C File Pointer

A file pointer is a variable that is used to refer to an opened file in a C program. The file pointer is actually a structure that stores the file data such as the file name, its location, mode, and the current position in the file. It is used in almost all the file operations in C such as opening, closing, reading, writing, etc.

Syntax

FILE *ptr;

Here, FILE is the typedef name of the predefined file pointer structure and ptr is a pointer variable of type FILE.

Example of File Pointer

C

// C Program to demonstrate the file pointer
#include <stdio.h>

int main()
{
    // declaring file pointer
    FILE* fptr;

    // trying to get the size of FILE datatype.
    printf("Size of FILE Structure: %d bytes",
           sizeof(FILE));

    return 0;
}

Output

Size of FILE Structure: 216 bytes

How File Pointer Works in C?

We use a file pointer to refer to the file opened using fopen() function and the behavior of a file pointer can vary depending on the access modes specified when opening the file using the fopen() function.

Let’s see how the C file pointer works in files with different access modes:

1. In Read Mode ( “r” )

Syntax

FILE *fp;
fp = fopen("fileName", "r");

• The position of the file pointer is initially at the beginning of the file.
• When we read data from a file using functions like fgetc(), fgets(), etc., the file pointer moves forward automatically to the next position after the read operation.
• We cannot perform write operations using file pointer referring to the file opened in read mode.

2. In Write Mode ( “w” )

Syntax

FILE *fp;
fp = fopen("fileName", "w");

• If the file exists, the position of the file pointer is initially at the beginning of the file.
• The existing content in the file is overwritten when we write data to the file.
• When we write data to the file using functions like fputc(), fprintf(), etc., the file pointer moves forward automatically to the next position after the write operation.
• Read operations like fgetc() or fgets() cannot be performed on file pointer pointing to these files.

3. Append Mode ( “a” )

Syntax

FILE *fp;
fp = fopen("fileName", "a");

• In append mode, the file pointer is positioned at the end of the file.
• The file pointer automatically moves forward to the next position after each write operation.

How File Pointer Works in fseek() function?

fseek() function in C is used to set or change the file pointer to a specific position within a file.

Syntax

fseek(FILE *filePointer, long offset, int origin);

Parameters

• filePointer: The file pointer we want to modify.
• offset: The number of bytes to move the file pointer that can be positive or negative.
• origin: The starting point from where the offset is calculated. It can take one of the following values:
  • SEEK_SET: Beginning of the file.
  • SEEK_CUR: Current position of the file pointer.
  • SEEK_END: End of the file.

When we call fseek() and provide the value of offset and origin, the position of the file pointer is changed accordingly. The new position is determined by adding the offset to the origin. It returns 0 if the operation is successful and it returns a non-zero value if an error occurred.

Conclusion

File pointers in C are important for performing input and output operations on files. They work as an interface between the program and the file, allowing us to read from and write to the file.