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fseek(3) Library Functions Manual fseek(3) NAME fseek, fseek_unlocked, rewind, ftell, fgetpos, fsetpos - Reposition the file pointer of a stream LIBRARY Standard C Library (libc.so, libc.a) SYNOPSIS #include <stdio.h> int fseek( FILE *stream, long int offset, int whence); int fseek_unlocked( FILE *stream, long int offset, int whence); void rewind( FILE *stream); long int ftell( FILE *stream); int fsetpos( FILE *stream, const fpos_t *position); int fgetpos( FILE *stream, fpos_t *position); STANDARDS Interfaces documented on this reference page conform to industry stan- dards as follows: fseek(), fgetpos(), fsetpos(), ftell(), rewind(): XPG4, XPG4-UNIX Refer to the standards(5) reference page for more information about in- dustry standards and associated tags. PARAMETERS Specifies the I/O stream. Determines the position of the next opera- tion. Determines the value for the file pointer associated with the stream parameter. Specifies the value of the file position indicator. DESCRIPTION The fseek() function sets the position of the next input or output op- eration on the I/O stream specified by the stream parameter. The posi- tion of the next operation is determined by the offset parameter, which can be either positive or negative. The fseek() function sets the file pointer associated with the speci- fied stream as follows: If the whence parameter is SEEK_SET(0), the pointer is set to the value of the offset parameter. If the whence pa- rameter is SEEK_CUR(1), the pointer is set to its current location plus the value of the offset parameter. If the whence parameter is SEEK_END(2), the pointer is set to the size of the file plus the value of the offset parameter. The fseek() function fails if attempted on a file that was not opened with the fopen() function. In particular, the fseek() function cannot be used on a terminal or on a file opened with the popen() function. A successful call to the fseek() function clears the End-of-File indi- cator for the stream and undoes any effects of the ungetc() function on the same stream. After a call to the fseek() function, the next opera- tion on an update stream may be either input or output. If the stream is writable, and buffered data was not written to the un- derlying file, the fseek() function causes the unwritten data to be written to the file and marks the st_ctime and st_mtime fields of the file for update. If the most recent operation (ignoring any ftell() operations) on a given stream was fflush(), then the fseek() function causes the file offset in the underlying open file descriptor to be adjusted to reflect the location specified by the fseek() function. The fseek() function allows the file-position indicator to be set be- yond the end of existing data in the file. If data is later written at this point, subsequent reads of data in the gap will return bytes with the value 0 (zero) until data is actually written into the gap. The rewind() function is equivalent to (void) fseek (stream, 0L, SEEK_SET), except that rewind() also clears the error indicator. The ftell() function obtains the current value of the file position in- dicator for the specified stream. The fgetpos() and fsetpos() functions are similar to the ftell() and fseek() functions, respectively. The fgetpos() function stores the cur- rent value of the file position indicator for the stream pointed to by the stream parameter in the object pointed to by the position parame- ter. The fsetpos function sets the file position indicator according to the value of the position parameter, returned by a prior call to the fgetpos() function. A successful call to the fsetpos() function clears the EOF indicator and undoes any effects of the ungetc() function. [Digital] The fseek_unlocked() function is functionally identical to the fseek() function, except that fseek_unlocked() may be safely used only within a scope that is protected by the flockfile() and funlock- file() functions used as a pair. The caller must ensure that the stream is locked before using these functions. RETURN VALUES Upon successful completion, the fseek() and fseek_unlocked() functions return a value of 0 (zero). If the fseek() or fseek_unlocked() function fails, a value of -1 is returned, and errno is set to indicate the er- ror. The rewind() function does not return a value. Upon successful completion, the ftell() function returns the offset of the current byte relative to the beginning of the file associated with the named stream. Otherwise, a value of -1 is returned, and errno is set to indicate the error. Upon successful completion, the fgetpos() and fsetpos() functions re- turn a value of 0 (zero). If the fgetpos() or the fsetpos() function fails, a value of -1 is returned, and errno is set to indicate the er- ror. ERRORS The fseek() or fseek_unlocked() function fails if either the stream is unbuffered, or the stream's buffer needed to be flushed and the call to fseek() or fseek_unlocked() caused an underlying lseek() or write() function to be invoked. In addition, if any of the following condi- tions occurs, the fseek() or fseek_unlocked( function sets errno to the value that corresponds to the condition. The O_NONBLOCK flag is set for the file descriptor underlying the stream parameter and the process would be delayed in the write operation. The file descriptor underly- ing the stream parameter is not a valid file descriptor open for writ- ing. An attempt was made to write to a file that exceeds the process's file size limit or the maximum file size. (See the ulimit(3) reference page.) The write operation was terminated by a signal, and either none, some, or all the data was transferred. If there is buffered I/O, it is recommended that you call the fflush() function before the fseek() function to guarantee that the buffer characters were written. The whence parameter is an invalid value, or the resulting file offset would be invalid. A physical I/O error has occurred, or the process is a member of a background process group attempting to write to its con- trolling terminal, the TOSTOP signal is set, the process is neither ig- noring nor blocking SIGTTOU, and the process group of the process is orphaned. There was no free space remaining on the device containing the file. The file descriptor underlying stream is associated with a pipe or FIFO. An attempt was made to write to a pipe or FIFO that is not open for reading by any process. A SIGPIPE signal will also be sent to the process. A request was made of a nonexistent device, or the request was outside the capabilities of the device. The rewind() function fails under the same conditions as the fseek() function, with the exception of [EINVAL], which does not apply. If the following conditions occur, the fgetpos(), fsetpos() or ftell() function sets errno to the value that corresponds to the condition. The file descriptor underlying the stream parameter is not a valid file descriptor. [Digital] The stream parameter does not point to a valid FILE structure or the position parameter is negative. An illegal at- tempt was made to get or set the file position of a pipe or FIFO. [XPG4-UNIX] The file descriptor underlying stream is associated with a socket. RELATED INFORMATION Functions: lseek(2), fopen(3) Standards: standards(5) delim off fseek(3)