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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, you must use one of these strings: |
| 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 |
If the mode string begins with one of the above sequences, the implementation might choose to ignore the
remaining characters, or it might use them to select different kinds of a file.
An implementation may define additional mode strings, but only the modes in the following table (adapted from the C99 standard) are fully portable and C99 compliant:
Risk Assessment
Using a non-standard mode will lead to undefined behavior, likely causing mode string that is not recognized by an implementation may cause the call to fopen() to fail.
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