6.18.25 fseek Function

Moves file pointer to a specific location.

Include

<stdio.h>

Prototype

int fseek(FILE * restrict stream, long offset, int mode);

Arguments

stream

stream in which to move the file pointer

offset

value to add to the current position

mode

type of seek to perform

Return Value

Returns 0 if successful; otherwise, returns a non-zero value and set errno.

Remarks

mode can be one of the following tabulated values.

Example

#include <stdio.h>

int main(void)
{
  FILE *myfile;
  char s[70];
  int  y;
  myfile = fopen("afile.out", "w+");
  if (myfile == NULL)
    printf("Cannot open afile.out\n");
  else
  {
    fprintf(myfile, "This is the beginning, this is the middle and this is the end.");

    y = fseek(myfile, 0L, SEEK_SET);
    if (y)
      perror("Fseek failed");
    else
    {
      fgets(s, 22, myfile);
      printf("\"%s\"\n\n", s);
    }

    y = fseek(myfile, 2L, SEEK_CUR);
    if (y)
      perror("Fseek failed");
    else
    {
      fgets(s, 70, myfile);
      printf("\"%s\"\n\n", s);
    }

    y = fseek(myfile, -16L, SEEK_END);
    if (y)
      perror("Fseek failed");
    else
    {
      fgets(s, 70, myfile);
      printf("\"%s\"\n", s);
    }
    fclose(myfile);
  }
}

Example Output

"This is the beginning"

"this is the middle and this is the end."

"this is the end."

Example Explanation

The file afile.out is created with the text, “This is the beginning, this is the middle and this is the end.”

The function fseek uses an offset of zero and SEEK_SET to set the file pointer to the beginning of the file. fgets then reads 22 characters which are “This is the beginning,” and adds a null character to the string.

Next, fseek uses an offset of two and SEEK_CURRENT to set the file pointer to the current position plus two (skipping the comma and space). fgets then reads up to the next 70 characters. The first 39 characters are “this is the middle and this is the end.” It stops when it reads EOF and adds a null character to the string.

Finally, fseek uses an offset of negative 16 characters and SEEK_END to set the file pointer to 16 characters from the end of the file. fgets then reads up to 70 characters. It stops at the EOF after reading 16 characters “this is the end,” and adds a null character to the string.