Random Access in File

Random access means you can move to any part of a file and read or write data from it without having to read through the entire file. There is no need to read each record sequentially if we want to access a particular record. C supports these functions for random access file processing.

Functions used for random access file are:

• fseek()
• ftell()
• rewind()

fseek()

• This function is used for seeking the pointer position in the file at the specified byte.
• Syntax:  fseek( file pointer, displacement, pointer position);
  Where
  ->file pointer: It is the pointer that points to the file.
  ->displacement: It is positive or negative. This is the number of bytes that are skipped backward (if negative) or forward (if positive) from the current position. This is attached to L because this is a long integer.
• Pointer position: This sets the pointer position in the file.
  SEEK_SET     Seeks from the beginning of the file
  SEEK_CUR    Seeks from the current position
  SEEK_END    Seeks from the end of file

Equivalently,

1. fseek( p,10L,0) or fseek(fp,10L, SEEK_SET);
   0 means pointer position is on the beginning of the file, from this statement pointer position is skipped 10 bytes from the beginning of the file.
2. fseek( p,5L,1) or fseek(fp,5L,SEEK_CUR);
   1 means the current position of the pointer position. From this statement pointer position is skipped 5 bytes forward from the current position.
3. fseek(p,-5L,1) or fseek(fp,-5L,SEEK_CUR);
   From this statement pointer position is skipped 5 bytes backward from the current position.

Example: fseek()
#include
int main()
{
FILE * fp;
f = fopen("myfile.txt", "w");
fputs("Hello World", fp);
fseek(fp, 6, SEEK_SET);
fputs(" Nepal", fp);
fclose(fp);
return 0;
}

ftell()

• This function returns the value of the current pointer position in the file.
• The value is count from the beginning of the file.
• Syntax: ftell(fptr); Where fptr is a file pointer.

Example:

#include
int main(void)
{
FILE *stream;
stream = fopen("MYFILE.TXT", "w");
fprintf(stream, "This is a test");
printf("The file pointer is at byte %ld\n", ftell(stream));
fclose(stream);
return 0;
}

rewind()

• This function is used to move the file pointer to the beginning of the given file.
• Syntax: rewind( fptr); Where fptr is a file pointer.

More Examples: