Opening a file in 'a+ 'mode

c file file-io fopen file-pointer
user191776 picture user191776 · Sep 5, 2010 · Viewed 32.4k times · Source

If a file is opened using the following command:

FILE *f1=fopen("test.dat","a+");

The man page reads:

a+

Open for reading and appending (writing at end of file). The file is created if it does not exist. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.

So does f1 have 2 separate offset pointers, one for read & another for write?

Answer

codaddict picture codaddict · Sep 5, 2010

No.

There is just one pointer which initially is at the start of the file but when a write operation is attempted it is moved to the end of the file. You can reposition it using fseek or rewind anywhere in the file for reading, but writing operations will move it back to the end of file.

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