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Why does fopen take a string as its second argument?

Asked 2010-03-25T20:50:24.843
19

It has always struck me as strange that the C function fopen() takes a const char * as the second argument. I would think it would be easier to both read your code and implement the library if there were bit masks defined in stdio.h, like IO_READ and such, so you could do things like:

FILE *myFile = fopen("file.txt", IO_READ | IO_WRITE);

Is there a programmatic reason for the way it actually is, or is it just historic? (i.e. 'That's just the way it is.')

c file-io fopen history c-standard-library
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2 Answers

12

I believe that one of the advantages of the character string instead of a simple bit-mask is that it allows for platform-specific extensions which are not bit-settings. Purely hypothetically:

FILE *fp = fopen("/dev/something-weird", "r+,bs=4096");

For this gizmo, the open() call needs to be told the block size, and different calls can use radically different sizes, etc. Granted, I/O has been organized pretty well now (such was not the case originally — devices were enormously diverse and the access mechanisms far from unified), so it seldom seems to be necessary. But the string-valued open mode argument allows for that extensibility far better.

On IBM's mainframe MVS o/s, the fopen() function does indeed take extra arguments along the general lines described here — as noted by Andrew Henle (thank you!). The manual page includes the example call (slightly reformatted):

FILE *fp = fopen("myfile2.dat", "rb+, lrecl=80, blksize=240, recfm=fb, type=record");

The underlying open() has to be augmented by the ioctl() (I/O control) call or fcntl() (file control) or functions hiding them to achieve similar effects.

answered 2010-03-26T14:19:06.640
8

One word: legacy. Unfortunately we have to live with it.

Just speculation: Maybe at the time a const char * seemed more flexible solution, because it is not limited in any way. A bit mask could only have 32 different values. Looks like a YAGNI to me now.

More speculation: Dudes were lazy and writing "rb" requires less typing than MASK_THIS | MASK_THAT :)

answered 2010-03-25T20:58:49.067

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