What is a good programming pattern for handling return values from stdio file writing functions

Question

I\'m working on some code that generates a lot of

ignoring return value of ‘size_t fwrite(const void*, size_t, size_t, FILE*)’, declared with attribute warn         


        

Answer 1

I'd do something along these lines:

FILE * file = fopen("foo", "wb");
if(!file) return FAILURE;

// assume failure by default
_Bool success = 0;

do
{
    if(!fwrite(&bar, sizeof(bar), 1, file))
        break;

    // [...]

    if(!fwrite(&baz, sizeof(baz), 1, file))
        break;

    // [...]

    success = 1;
} while(0);

fclose(file);

return success ? SUCCESS : FAILURE;

With a little C99 macro magic

#define with(SUBJECT, FINALIZE, ...) do { \
    if(SUBJECT) do { __VA_ARGS__ } while(0); if(SUBJECT) FINALIZE; \
} while(0)

and using ferror() instead of our own error flag as suggested by Jonathan Leffler, this can be written as

FILE * file = fopen("foo", "wb");
with(file, fclose(file),
{
    if(!fwrite(&bar, sizeof(bar), 1, file))
        break;

    // [...]

    if(!fwrite(&baz, sizeof(baz), 1, file))
        break;

    // [...]
});

return file && !ferror(file) ? SUCCESS : FAILURE;

If there are other error conditions aside from io errors, you'll still have to track them with one or more error variables, though.

Also, your check against sizeof(blah) is wrong: fwrite() returns the count of objects written!