Having scanf in C return void instead of int

If I have a program that is using scanf t开发者_高级运维o get user inputted data:

    scanf("%d",&val);

When I run it through lint I get warnings saying that scanf returns a int and it is not being assigned anywhere. What is the c practice to solve this problem? Do I type cast it as void?

    (void)scanf("%d",&val);

The C best practice to solve this warnings is simple: Check the resultcode. The scanf function returns in the int if the operation has gone ok or not, and stores the error usually in errno.

The lack of resultcode checks usually results in big errors in certain situations.

The proper answer, of course, is to not ignore the return value. For a robust program, you should always (EDIT: usually) check that the return value indicates success, and fail gracefully when it does not.

Ideally you would do something like

int scanVal = scanf("%d", &val);
if (scanVal != 1) {
  // wait a minute, scanf couldn't match input to a "%d"!
}

and then act on the output in a sensible manner.

Instead of silencing the lint message, just check the return value to make sure nothing has gone wrong. From the man page:

RETURN VALUE

These functions return the number of input items successfully matched and assigned, which can be fewer than provided for, or even zero in the event of an early matching failure.

The value EOF is returned if the end of input is reached before either the first successful conversion or a matching failure occurs. EOF is also >returned if a read error occurs, in which case the error indicator for the stream (see ferror(3)) is set, and errno is set indicate the error.

scanf returns the number of input items successfully matched and assigned. Compare its return value to the number you expect in order to detect errors.

while (scanf("%d",&val) != 1)
    printf("Try again.\n");

Use

#pragma warning(disable:<warning number>)
scanf("%d",&val);
#pragma warning(default:<warning number>)

And your compiler will suppress that warning.

The best practice is to assign the return value of scanf to see if all (or how many) items were read. In this particular case, if it returns anything but 1, something went wrong (e. g. you wanted a number but the user is giving you unprintable characters) and you should handle it appropriately.

If you want your code to be robust in the presence of bad input, don't use scanf("%d", ...).

For most errors, scanf("%d", &val) will return 0 to indicate that it wasn't able to read an int value (for example, if the input is "foo" rather than "123").

But if the input is syntactically valid but out of range (for example 9999999999999999999), the behavior is undefined.

Most implementations will probably behave sanely, but I don't know how much consistency there is.

To read an int value safely, use fgets() (not gets()) followed by strtol(). (fgets() can have problems with very long input lines, but there are ways to deal with that.)