.\" $NetBSD: fopen.3,v 1.35.6.1 2019/09/05 08:19:40 martin Exp $ .\" .\" Copyright (c) 1990, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" This code is derived from software contributed to Berkeley by .\" Chris Torek and the American National Standards Committee X3, .\" on Information Processing Systems. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)fopen.3 8.1 (Berkeley) 6/4/93 .\" .Dd September 2, 2019 .Dt FOPEN 3 .Os .Sh NAME .Nm fopen , .Nm fdopen , .Nm freopen .Nd stream open functions .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In stdio.h .Ft FILE * .Fn fopen "const char * restrict path" "const char * restrict mode" .Ft FILE * .Fn fdopen "int fildes" "const char *mode" .Ft FILE * .Fn freopen "const char * restrict path" "const char * restrict mode" "FILE * restrict stream" .Sh DESCRIPTION The .Fn fopen function opens the file whose name is the string pointed to by .Fa path and associates a stream with it. .Pp The argument .Fa mode points to a string beginning with one of the following sequences, which may be followed by additional modifiers as indicated below: .Bl -tag -width 4n .It Dq Li a Append; open an existing or new file for writing in append mode. The file is created if it does not exist. .It Dq Li a+ Open for reading and writing in append mode. The file is created if it does not exist. .It Dq Li r Read; open an existing file for reading. .It Dq Li r+ Open an existing file for reading and writing. .It Dq Li w Write; open an empty file for writing. Truncate an existing file to zero length or create a new file. .It Dq Li w+ Open an empty file for reading and writing. Truncate file to zero length or create file. .El .Pp After one of those, the .Fa mode string can also include one or more of the following modifier letters: .Bl -tag -width 4n .It Sq b The letter .Sq b may appear either as a last character or as a character between the characters in any of the two-character strings described above. This is strictly for compatibility with .St -ansiC , where it means open in .Dq binary mode which is identical to text mode here, so has no effect; the .Sq b is ignored. .It Sq e The letter .Sq e in the mode string sets the close-on-exec .Pq Dv O_CLOEXEC flag of the file descriptor, which means that it will not be available after an .Xr exec 3 system call. This is a non .St -ansiC extension. .It Sq f The letter .Sq f in the mode string restricts .Fn fopen to regular files; if the file opened is not a regular file, .Fn fopen will close it, and fail. This is a non .St -ansiC extension. .It Sq l The letter .Sq l in the mode string turns the don't-follow-symlinks .Pq Dv O_NOFOLLOW flag of the file descriptor, which means that if the last path component is a symbolic link, it will not be followed. This is a non .St -ansiC extension. .It Sq x The letter .Sq x in the mode turns on exclusive open mode to the file .Pq Dv O_EXCL which means that the file will not be created if it already exists. In that case .Fn fopen will fail. .El .Pp Any created files will have mode .Pf \*q Dv S_IRUSR \&| .Dv S_IWUSR \&| .Dv S_IRGRP \&| .Dv S_IWGRP \&| .Dv S_IROTH \&| .Dv S_IWOTH Ns \*q .Pq Li 0666 , as modified by the process' .Xr umask 2 value. .Pp Opening a file with append mode causes all subsequent writes to it to be forced to the then current end of file, regardless of intervening repositioning of the stream. .Pp The .Fn fopen and .Fn freopen functions initially position the stream at the start of the file unless the file is opened with append mode, in which case the stream is initially positioned at the end of the file. .\" PR 6072 claims this paragraph is not correct. .\" .Pp .\" Reads and writes may be intermixed on read/write streams in any order, .\" and do not require an intermediate seek as in previous versions of .\" .Em stdio . .\" This is not portable to other systems, however; .\" .Tn ANSI C .\" requires that .\" a file positioning function intervene between output and input, unless .\" an input operation encounters end-of-file. .Pp The .Fn fdopen function associates a stream with the existing file descriptor, .Fa fildes . The .Fa mode of the stream must be compatible with the mode of the file descriptor. The stream is positioned at the file offset of the file descriptor. .Pp The .Fn freopen function opens the file whose name is the string pointed to by .Fa path and associates the stream pointed to by .Fa stream with it. The original stream (if it exists) is closed. The .Fa mode argument is used just as in the .Fn fopen function. The primary use of the .Fn freopen function is to change the file associated with a standard text stream .Pf ( Em stderr , .Em stdin , or .Em stdout ) . .Pp Input and output against the opened stream will be fully buffered, unless it refers to an interactive terminal device, or a different kind of buffering is specified in the environment. See .Xr setvbuf 3 for additional details. .Sh RETURN VALUES Upon successful completion .Fn fopen , .Fn fdopen and .Fn freopen return a FILE pointer. Otherwise, .Dv NULL is returned and the global variable .Va errno is set to indicate the error. .Sh ERRORS The functions may fail if: .Bl -tag -width Er .It Bq Er EFTYPE The file is not a regular file and the character ``f'' is specified in the mode. .It Bq Er EINVAL The specified .Fa mode was invalid. .El .Pp The .Fn fopen , .Fn fdopen and .Fn freopen functions may also fail and set .Va errno for any of the errors specified for the routine .Xr malloc 3 . .Pp The .Fn fopen function may also fail and set .Va errno for any of the errors specified for the routine .Xr open 2 . .Pp The .Fn fdopen function may also fail and set .Va errno for any of the errors specified for the routine .Xr fcntl 2 . .Pp The .Fn freopen function may also fail and set .Va errno for any of the errors specified for the routines .Xr open 2 , .Xr fclose 3 and .Xr fflush 3 . .Sh SEE ALSO .Xr open 2 , .Xr fclose 3 , .Xr fileno 3 , .Xr fseek 3 , .Xr funopen 3 .Sh STANDARDS The .Fn fopen and .Fn freopen functions conform to .St -ansiC . All three functions are specified in .St -p1003.1-2008 . .Sh HISTORY An .Fn fopen function appeared in .At v1 . .Sh CAVEATS Proper code using .Fn fdopen with error checking should .Xr close 2 .Fa fildes in case of failure, and .Xr fclose 3 the resulting FILE * in case of success. .Bd -literal FILE *file; int fd; if ((file = fdopen(fd, "r")) != NULL) { /* perform operations on the FILE * */ fclose(file); } else { /* failure, report the error */ close(fd); } .Ed