fseek

Repositions a file pointer on a stream.

Defined in header <stdio.h>

Syntax

#include <stdio.h>
int fseek(FILE *stream, long offset,int whence);

Portability

Remarks

fseek sets the file pointer associated with stream to a new position that is offset bytes from the file location given by whence. For text mode streams, offset should be 0 or a value returned by ftell.

whence must be one of the values 0, 1, or 2, which represent three symbolic constants (defined in stdio.h) as follows:

fseek discards any character pushed back using ungetc.

fseek is used with stream I/O; for file handle I/O, use lseek.

After fseek, the next operation on an update file can be either input or output.

Return value

fseek returns 0 if the pointer is successfully moved and a nonzero on failure.

Note: fseek can return a zero, indicating that the pointer has been moved successfully, when in fact it has not been. This is because DOS, which actually resets the pointer, does not verify the setting. fseek returns an error code only on an unopened file or device.

See also

• fgetpos
• fopen
• fsetpos
• ftell
• lseek
• rewind
• setbuf
• tell

Example

#include <stdio.h>

long filesize(FILE *stream);

int main(void)
{
   FILE *stream;
   stream = fopen("MYFILE.TXT", "w+");
   fprintf(stream, "This is a test");
   printf("Filesize of MYFILE.TXT is %ld bytes\n", filesize(stream));
   fclose(stream);
   return 0;
}

long filesize(FILE *stream) {
   long curpos, length;

   /* save the current location in the file */
   curpos = ftell(stream);

   /* seek to the end of the file */
   fseek(stream, 0L, SEEK_END);

   /* get the current offset into the file */
   length = ftell(stream);

   /* restore saved cursor position */
   fseek(stream, curpos, SEEK_SET);
   return length;
}

Differences from modern implementations

Modern references

• cppreference: fseek
• POSIX: fseek, fseeko
• Microsoft CRT: fseek, _fseeki64

Stable link: /3.1/stdio.h/fseek/ · short form /3.1/fseek/