How can I fix the scanf to take data into the array? - c

Can someone please advise regarding the scanf?
"message" is an array of 4 rows by 16 columns.
After the user enters the numbers (all 1 digit integers), when hitting "Enter" there is an error message. As said - probably something with the scanf.
for (int i = 0; i < M; i++) {
for (int j = 0; j < columns; j++) {
scanf("%d", message[i][j]);
}
}

[Answer adapted from a now-deleted former question.]
scanf seems like a nice, simple, easy, convenient way to read input. And in some ways it is, but it turns out that using it correctly can be surprisingly difficult, and it's chock-full of pitfalls.
You're probably not going to want to learn about all of scanf's intricacies at first. So for now, here are some guidelines for using a subset of scanf's functionality, safely. (For reasons I fail to understand, everybody tells beginners to use scanf, but nobody ever teaches rules like these along with it.)
Remember to always use & in front of the variables you're trying to read. (This is a special rule, for scanf only. Don't try to use that & on the variables you pass to printf, for example.)
Exception to rule 1: Do not use the & when you are reading strings with %s.
If you are reading strings using %s, make sure the variable you read into is either a character array that's big enough for what the user is likely to type, or a pointer to malloc'ed memory that's big enough for what the user is likely to type. (See also note below about avoiding overflow.)
Be aware that %s reads strings that don't contain space characters. You can't use %s to read strings (like full names) that might contain spaces. (For now, please don't worry about how you might read a string that might contain spaces. See also rule 14.)
Don't try to use scanf and fgets together in the same program.
Don't try to use scanf and getchar together in the same program.
Try to limit yourself to only the format specifiers %d, %s, %f, and %lf, to read into variables of type int, string (see rule 3), float, and double, respectively.
If you want to read a character into a variable of type char, you can use " %c", but the mysterious extra explicit space character there is vital.
Use only one % sign in the format string, to read one variable. Don't try to read two or more variables in one scanf call.
Always check scanf's return value. If it returns 0, or the negative value EOF, that means it didn't successfully read anything. If it returns 1, that means it successfully read one value. (And if you break rule 9, and try to read multiple values, it'll return the number of values it did successfully read, anywhere between 0 and the number you asked for.)
If scanf returns 0 or EOF, indicating that the user did not type a valid value, just print an error message and exit. Don't try to write code that asks the user to try again, because the user's wrong input is still sitting on the input stream, and there's no good, simple way to get rid of it. (If you really want to write user-friendly code that re-prompts in case of error, scanf is not the right tool for the job.)
Never put whitespace after the format string. That includes the newline character \n. (That is, use "%d", not "%d " or "%d\n".)
Don't try to use the %[…] specifier.
If you break rule 13 (perhaps because someone told you that %[…] might be a way to read a string containing spaces, or a whole line), do not put an s after it. The format is %[…], not %[…]s.
These rules may seem restrictive, but if you follow these rules, you should be able to simply and easily and reliably get simple inputs into your simple programs, which is the goal here. scanf is otherwise remarkably hard to use, and experience has shown that there are something like 17 different horribly frustrating problems that tend to come up, and trying to solve them is a completely unnecessary distraction from your goal of learning C by writing simple C programs.
When you're reading strings with %s or %[…], there's a danger: no matter how big the array or malloc'ed buffer you're reading into is, the user might type more than that. To avoid overflow, you can use a format like %19s to explicitly tell scanf how many characters it's allowed to read. (And remember to subtract 1, to leave room for the terminating '\0'. That is, you'd use %19s to read into char str[20]; or char *p = malloc(20);.)
These rules are somewhat numerous. For a simpler set of rules, see this answer. Putting rules 7, 8, 9, 12, and 13 together, there are really only five complete format strings you want to use with scanf: "%d", "%s", "%f", "%lf", or " %c". (But no commas, no fixed strings, no whitespace other than the explicit space in " %c", nothing else.) Exception: as mentioned above, it's fine to use something like %19s to limit the length of a string being read.
If you need to do something more complicated, that you can't do while staying within these rules, it's time to either:
Learn enough about how scanf really works (and doesn't work) so that you can try to do the thing you want, but without getting stuck on one of those 17 problems, or
Learn how to do input using better and more powerful methods than scanf, perhaps by reading whole lines using fgets and then parsing them.

