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Reading from Files

1. Introduction

Reading from files in C means retrieving data stored in a file and using it in a program. Files are used to store data permanently. When a program needs old or saved data, it must read the data from a file.

Reading from files is commonly used in:

  • Student record systems

  • Report generation

  • Banking and accounting software

  • Database-based applications

C provides many file handling functions to read data from files efficiently.

2. Steps to Read from a File in C

To read data from a file, the following steps are followed:

  1. Declare a file pointer

  2. Open the file in read mode ("r")

  3. Read data using file reading functions

  4. Close the file

3. File Pointer for Reading

A file pointer is required to access a file.

Declaration

FILE *fp;

  • FILE is a structure defined in stdio.h

  • fp stores the address of the file

4. Opening a File in Read Mode

To read a file, it must be opened using the fopen() function.

Syntax

fp = fopen("filename", "r");

Example

fp = fopen("data.txt", "r");

âš  Important Points:

  • If the file does not exist, fopen() returns NULL

  • Always check whether the file opened successfully

Example with Checking

if(fp == NULL) {
printf("File not found");
}

5. Reading Data Using fgetc()

Description

  • Reads one character at a time

  • Returns the character read

  • Returns EOF at end of file

Syntax

ch = fgetc(fp);

Example Program

#include <stdio.h>

int main() {
FILE *fp;
char ch;

fp = fopen(“data.txt”, “r”);

while((ch = fgetc(fp)) != EOF) {
printf(“%c”, ch);
}

fclose(fp);
return 0;
}

Use

  • Useful for reading character-by-character

  • Best for text files

6. Reading Data Using fgets()

Description

  • Reads one line at a time

  • Stops at newline or end of file

Syntax

fgets(string, size, fp);

Example Program

#include <stdio.h>

int main() {
FILE *fp;
char line[100];

fp = fopen(“data.txt”, “r”);

while(fgets(line, 100, fp) != NULL) {
printf(“%s”, line);
}

fclose(fp);
return 0;
}

Advantages

  • Safe for reading strings

  • Prevents buffer overflow

7. Reading Data Using fscanf()

Description

  • Reads formatted data

  • Works like scanf(), but reads from a file

Syntax

fscanf(fp, "format", &variables);

Example Program

#include <stdio.h>

int main() {
FILE *fp;
char name[20];
int marks;

fp = fopen(“student.txt”, “r”);

fscanf(fp, “%s %d”, name, &marks);

printf(“Name: %s\nMarks: %d”, name, marks);

fclose(fp);
return 0;
}

Limitation

  • Stops reading at spaces

  • Not suitable for full lines

8. End of File (EOF)

Definition

EOF (End of File) is a special constant used to detect when the file has no more data.

Example

while(fscanf(fp, "%d", &num) != EOF) {
printf("%d", num);
}

9. Reading Binary Files

Binary files store data in binary format and are faster than text files.

Function Used: fread()

Syntax

fread(&variable, sizeof(variable), 1, fp);

Example: Reading a Structure from a Binary File

#include <stdio.h>

struct student {
int roll;
float marks;
};

int main() {
FILE *fp;
struct student s;

fp = fopen(“data.bin”, “rb”);

fread(&s, sizeof(s), 1, fp);

printf(“Roll: %d\nMarks: %.2f”, s.roll, s.marks);

fclose(fp);
return 0;
}

10. Comparison of File Reading Functions

Function Reads Best Use
fgetc() Character Character-wise reading
fgets() Line Line-wise reading
fscanf() Formatted data Structured data
fread() Binary data Binary files

11. Common Errors While Reading Files

  • File not opened properly

  • Reading beyond end of file

  • Using wrong file mode

  • Forgetting to close the file

12. Advantages of Reading from Files

  • Access to permanent data

  • Useful for large data handling

  • Reduces memory usage

  • Makes programs more efficient

13. Conclusion

Reading from files in C allows programs to retrieve stored data efficiently. By using different file reading functions like fgetc(), fgets(), fscanf(), and fread(), programmers can read data in various formats. Proper use of file reading techniques helps in building reliable and practical applications.