[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 POSIX.1-2017 defers to the ISO C standard. 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 POSIX.1-2017 defers to the ISO C standard.

The fopen() function shall open the file whose pathname is the string pointed to by pathname, 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, fopen() shall mark for update the last data access, last data modification, and last file status change timestamps of the file and the last file status change and last data modification timestamps of the parent directory.

If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file did not previously exist, the fopen() function shall create a file as if it called the creat() function with a value appropriate for the path argument interpreted from pathname and a value of S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH for the mode argument.

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 last data modification and last file status change timestamps of the file.

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 file descriptor associated with the opened stream shall be allocated and opened as if by a call to open() with the following flags: