fsetpos() — Set File Position

Format

#include <stdio.h>
int fsetpos(FILE *stream, const fpos_t *pos);

Language Level: ANSI

Threadsafe: Yes.

Description

The fsetpos() function moves any file position that is associated with stream to a new location within the file according to the value pointed to by pos. The value of pos was obtained by a previous call to the fgetpos() library function.

If successful, fsetpos() clears the end-of-file indicator, and undoes the effect of any previous ungetc() function on the same stream.

After the fsetpos() call, the next operation on a stream in update mode can be input or output.

Return Value

If fsetpos() successfully changes the current position of the file, it returns 0. A nonzero return value indicates an error.

The value of errno can be set to:

Value: Meaning
EBADF: The file pointer or descriptor is invalid.
EBADPOS: The position that is specified is not valid.
EINVAL: The value specified for the argument is not correct. You might receive this errno when you compile your program with *IFSIO, and you are working with a file in the QSYS file system. For example, "/qsys.lib/qtemp.lib/myfile.file/mymem.mbr"
ENODEV: Operation was attempted on a wrong device.
ENOPOS: No record at the specified position.
ERECIO: The file is open for record I/O.
ESTDERR: stderr cannot be opened.
ESTDIN: stdin cannot be opened.
ESTDOUT: stdout cannot be opened.
EIOERROR: A non-recoverable I/O error occurred.
EIORECERR: A recoverable I/O error occurred.

The fsetpos() function cannot be used for files that are opened with type=record. Also, the fsetpos() function can only support setting the position to the beginning of the file if:

your program is compiled with *IFSIO, and
you are working on a file in the QSYS file system.

Example that uses fsetpos()

This example opens a file mylib/myfile for reading. After performing input operations, fsetpos() moves the file pointer to the beginning of the file and rereads the first byte.

#include  <stdio.h>
 
FILE *stream;
 
int main(void)
{
   int retcode;
   fpos_t pos;
   char ptr[20];  /* existing file 'mylib/myfile' has 20 byte records */
   int  i;
 
   /* Open file, get position of file pointer, and read first record */
 
   stream = fopen("mylib/myfile", "rb");
   fgetpos(stream,Point-of-Sale);
   if (!fread(ptr,sizeof(ptr),1,stream))
       perror("fread error");
   else printf("1st record: %s\n", ptr);
 
   /* Perform another read operation on the second record       */
   /*  - the value of 'pos' changes                             */
   if (!fread(ptr,sizeof(ptr),1,stream))
       perror("fread error");
   else printf("2nd record: %s\n", ptr);
 
   /* Re-set pointer to start of file and re-read first record  */
   fsetpos(stream,Point-of-Sale);
   if (!fread(ptr,sizeof(ptr),1,stream))
       perror("fread error");
   else printf("1st record again: %s\n", ptr);
 
   fclose(stream);
}

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