The C standard specifies identifies specific strings to use for the mode for the function on calls to fopen(). An implementation may define extra strings that define additional modesadditional mode strings, but only the modes in the following table (adapted from the C99 standard) are fully portable and C99 compliant:
| 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 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.
Risk Assessment
Using a non-standard mode will lead to undefined behavior, likely causing the call to fopen() to fail.
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