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The C standard identifies specific strings to use for the {{mode}} on calls to {{fopen()}} \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]. To be strictly conforming and portable, one of the strings from the following table (adapted from the C standard) must be used:
Strings to use for the mode on calls to {{fopen()}}

mode string

Result

r

open text file for reading

w

truncate to zero length or create text file for writing

a

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

rb

open binary file for reading

wb

truncate to zero length or create binary file for writing

ab

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

r+

open text file for update (reading and writing)

w+

truncate to zero length or create text file for update

a+

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

r+b or rb+

open binary file for update (reading and writing)

w+b or wb+

truncate to zero length or create binary file for update

a+b or ab+

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

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