cant write to file after using fgetc/getc (Windows)

Question

I'm developing on Windows (VS 2010) a simple program using fgetc and I recognized that after I'm using this function (same behavior with getc) I can't write to the file although I opened it with appropriate permissions.

Here is a simple program I wrote to check what the hell is going on:

int main(int argc, char * argv[]) 
{
    FILE *f = NULL;
    char ch = 0;
    int bytes = 0;

    f = fopen(argv[1], "r+");
    ch = fgetc(f);
    bytes = fwrite("1234", sizeof(char), 4, f);
    printf("%d", bytes);
    fflush(f);
    fclose(f);

    system("PAUSE");
    return 0;
}

The program prints 4 and exit without any error, but the file content remain the same. After I research it a little bit I found out that if I add this line (that doing "nothing"):

fseek(f, 0, SEEK_CUR);

The program working as I expected, and the file content changed.

all the code after adding this line:

int main(int argc, char * argv[]) 
{
    FILE *f = NULL;
    char ch = 0;
    int bytes = 0;

    f = fopen(argv[1], "r+");
    ch = fgetc(f);
    fseek(f, 0, SEEK_CUR)
    bytes = fwrite("1234", sizeof(char), 4, f);
    printf("%d", bytes);
    fflush(f);
    fclose(f);

    system("PAUSE");
    return 0;
}

Does anyone know why is this happen and how to solve it?

Thanks a lot, Hanan.

First of all [`fgetc`](http://en.cppreference.com/w/c/io/fgetc) doesn't return a `char`. This is actually very important. — Some programmer dude, May 13 '16 at 07:45
As for your problem, what is the original content of the file? Just a single byte? — Some programmer dude, May 13 '16 at 07:45
[some stuff that might happen if you use `fgetc` and friends with `char`](http://stackoverflow.com/a/35356684/918959) — Antti Haapala -- Слава Україні, May 13 '16 at 07:52
And `sizeof(char)` is 1, since return value of `sizeof` is number of `char`s ("bytes") — Antti Haapala -- Слава Україні, May 13 '16 at 07:53
BTW, is your intention really to open the file in text mode? — Antti Haapala -- Слава Україні, May 13 '16 at 07:55
I wrote the example only to demonstrate my problem I didn't really care about those problems since the program does works. Thanks for the good practice tips. — hanrok, May 13 '16 at 08:28

Antti Haapala -- Слава Україні · Answer 1 · 2016-05-13T08:05:52.567

2

It does not work because the C standard (C11 draft n1570 7.21.5.3 in The fopen function paragraph 7; and all previous versions as well) says it does not have to work.

When a file is opened with update mode ( + as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function ( fseek , fsetpos ,or rewind ), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file. [--]

Notice that if the file was initially empty, then fgetc would have returned EOF and "1234" would have been written.

edited May 13 '16 at 08:05

answered May 13 '16 at 08:00

Antti Haapala -- Слава Україні

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So what do I need to do if I want to write something in the file based on what I read. for example, if I want to replace every byte after 'a' char in the file with 'b' (if the content of the file was "asdfasdf" I want it to be "abdfabdf") – hanrok May 13 '16 at 08:33
ah now that is trickier. You cannot seek backwards; instead standardwise, you'd have to use `ftell` to tell the file position *before* `fgetc`; then after `fgetc` seek to it using `fseek` with `SEEK_SET`. All in all, if you need to do these modifications a lot, it'd be more worthwhile to use an operating-system specific mechanism for memory-mapping the file. – Antti Haapala -- Слава Україні May 13 '16 at 08:40

score 0 · Accepted Answer · answered May 13 '16 at 08:00

From https://msdn.microsoft.com/en-us/library/yeby3zcb.aspx

When the "r+", "w+", or "a+" access type is specified, both reading and writing are enabled (the file is said to be open for "update"). However, when you switch from reading to writing, the input operation must encounter an EOF marker. If there is no EOF, you must use an intervening call to a file positioning function. The file positioning functions are fsetpos, fseek, and rewind. When you switch from writing to reading, you must use an intervening call to either fflush or to a file positioning function

So because you probably haven't hit EOF then you need to position the seek position so your call to fseek is exactly what you needed.

The comments on the return code from fgetc are important too but aren't affecting your code at the moment.

cant write to file after using fgetc/getc (Windows)

2 Answers2