The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here

NAME

fopen - open a stream

SYNOPSIS

#include <stdio.h>

FILE *fopen(const char *restrict
filename, const char *restrict mode);

DESCRIPTION

[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

The fopen() function shall open the file whose pathname is the string pointed to by filename, and associates a stream with it.

The mode argument points to a string. If the string is one of the following, the file shall be opened in the indicated mode. Otherwise, the behavior is undefined.

r or rb
Open file for reading.
w or wb
Truncate to zero length or create file for writing.
a or ab
Append; open or create file for writing at end-of-file.
r+ or rb+ or r+b
Open file for update (reading and writing).
w+ or wb+ or w+b
Truncate to zero length or create file for update.
a+ or ab+ or a+b
Append; open or create file for update, writing at end-of-file.

[CX] [Option Start] The character 'b' shall have no effect, but is allowed for ISO C standard conformance. [Option End] Opening a file with read mode (r as the first character in the mode argument) shall fail if the file does not exist or cannot be read.

Opening a file with append mode (a as the first character in the mode argument) shall cause all subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening calls to fseek().

When a file is opened with update mode ( '+' as the second or third character in the mode argument), both input and output may be performed on the associated stream. However, the application shall ensure that output is not directly followed by input without an intervening call to fflush() or to a file positioning function ( fseek(), fsetpos(), or rewind()), and input is not directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.

When opened, a stream is fully buffered if and only if it can be determined not to refer to an interactive device. The error and end-of-file indicators for the stream shall be cleared.

[CX] [Option Start] If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file did not previously exist, upon successful completion, the fopen() function shall mark for update the st_atime, st_ctime, and st_mtime fields of the file and the st_ctime and st_mtime fields of the parent directory.

If mode is w, wb, w+, wb+, or w+b, and the file did previously exist, upon successful completion, fopen() shall mark for update the st_ctime and st_mtime fields of the file. The fopen() function shall allocate a file descriptor as open() does. [Option End]

After a successful call to the fopen() function, the orientation of the stream shall be cleared, [XSI] [Option Start]  the encoding rule shall be cleared, [Option End] and the associated mbstate_t object shall be set to describe an initial conversion state.

[CX] [Option Start] The largest value that can be represented correctly in an object of type off_t shall be established as the offset maximum in the open file description. [Option End]

RETURN VALUE

Upon successful completion, fopen() shall return a pointer to the object controlling the stream. Otherwise, a null pointer shall be returned, [CX] [Option Start]  and errno shall be set to indicate the error. [Option End]

ERRORS

The fopen() function shall fail if:

[EACCES]
[CX] [Option Start] Search permission is denied on a component of the path prefix, or the file exists and the permissions specified by mode are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created. [Option End]
[EINTR]
[CX] [Option Start] A signal was caught during fopen(). [Option End]
[EISDIR]
[CX] [Option Start] The named file is a directory and mode requires write access. [Option End]
[ELOOP]
[CX] [Option Start] A loop exists in symbolic links encountered during resolution of the path argument. [Option End]
[EMFILE]
[CX] [Option Start] {OPEN_MAX} file descriptors are currently open in the calling process. [Option End]
[ENAMETOOLONG]
[CX] [Option Start]
The length of the filename argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}. [Option End]
[ENFILE]
[CX] [Option Start] The maximum allowable number of files is currently open in the system. [Option End]
[ENOENT]
[CX] [Option Start] A component of filename does not name an existing file or filename is an empty string. [Option End]
[ENOSPC]
[CX] [Option Start] The directory or file system that would contain the new file cannot be expanded, the file does not exist, and the file was to be created. [Option End]
[ENOTDIR]
[CX] [Option Start] A component of the path prefix is not a directory. [Option End]
[ENXIO]
[CX] [Option Start] The named file is a character special or block special file, and the device associated with this special file does not exist. [Option End]
[EOVERFLOW]
[CX] [Option Start] The named file is a regular file and the size of the file cannot be represented correctly in an object of type off_t. [Option End]
[EROFS]
[CX] [Option Start] The named file resides on a read-only file system and mode requires write access. [Option End]

The fopen() function may fail if:

[EINVAL]
[CX] [Option Start] The value of the mode argument is not valid. [Option End]
[ELOOP]
[CX] [Option Start] More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument. [Option End]
[EMFILE]
[CX] [Option Start] {FOPEN_MAX} streams are currently open in the calling process. [Option End]
[EMFILE]
[CX] [Option Start] {STREAM_MAX} streams are currently open in the calling process. [Option End]
[ENAMETOOLONG]
[CX] [Option Start]
Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}. [Option End]
[ENOMEM]
[CX] [Option Start] Insufficient storage space is available. [Option End]
[ETXTBSY]
[CX] [Option Start] The file is a pure procedure (shared text) file that is being executed and mode requires write access. [Option End]

The following sections are informative.

EXAMPLES

Opening a File

The following example tries to open the file named file for reading. The fopen() function returns a file pointer that is used in subsequent fgets() and fclose() calls. If the program cannot open the file, it just ignores it.

#include <stdio.h>
...
FILE *fp;
...
void rgrep(const char *file)
{
...
    if ((fp = fopen(file, "r")) == NULL)
        return;
...
}

APPLICATION USAGE

None.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

fclose(), fdopen(), freopen(), the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

Large File Summit extensions are added.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

The following changes are made for alignment with the ISO/IEC 9899:1999 standard:

The wording of the mandatory [ELOOP] error condition is updated, and a second optional [ELOOP] error condition is added.

End of informative text.

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