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Contents
The stdio.h library has five functions that affect the file position indicator besides those that do reading or writing: fgetpos, fseek, fsetpos, ftell, and rewind.
The fseek and ftell functions are older than fgetpos and fsetpos.
#include <stdio.h>
int fgetpos(FILE *stream, fpos_t *pos);
int fsetpos(FILE *stream, const fpos_t *pos);
The fgetpos function stores the current value of the file position indicator for the stream pointed to by stream in the object pointed to by pos. The value stored contains unspecified information usable by the fsetpos function for repositioning the stream to its position at the time of the call to the fgetpos function.
If successful, the fgetpos function returns zero; on failure, the fgetpos function returns nonzero and stores an implementation-defined positive value in errno.
The fsetpos function sets the file position indicator for the stream pointed to by stream according to the value of the object pointed to by pos, which shall be a value obtained from an earlier call to the fgetpos function on the same stream.
A successful call to the fsetpos function clears the end-of-file indicator for the stream and undoes any effects of the ungetc function on the same stream. After an fsetpos call, the next operation on an update stream may be either input or output.
If successful, the fsetpos function returns zero; on failure, the fsetpos function returns nonzero and stores an implementation-defined positive value in errno.
#include <stdio.h>
int fseek(FILE *stream, long int offset, int whence);
long int ftell(FILE *stream);
The fseek function sets the file position indicator for the stream pointed to by stream.
For a binary stream, the new position, measured in characters from the beginning of the file, is obtained by adding offset to the position specified by whence. Three macros in stdio.h called SEEK_SET, SEEK_CUR, and SEEK_END expand to unique values. If the position specified by whence is SEEK_SET, the specified position is the beginning of the file; if whence is SEEK_END, the specified position is the end of the file; and if whence is SEEK_CUR, the specified position is the current file position. A binary stream need not meaningfully support fseek calls with a whence value of SEEK_END.
For a text stream, either offset shall be zero, or offset shall be a value returned by an earlier call to the ftell function on the same stream and whence shall be SEEK_SET.
The fseek function returns nonzero only for a request that cannot be satisfied.
The ftell function obtains the current value of the file position indicator for the stream pointed to by stream. For a binary stream, the value is the number of characters from the beginning of the file; for a text stream, its file position indicator contains unspecified information, usable by the fseek function for returning the file position indicator for the stream to its position at the time of the ftell call; the difference between two such return values is not necessarily a meaningful measure of the number of characters written or read.
If successful, the ftell function returns the current value of the file position indicator for the stream. On failure, the ftell function returns -1L and stores an implementation-defined positive value in errno.
#include <stdio.h>
void rewind(FILE *stream);
The rewind function sets the file position indicator for the stream pointed to by stream to the beginning of the file. It is equivalent to
(void)fseek(stream, 0L, SEEK_SET)
except that the error indicator for the stream is also cleared.
All text is available under the terms of the GNU Free Documentation License
Source : Wikibooks