You're missing a "&" before "message[i][j]".
for (int i = 0; i < M; i++) {
for (int j = 0; j < columns; j++) {
scanf("%d", &message[i][j]);
}
}

Related

How I can solve this problem during debugging?(unhandled exception at 0xFEFEFEFE)

It seems like there is problem in scanf_s
Here is my code.
#include <stdio.h>
#include "stack.h"
int main(){
int disk;
int hanoi[3][9];
char input[3] = { 0,0,0 };
int moveDisk;
for (int i = 0; i < 9; i++) {
hanoi[0][i] = i + 1;
hanoi[1][i] = 0;
hanoi[2][i] = 0;
}
printf("Insert the number of disks(1~9): ");
scanf_s("%d", &disk);
while (input[0] != 'q') {
printf("%3c %3c %3c\n", 'A', 'B', 'C');
for (int i = 0; i < disk; i++) {
printf("%3d %3d %3d\n", hanoi[0][i], hanoi[1][i], hanoi[2][i]);
}
scanf_s("%s", &input); //getting moving disk -- LOCATION OF ERROR
}
}
I have no idea how to solve this
No doubt you tried to use scanf() in the normal way and Visual Studio reported an error instructing you to use scanf_s()? It is not a direct replacement. For all %c, %s and %[ format specifiers you must provide two arguments - the target receiving the input, and the size of target (or strictly the number of elements).
In VS2019 even at /W1 warning level, it issues a clear explanation of the problem in this case:
warning C4473: 'scanf_s' : not enough arguments passed for format string
message : placeholders and their parameters expect 2 variadic arguments, but 1 were provided
message : the missing variadic argument 2 is required by format string '%s'
message : this argument is used as a buffer size
Don't ignore the warnings, and certainly don't disable them globally (/W0).
So in this case:
scanf_s("%s", input, sizeof(input) ) ;
again more strictly:
scanf_s("%s", input, sizeof(input)/sizeof(*input) ) ;
but the latter is really only necessary for wscanf_s (wide characters). In both cases you could use the _countof() macro, but it is Microsoft specific.
scanf_s("%s", input, _countof(input) ) ;
Note also the lack of an & before input. You don't need it for an argument that is already array or pointer. That is true of scanf() too.
Whilst there are arguments for using scanf_s() over scanf() (which is intrinsically more dangerous), it can just make life difficult if you are learning from standard examples or using a different toolchain. The simpler solution is just to disable the warning, and understand that it is unsafe:
You cited the line
scanf_s("%s", &input);
There are several things wrong with this line:
You are reading a string into a character array. This is an exception to the normal pattern for scanf, in that you do not need the &.
You are using the semistandard scanf_s, instead of the normal scanf. scanf_s is supposed to be "safer", but in order for it to provide its safetiness guarantees you have to call it differently than normal scanf, too. You have to tell it the size of the array you're reading the string into. Combined with #1 above, I believe a more correct call would be scanf_s("%s", input, 3);.
For most purposes, a string of size 3 would be far too small for reading a line of input from the user. Since in this case I guess you're only reading a "line" to give yourself an opportunity to hit RETURN before the program makes another trip through its loop, I guess it's okay.
As I mentioned, scanf_s is not quite Standard, so using it is a mixed bag. Pros: 1. It's allegedly safer. 2. Some people (including perhaps your instructor) will recommend always using it for that reason. Cons: 3. It's nut fully standard (it's an optional part of the standard) meaning that not all C compilers and libraries will support it. 4. Its calling patterns are necessarily quite different than normal scanf; it is not a drop-in replacement, so confusion is likely. (I'm not saying "don't use scanf_s", but you should be aware of its somewhat dubious status.)
If you want to read a line of input from the user before continuing, and if the line might be a "q" or something else, scanf (of any variety) might not be the best choice. In particular, %s wants to read a non-whitespace string, so if you just hit the Return key, it's going to keep waiting. This might or might not be a problem for you. (Or it might not be something you need to worry about right now; you may have bigger fish to fry.)
How can I solve this problem during debugging?
Run your program step by step using the debugger. Then when you get the exception, you've found the line causing it.
Restart your program and go up to the line where the exception will occur. That is stop on that line without execution it.
Then with the debugger, you can look at all variables and try to understand if their value is what you expect.
Does this answered your question?
BTW: The compiler should at least emitted some warnings. You really should first fix those warnings. If you have no warning, make sure you have turned on all warnings in the compiler options.

scanf("%[^\n]s",a) vs gets(a)

