fseek

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Header File

stdio.h

Category

Input/output Routines

Prototype

int fseek(FILE *stream, long offset, int whence);
int _fseeki64(FILE *stream, __int64 offset, int whence);

Description

Repositions a file pointer on a stream.

fseek sets the file pointer associated with the 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:

SEEK_SET	0	File beginning If stream is in binary mode, then fseek sets the file pointer to offset bytes from the beginning of the file. If stream is in text mode, then fseek sets the file pointer according to the offset value. offset can be either zero (representing the beginning of the file) or a value returned by an earlier successful call to ftell (representing another position in the file). The offset value is not a byte count, so you cannot add to it or subtract from it.
SEEK_CUR	1	Current file pointer position If stream is in binary mode, then fseek adds offset to the current file pointer value. If stream is in text mode, then the offset must be zero (leaves the file pointer at its current value -- keeping the current position in the file.)
SEEK_END	2	End-of-file If stream is in binary mode, then fseek adds offset bytes from the end of the file (possibly positioning the file pointer after an arbitrary number of null characters). If stream is in text mode, then the offset must be zero (representing the end of the file).

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 nonzero on failure.

In the event of an error return, the global variable errno is set to one of the following values:

EBADF	Bad file pointer
EINVAL	Invalid argument
ESPIPE	Illegal seek on device

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;
   curpos = ftell(stream);
   fseek(stream, 0L, SEEK_END);
   length = ftell(stream);
   fseek(stream, curpos, SEEK_SET);
   return length;
}

Portability

POSIX	Win32	ANSI C	ANSI C++
+	+	+	+