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The C standard Standard identifies specific strings to use for the mode on calls to fopen() [ISO/IEC 9899:1999]. To to fopen() and fopen_s(). C11 provides a new mode flag, x, that provides the mechanism needed to determine if the file that is to be opened exists. To be strictly conforming and portable, one of the strings from the following table (adapted from the C standardStandard, subclause 7.21.5.2 [ISO/IEC 9899:2011]) must be used:

Strings to use Use for the mode on calls to fopenMode on Calls to fopen() and fopen_s()

`mode`

string

String	Result
`r`

open

Open text file for reading

w

truncate

Truncate to zero length or create text file for writing
`wx`	Create text file for writing
`a`

append

Append; open or create text file for writing at end-of-file

rb

open

Open binary file for reading

wb

truncate

Truncate to zero length or create binary file for writing
`wbx`	Create binary file for writing
`ab`

append

Append; open or create binary file for writing at end-of-file

r+

open

Open text file for update (reading and writing)

w+

truncate

Truncate to zero length or create text file for update
`w+x`	Create text file for update
`a+`

append

Append; open or create text file for update, writing at end-of-file

r+b or rb+

open

Open binary file for update (reading and writing)

w+b or wb+

truncate

Truncate to zero length or create binary file for update
`w+bx` or `wb+x`	Create binary file for update
`a+b` or `ab+`

append

Append; open or create binary file for update, writing at end-of-file

If the mode string begins with one of these sequences, the the implementation might choose to ignore the remaining characters, or it might use them to select different kinds of files.

When calling fopen_s(), any of the mode strings used for writing (w or a) may be prefixed with the u character to give the file system default access permissions.

An implementation may define additional mode strings, but only the modes shown in the table are fully portable and C99 compliant [ISO/IEC 9899:1999C compliant. Beware that Microsoft Visual Studio 2012 and earlier do not support the x or u mode characters [MSDN].