the previously saved
position
if (fsetpos(file, &position) != 0) {
perror("Error setting file position");
return 1;
}
// Read and print lines from the saved position
while (fgets(buffer, sizeof(buffer), file) != NULL) {
printf("Restored Position: %s", buffer);
}
fclose(file);
return 0;
}
Note - Please refer to the man pages for more detailed description and usage.
File Operations Page 8
Input Output Redirections -
In Linux, whenever an individual runs a command, it can take input, give output, or
do both. Redirection helps us redirect these input and output functionalities to the
files or folders we want, and we can use special commands or characters to do so.
Types of Redirection -
1. Overwrite Redirection
Overwrite redirection is useful when you want to store/save the output of a command
to a file and replace all the existing content of that file.
Types -
“>” standard output
So whatever you will write after running this command, will be redirected and
copied to the “file.txt”. This is standard output redirection.
For Example -
cat > sample.txt
I love Operating Systems.
This will overwrite the existing contents of the file sample.txt and write “I love
Operating Systems.” to it.
To verify use,
cat sample.txt
I love Operating Systems.
To close the command press Ctrl + D.
“<” standard input
For Example -
cat < sample.txt
I love Operating Systems.
Now, this is standard input redirection, cat command will take the input from
“sample.txt” and print it to the terminal screen.
File Operations Page 9
2. Append Redirection:
With the help of this Redirection, you can append the output to the file without
compromising the existing data of the file.
“>>” standard output
For Example -
cat >> sample.txt
I love C Programming.
This will append to the existing contents of the file sample.txt and write “I love C
Programming”. to it.
To verify use,
cat sample.txt
I love Operating Systems.
I love C Programming.
“<<” standard input
Example -
cat << EndOfText
This is an example of a document here.
You can input multiple lines of text here.
EndOfText
This is an example of a document here.
You can input multiple lines of text here.
3. Merge Redirection:
This allows you to redirect the output of a command or a program to a specific file
descriptor instead of standard output.
The syntax for using this is “>&” operator followed by the file descriptor number.
● “p >& q” Merges output from stream p with stream q
● “p <& q” Merges input from stream p with stream q
File Operations Page 10
Some Useful commands -
● man - man command in Linux is used to display the user manual of any
command that we can run on the terminal.
Syntax - $ man [COMMAND NAME]
Example - $ man fprintf
● apropos - apropos command helps the user when they don’t remember the
exact command but knows a few keywords related to the command that
define its uses or functionality. It searches the Linux man page with the help
of the keyword provided by the user to find the command and its functions.
Syntax - apropos [OPTION..] KEYWORD…
Example - $ apropos print first lines
File Operations Page 11
