File

Introduction

• File handling in C allows us to store and retrieve data from files.
• This is very useful for creating programs that need to save data permanently.

Basic operations on File

File handling refers to working with files for tasks like:

• Reading data from files(Read).
• Writing data to files(Write).
• Updating or appending data to existing files(Update).
• Delete data from files(Delete).
• FILE * pointer is used to access a file.

Opening a File

• To open a file, we use the fopen() function.
• Structure is:

  FILE *file = fopen("file_name.extension", "mode")
  

• mode specifies how the file should be opened. Some are
  • r
    • Read mode.
    • Returns NULL if file can't be opened or doesn't exist.
    • It doesn't create new file.
  • w
    • Write mode
    • It creates a new file or overwrites an existing one.
    • Overwrites deletes the old content and make the file blank.
  • a
    • Append mode
    • It opens the existing file or create new one then sets the pointer to the last character.
    • Unlike w, it doesn't overwrite the file content.
  • r+
    • Read and write.
    • Sets the pointer to the first character of the file.
    • It doesn't create new file.
  • w+
    • Read and write.
    • It creates new file or overwrite existing content.
  • a+
    • Append and read.
    • Creates new file if doesn't exists.
    • Sets the pointer to the end same as a.
  • rb, wb, rb+, wb+
    • These are for binary files.
    • Used for reading non-text data like image, audio etc.

Reading file

• Reading by character by character

  • File content (data.txt)

    ASDFGH
    QWERTY
    

  • Code

    #include<stdio.h>
    int main(){
    
        FILE* file = fopen("files/data.txt", "r");
        if(file == NULL) { printf("File not found\n"); return 1; }
    
        char ch;
        while( (ch = fgetc(file)) != EOF) {
            printf("%c", ch);
        }
        fclose(file);
    
        return 0;
    }
    

    • fgetc takes the file pointer and returns a char or EOF at that position.
  • Output

    ASDFGH
    QWERTY
    

• Reading by line by line

  • File content (data.txt)

    ABC CBA
    123
    DEF
    321
    

  • Code

    #include<stdio.h>
    
    int main(){
    
        FILE* file = fopen("files/data.txt", "r");
        if(file == NULL) { printf("File not found\n"); return 1; }
    
        char line[50];
        int lineNo = 1;
        while( fgets(line, 50, file) ) {
            printf("%d: %s", lineNo, line);
            lineNo++;
        }
        fclose(file);
    
        return 0;
    }
    

    • fgets
      • Structure

        fgets(char * destination, int size, FILE * fptr);
        

      • It will read at most size - 1 character from that line.

  • Output

    1: ABC CBA
    2: 123
    3: DEF
    4: 321
    

• Reading file data using format specifiers

  • We can read data like scanf function using fscanf function.
  • fscanf is similar to scanf, but it takes the file ptr as parameter followed by others.

    int fscanf(FILE *ptr, const char *format, ...);
    

  • Assume file(data.txt) contains roll name age in each line.

    1907002 Sina 21
    1907030 Abdullah 22
    1907046 Saif 23
    

  • Code for reading:

    #include<stdio.h>
    int main(){
    
        FILE* file = fopen("files/data.txt", "r");
        if(file == NULL) { printf("File not found\n"); return 1; }
    
        int roll, age;
        char name[50];
    
        printf("%-10s %-20s %-5s\n", "Roll", "Name", "Age");
        while ( fscanf(file, "%d %s %d", &roll, name, &age) != EOF) {
            printf("%-10d %-20s %-5d\n", roll, name, age);
        }
        fclose(file);
        return 0;
    }
    

  • Output

    Roll       Name                 Age  
    1907002    Sina                 21
    1907030    Abdullah             22
    1907046    Saif                 23
    

Writing to a File

• Use fprintf() or fputs() to write to a file.

• Simple writing

  • Code

    #include<stdio.h>
    int main(){
        FILE* file = fopen("files/content.txt", "w");
        if(file == NULL) { printf("Error\n"); return 1; }
    
        fprintf(file, "%d %s %d\n", 1, "John", 20);
        fprintf(file, "%d %s %d\n", 2, "Doe", 25);
    
        fclose(file);
        return 0;
    }
    

  • File content(content.txt)

    1 John 20
    2 Doe 25
    

• Appending to end of the file

  • Suppose we want to write 3 Sun 53 at the end of the previos file(content.txt). Code

    #include<stdio.h>
    int main(){
        FILE* file = fopen("files/content.txt", "a");
        if(file == NULL) { printf("Error\n"); return 1; }
    
        fprintf(file, "%d %s %d\n", 3, "Sun", 53);
    
        fclose(file);
        return 0;
    }
    

  • Updated file content

    1 John 20
    2 Doe 25
    3 Sun 53
    

    • See, only the last line is added. Rest remains the same.