I have been told that scanf should not be used when user inputs a string. Instead, go for gets() by most of the experts and also the users on StackOverflow. I never asked it on StackOverflow why one should not use scanf over gets for strings. This is not the actual question but answer to this question is greatly appreciated.
Now coming to the actual question. I came across this type of code -
scanf("%[^\n]s",a);
This reads a string until user inputs a new line character, considering the white spaces also as string.
Is there any problem if I use
scanf("%[^\n]s",a);
instead of gets?
Is gets more optimized than scanf function as it sounds, gets is purely dedicated to handle strings. Please let me know about this.
Update
This link helped me to understand it better.
gets(3) is dangerous and should be avoided at all costs. I cannot envision a use where gets(3) is not a security flaw.
scanf(3)'s %s is also dangerous -- you must use the "field width" specifier to indicate the size of the buffer you have allocated. Without the field width, this routine is as dangerous as gets(3):
char name[64];
scanf("%63s", name);
The GNU C library provides the a modifier to %s that allocates the buffer for you. This non-portable extension is probably less difficult to use correctly:
The GNU C library supports a nonstandard extension that
causes the library to dynamically allocate a string of
sufficient size for input strings for the %s and %a[range]
conversion specifiers. To make use of this feature, specify
a as a length modifier (thus %as or %a[range]). The caller
must free(3) the returned string, as in the following
example:
char *p;
int n;
errno = 0;
n = scanf("%a[a-z]", &p);
if (n == 1) {
printf("read: %s\n", p);
free(p);
} else if (errno != 0) {
perror("scanf");
} else {
fprintf(stderr, "No matching characters\n"):
}
As shown in the above example, it is only necessary to call
free(3) if the scanf() call successfully read a string.
Firstly, it is not clear what that s is doing in your format string. The %[^\n] part is a self-sufficient format specifier. It is not a modifier for %s format, as you seem to believe. This means that "%[^\n]s" format string will be interpreted by scanf as two independent format specifiers: %[^\n] followed by a lone s. This will direct scanf to read everything until \n is encountered (leaving \n unread), and then require that the next input character is s. This just doesn't make any sense. No input will match such self-contradictory format.
Secondly, what was apparently meant is scanf("%[^\n]", a). This is somewhat close to [no longer available] gets (or fgets), but it is not the same. scanf requires that each format specifiers matches at least one input character. scanf will fail and abort if it cannot match any input characters for the requested format specifier. This means that scanf("%[^\n]",a) is not capable of reading empty input lines, i.e. lines that contain \n character immediately. If you feed such a line into the above scanf, it will return 0 to indicate failure and leave a unchanged. That's very different from how typical line-based input functions work.
(This is a rather surprising and seemingly illogical properly of %[] format. Personally, I'd prefer %[] to be able to match empty sequences and produce empty strings, but that's not how standard scanf works.)
If you want to read the input in line-by-lane fashion, fgets is your best option.

scanf error handling C

Say If i want an input to be
[Name] [Name]
How would I detect
[Name] [Name] [Name]
and return error?
Here is what I have so far,
char in[20];
char out[20];
scanf(" %s %s", out, in);
scanf returns the number of validly converted arguments. So in your first case, the return value would be 2, in the latter case 3.
To check the right amount of parameters, this might help:
char in[20];
char out[20];
char error[20];
int check;
check = scanf(" %s %s %s", out, in, error);
if(check != 2)
{
// TODO: error handling
}
EDIT: now it should be working, see comments below.
Of course, as stated by other posters: scanf is not considered a quite safe function since buffer overflows can occur, and you should avoid using it. It is better to read the inputs to a buffer with fgets() and the try to parse the arguments you want.
This is homework, so you might be required to work under certain (arbitrary) restrictions. However, the phrase "scanf error handling" is something of an oxymoron in the C programming language.
The best way to do this is to read in a line/other suitable chunk and parse it with C string functions. You can do it in one line of scanf but there are many drawbacks:
You can guard against buffer overflows, but if a buffer isn't large enough you can't recover.
You can specify specific character ranges for your strings, but it starts to look a little regexy, and the behavior of the "%[" format of scanf isn't mandated in the standard.
You can check for a third name, but the code looks unintuitive - it doesn't look like you only want two names, it looks like you want three. scanf also gives you very little control over how you handle whitespace.
EDIT: I initially thought from your question that the names were contained in brackets (a la "[Bruce] [Wayne]") but it now appears that was merely your convention for denoting a placeholder.
Anyway, despite my intense dislike of scanf, it has its uses. The biggest killer (for me) is the inability to distinguish between line endings and simple space separation. To fix that, you can call fgets to read the data into a buffer, then call sscanf on the buffer. This gives you both a) safer reading (scanf messes with the ability of other more straightforward functions to read from a buffer) and b) the benefits of scanf formats.
If you have to use scanf, your format basically be this:
" %s %s %s"
With the third being undesirable. As #Constantinius's answer shows, you'd need to read data into three buffers, and check whether or not the third passed. However, if you're reading multiple consecutive lines of this data, then the first entry of the next line would satisfy the third slot, falsely giving you an error. I highly recommend using fgets and sscanf or ditching the sscanf for more precise manual parsing in this case.
Here's a link to the fgets man page if you missed the one I snuck in earlier. If you decide to ditch sscanf, here are some other functions to look into: strchr (or strspn, or strcspn) to find how long the name is, strcpy or memcpy (but please not strncpy, it's not what you think it is) to copy data into the buffers.

