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C Library - ftell() function

The C library ftell() function returns the current file position of the given stream. This function is important for determining where in the file the next read or write operation will occur.

Following is the C Library syntax of ftell() function −

long ftell(FILE *stream);

stream : A pointer to a FILE object that specifies the file stream. The FILE object is typically obtained by using the fopen() function.

On success, ftell() returns the current file position as a long integer. On failure, it returns -1L and sets the global variable errno to indicate the error.

Basic Usage of ftell()

In this example, we open a file, move the file position indicator, and then use ftell() to get the current position.

#include <stdio.h>
int main() {
   FILE *file = fopen("example.txt", "r");
   if (file == NULL) {
      perror("Failed to open file");
      return 1;
   }

   // Move the file position indicator to the 10th byte
   fseek(file, 10, SEEK_SET);

   // Get the current position
   long position = ftell(file);
   if (position == -1L) {
      perror("ftell failed");
   } else {
      printf("Current file position: %ld\n", position);
   }
   fclose(file);
   return 0;
}

The above code produces following result−

Current file position: 10

Using ftell() to Determine File Length

This example shows how to use ftell() to find the length of a file by seeking to the end of the file.

#include <stdio.h>
int main() {
   FILE *file = fopen("example.txt", "r");
   if (file == NULL) {
      perror("Failed to open file");
      return 1;
   }

   // Move the file position indicator to the end of the file
   fseek(file, 0, SEEK_END);

   // Get the current position, which is the file size
   long fileSize = ftell(file);
   if (fileSize == -1L) {
      perror("ftell failed");
   } else {
      printf("File size: %ld bytes\n", fileSize);
   }
   fclose(file);
   return 0;
}

After execution of above code, we get the following result

File size: 150 bytes

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