Updating the content of a file

• Let's say we want to round the age by 10. Like: 25 will be replaced by 30, 57 will be replaced by 60.
• For updating a file, we will create a temporary file with our updated data. Then remove the old data, and rename temporary file.
• Code(Way-1)(Using a temporary file):

  #include<stdio.h>
  
  int main(){
  
      FILE* file = fopen("files/content.txt", "r+");
      if(file == NULL) { printf("Error\n"); return 1; }
  
      int roll, age;
      char name[50];
  
      FILE *temp = fopen("files/temp.txt", "w");
  
      while ( fscanf(file, "%d %s %d", &roll, name, &age) != EOF) {
          
          int rem = age % 10;
          if(rem < 5) age -= rem;
          else age += (10 - rem);
  
          // age = ((age + 5) / 10) * 10;
          fprintf(temp, "%d %s %d\n", roll, name, age);
      }
  
      fclose(file);
      fclose(temp);
  
      remove("files/content.txt");
      rename("files/temp.txt", "files/content.txt");
      return 0;
  }
  

• Code(Way-2)(Using fseek)

  #include<stdio.h>
  
  int getNumLength(int num){
      int length = 0;
      while( num > 0 ){
          num /= 10;
          length++;
      }
      return length;
  }
  
  int main(){
  
      FILE* file = fopen("files/content.txt", "r+");
      if(file == NULL) { printf("Error\n"); return 1; }
  
      int roll, age;
      char name[50];
  
      while ( fscanf(file, "%d %s %d", &roll, name, &age) != EOF) {
          int oldAge = age;
          
          age = ((age + 5) / 10) * 10;
  
          int numLength = getNumLength(age);
          int newNumLength = getNumLength(oldAge);
          
          // moving the file pointer to the correct position
          int position = ftell(file) - numLength;
          fseek(file, position, SEEK_SET);
  
          // writing the new age
          fprintf(file, "%d", age);
  
          // moving the file pointer to the next line
          position += newNumLength;
          fseek(file, position, SEEK_SET);
      }
  
      fclose(file);
      return 0;
  }
  

• Output

  1 John 20
  2 Doe 30
  3 Sun 50
  

Closing a File

• Always close a file using fclose() when done.
• It ensures that file is closed properly and data from buffer is cleared and written in file.

  fclose(file);
  

Reading and writing binary files

• Why do we need binary format?

  • It represents data exactly as it was.
    • Unlike text files, binary files do not interpret or translate the data (e.g., newline conversions like \n to \r\n on Windows).
    • An integer 1234 in binary format stores its actual 4-byte binary representation, not its ASCII text equivalent.
  • It is faster and can read/write as chunks.
  • It takes relatively less storage than normal representation.
  • It allows us to store and retrive complex data structure like struct without conversion.

• Saving & Reading entire array

  • We can do it using fwrite function.

  • Code:

    #include<stdio.h>
    int main(){
    
        FILE* file = fopen("files/content.bin", "wb");
        if(file == NULL) { printf("Error\n"); return 1; }
    
        int arr[5] = {1, 2, 3, 4, 5};
    
        fwrite(arr, sizeof(int), 5, file); // 5 is the len of the array
        fclose(file);
        return 0;
    }
    

  • File content(content.bin)

    

    • See content is not redable while previewing since data are in binary format.

  • But if we read the array using code like this:

    #include<stdio.h>
    int main(){
        FILE* file = fopen("files/content.bin", "rb");
        if(file == NULL) { printf("Error\n"); return 1; }
    
        int arr[5];
        fread(arr, sizeof(int), 5, file);
        
        for(int i = 0; i < 5; i++){
            printf("%d ", arr[i]);
        }
        printf("\n");
        fclose(file);
        return 0;
    }
    

  • Output:

    1 2 3 4 5
    

    • See, this was our actual array.

• Saving & Reading struct

  • As like before, we can do it using fwrite.
  • Code:

    #include<stdio.h>
    
    struct Student{
        char name[20];
        int age;
        float marks;
    };
    
    int main(){
    
        struct Student sami = {"Sami", 21, 90.5};
        struct Student panda = {"Panda", 22, 85.5};
    
        FILE* file = fopen("files/student.bin", "wb");
        if(file == NULL) { printf("Error\n"); return 1; }
    
        fwrite(&sami, sizeof(struct Student), 1, file);
        fwrite(&panda, sizeof(struct Student), 1, file);
        
        fclose(file);
        return 0;
    }
    

  • Content on file(student.bin)

    慓業          䊵慐摮a         䊫
    

  • Retrieving from file

    #include<stdio.h>
    
    struct Student{
        char name[20];
        int age;
        float marks;
    };
    
    int main(){
    
        struct Student sami, panda;
    
        FILE* file = fopen("files/student.bin", "rb");
        if(file == NULL) { printf("Error\n"); return 1; }
    
        fread(&sami, sizeof(struct Student), 1, file);
        fread(&panda, sizeof(struct Student), 1, file);
        
        printf("%s %d %.2f\n", sami.name, sami.age, sami.marks);
        printf("%s %d %.2f\n", panda.name, panda.age, panda.marks);
        
        fclose(file);
        return 0;
    }
    

  • Output:

    Sami 21 90.50
    Panda 22 85.50
    

That's the end of our C journey. Welcome to the computer world.

"The only way to do great work is to love what you do."
— Steve Jobs