Using scanf to read in certain amount of characters in C?

I am having trouble accepting input from a text file. My program is supposed to read in a string specified by the user and the length of that string is determined at runtime. It works fine when the user is running the program (manually inputting the values) but when I run my teacher's text file, it runs into an infinite loop.
For this example, it fails when I am taking in 4 characters and his input in his file is "ABCDy". "ABCD" is what I am supposed to be reading in and 'y' is supposed to be used later to know that I should restart the game. Instead when I used scanf to read in "ABCD", it also reads in the 'y'. Is there a way to get around this using scanf, assuming I won't know how long the string should be until runtime?
Normally, you'd use something like "%4c" or "%4s" to read a maximum of 4 characters (the difference is that "%4c" reads the next 4 characters, regardless, while "%4s" skips leading whitespace and stops at a whitespace if there is one).
To specify the length at run-time, however, you have to get a bit trickier since you can't use a string literal with "4" embedded in it. One alternative is to use sprintf to create the string you'll pass to scanf:
char buffer[128];
sprintf(buffer, "%%%dc", max_length);
scanf(buffer, your_string);
I should probably add: with printf you can specify the width or precision of a field dynamically by putting an asterisk (*) in the format string, and passing a variable in the appropriate position to specify the width/precision:
int width = 10;
int precision = 7;
double value = 12.345678910;
printf("%*.*f", width, precision, value);
Given that printf and scanf format strings are quite similar, one might think the same would work with scanf. Unfortunately, this is not the case--with scanf an asterisk in the conversion specification indicates a value that should be scanned, but not converted. That is to say, something that must be present in the input, but its value won't be placed in any variable.
Try
scanf("%4s", str)
You can also use fread, where you can set a read limit:
char string[5]={0};
if( fread(string,(sizeof string)-1,1,stdin) )
printf("\nfull readed: %s",string);
else
puts("error");
You might consider simply looping over calls to getc().

