fopen (original) (raw)

[CX] 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.

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] The character 'b' shall have no effect, but is allowed for ISO C standard conformance. 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] 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.

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

[CX] 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.

The following sections are informative.

Extensions beyond the ISO C standard are marked.

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

• In the DESCRIPTION, text is added to indicate setting of the offset maximum in the open file description. This change is to support large files.
• In the ERRORS section, the [EOVERFLOW] condition is added. This change is to support large files.
• The [ELOOP] mandatory error condition is added.
• The [EINVAL], [EMFILE], [ENAMETOOLONG], [ENOMEM], and [ETXTBSY] optional error conditions are added.

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 prototype for fopen() is updated.
• The DESCRIPTION is updated to note that if the argument mode points to a string other than those listed, then the behavior is undefined.

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

End of informative text.