Disadvantages of scanf

I want to know the disadvantages of scanf().
In many sites, I have read that using scanf might cause buffer overflows. What is the reason for this? Are there any other drawbacks with scanf?
Most of the answers so far seem to focus on the string buffer overflow issue. In reality, the format specifiers that can be used with scanf functions support explicit field width setting, which limit the maximum size of the input and prevent buffer overflow. This renders the popular accusations of string-buffer overflow dangers present in scanf virtually baseless. Claiming that scanf is somehow analogous to gets in the respect is completely incorrect. There's a major qualitative difference between scanf and gets: scanf does provide the user with string-buffer-overflow-preventing features, while gets doesn't.
One can argue that these scanf features are difficult to use, since the field width has to be embedded into format string (there's no way to pass it through a variadic argument, as it can be done in printf). That is actually true. scanf is indeed rather poorly designed in that regard. But nevertheless any claims that scanf is somehow hopelessly broken with regard to string-buffer-overflow safety are completely bogus and usually made by lazy programmers.
The real problem with scanf has a completely different nature, even though it is also about overflow. When scanf function is used for converting decimal representations of numbers into values of arithmetic types, it provides no protection from arithmetic overflow. If overflow happens, scanf produces undefined behavior. For this reason, the only proper way to perform the conversion in C standard library is functions from strto... family.
So, to summarize the above, the problem with scanf is that it is difficult (albeit possible) to use properly and safely with string buffers. And it is impossible to use safely for arithmetic input. The latter is the real problem. The former is just an inconvenience.
P.S. The above in intended to be about the entire family of scanf functions (including also fscanf and sscanf). With scanf specifically, the obvious issue is that the very idea of using a strictly-formatted function for reading potentially interactive input is rather questionable.
The problems with scanf are (at a minimum):
using %s to get a string from the user, which leads to the possibility that the string may be longer than your buffer, causing overflow.
the possibility of a failed scan leaving your file pointer in an indeterminate location.
I very much prefer using fgets to read whole lines in so that you can limit the amount of data read. If you've got a 1K buffer, and you read a line into it with fgets you can tell if the line was too long by the fact there's no terminating newline character (last line of a file without a newline notwithstanding).
Then you can complain to the user, or allocate more space for the rest of the line (continuously if necessary until you have enough space). In either case, there's no risk of buffer overflow.
Once you've read the line in, you know that you're positioned at the next line so there's no problem there. You can then sscanf your string to your heart's content without having to save and restore the file pointer for re-reading.
Here's a snippet of code which I frequently use to ensure no buffer overflow when asking the user for information.
It could be easily adjusted to use a file other than standard input if necessary and you could also have it allocate its own buffer (and keep increasing it until it's big enough) before giving that back to the caller (although the caller would then be responsible for freeing it, of course).
#include <stdio.h>
#include <string.h>
#define OK 0
#define NO_INPUT 1
#define TOO_LONG 2
#define SMALL_BUFF 3
static int getLine (char *prmpt, char *buff, size_t sz) {
int ch, extra;
// Size zero or one cannot store enough, so don't even
// try - we need space for at least newline and terminator.
if (sz < 2)
return SMALL_BUFF;
// Output prompt.
if (prmpt != NULL) {
printf ("%s", prmpt);
fflush (stdout);
}
// Get line with buffer overrun protection.
if (fgets (buff, sz, stdin) == NULL)
return NO_INPUT;
// Catch possibility of `\0` in the input stream.
size_t len = strlen(buff);
if (len < 1)
return NO_INPUT;
// If it was too long, there'll be no newline. In that case, we flush
// to end of line so that excess doesn't affect the next call.
if (buff[len - 1] != '\n') {
extra = 0;
while (((ch = getchar()) != '\n') && (ch != EOF))
extra = 1;
return (extra == 1) ? TOO_LONG : OK;
}
// Otherwise remove newline and give string back to caller.
buff[len - 1] = '\0';
return OK;
}
And, a test driver for it:
// Test program for getLine().
int main (void) {
int rc;
char buff[10];
rc = getLine ("Enter string> ", buff, sizeof(buff));
if (rc == NO_INPUT) {
// Extra NL since my system doesn't output that on EOF.
printf ("\nNo input\n");
return 1;
}
if (rc == TOO_LONG) {
printf ("Input too long [%s]\n", buff);
return 1;
}
printf ("OK [%s]\n", buff);
return 0;
}
Finally, a test run to show it in action:
$ printf "\0" | ./tstprg # Singular NUL in input stream.
Enter string>
No input
$ ./tstprg < /dev/null # EOF in input stream.
Enter string>
No input
$ ./tstprg # A one-character string.
Enter string> a
OK [a]
$ ./tstprg # Longer string but still able to fit.
Enter string> hello
OK [hello]
$ ./tstprg # Too long for buffer.
Enter string> hello there
Input too long [hello the]
$ ./tstprg # Test limit of buffer.
Enter string> 123456789
OK [123456789]
$ ./tstprg # Test just over limit.
Enter string> 1234567890
Input too long [123456789]
From the comp.lang.c FAQ: Why does everyone say not to use scanf? What should I use instead?
scanf has a number of problems—see questions 12.17, 12.18a, and 12.19. Also, its %s format has the same problem that gets() has (see question 12.23)—it’s hard to guarantee that the receiving buffer won’t overflow. [footnote]
More generally, scanf is designed for relatively structured, formatted input (its name is in fact derived from “scan formatted”). If you pay attention, it will tell you whether it succeeded or failed, but it can tell you only approximately where it failed, and not at all how or why. You have very little opportunity to do any error recovery.
Yet interactive user input is the least structured input there is. A well-designed user interface will allow for the possibility of the user typing just about anything—not just letters or punctuation when digits were expected, but also more or fewer characters than were expected, or no characters at all (i.e., just the RETURN key), or premature EOF, or anything. It’s nearly impossible to deal gracefully with all of these potential problems when using scanf; it’s far easier to read entire lines (with fgets or the like), then interpret them, either using sscanf or some other techniques. (Functions like strtol, strtok, and atoi are often useful; see also questions 12.16 and 13.6.) If you do use any scanf variant, be sure to check the return value to make sure that the expected number of items were found. Also, if you use %s, be sure to guard against buffer overflow.
Note, by the way, that criticisms of scanf are not necessarily indictments of fscanf and sscanf. scanf reads from stdin, which is usually an interactive keyboard and is therefore the least constrained, leading to the most problems. When a data file has a known format, on the other hand, it may be appropriate to read it with fscanf. It’s perfectly appropriate to parse strings with sscanf (as long as the return value is checked), because it’s so easy to regain control, restart the scan, discard the input if it didn’t match, etc.
Additional links:
longer explanation by Chris Torek
longer explanation by yours truly
References: K&R2 Sec. 7.4 p. 159
It is very hard to get scanf to do the thing you want. Sure, you can, but things like scanf("%s", buf); are as dangerous as gets(buf);, as everyone has said.
As an example, what paxdiablo is doing in his function to read can be done with something like:
scanf("%10[^\n]%*[^\n]", buf));
getchar();
The above will read a line, store the first 10 non-newline characters in buf, and then discard everything till (and including) a newline. So, paxdiablo's function could be written using scanf the following way:
#include <stdio.h>
enum read_status {
OK,
NO_INPUT,
TOO_LONG
};
static int get_line(const char *prompt, char *buf, size_t sz)
{
char fmt[40];
int i;
int nscanned;
printf("%s", prompt);
fflush(stdout);
sprintf(fmt, "%%%zu[^\n]%%*[^\n]%%n", sz-1);
/* read at most sz-1 characters on, discarding the rest */
i = scanf(fmt, buf, &nscanned);
if (i > 0) {
getchar();
if (nscanned >= sz) {
return TOO_LONG;
} else {
return OK;
}
} else {
return NO_INPUT;
}
}
int main(void)
{
char buf[10+1];
int rc;
while ((rc = get_line("Enter string> ", buf, sizeof buf)) != NO_INPUT) {
if (rc == TOO_LONG) {
printf("Input too long: ");
}
printf("->%s<-\n", buf);
}
return 0;
}
One of the other problems with scanf is its behavior in case of overflow. For example, when reading an int:
int i;
scanf("%d", &i);
the above cannot be used safely in case of an overflow. Even for the first case, reading a string is much more simpler to do with fgets rather than with scanf.
Yes, you are right. There is a major security flaw in scanf family(scanf,sscanf, fscanf..etc) esp when reading a string, because they don't take the length of the buffer (into which they are reading) into account.
Example:
char buf[3];
sscanf("abcdef","%s",buf);
clearly the the buffer buf can hold MAX 3 char. But the sscanf will try to put "abcdef" into it causing buffer overflow.
Problems I have with the *scanf() family:
Potential for buffer overflow with %s and %[ conversion specifiers. Yes, you can specify a maximum field width, but unlike with printf(), you can't make it an argument in the scanf() call; it must be hardcoded in the conversion specifier.
Potential for arithmetic overflow with %d, %i, etc.
Limited ability to detect and reject badly formed input. For example, "12w4" is not a valid integer, but scanf("%d", &value); will successfully convert and assign 12 to value, leaving the "w4" stuck in the input stream to foul up a future read. Ideally the entire input string should be rejected, but scanf() doesn't give you an easy mechanism to do that.
If you know your input is always going to be well-formed with fixed-length strings and numerical values that don't flirt with overflow, then scanf() is a great tool. If you're dealing with interactive input or input that isn't guaranteed to be well-formed, then use something else.
Many answers here discuss the potential overflow issues of using scanf("%s", buf), but the latest POSIX specification more-or-less resolves this issue by providing an m assignment-allocation character that can be used in format specifiers for c, s, and [ formats. This will allow scanf to allocate as much memory as necessary with malloc (so it must be freed later with free).
An example of its use:
char *buf;
scanf("%ms", &buf); // with 'm', scanf expects a pointer to pointer to char.
// use buf
free(buf);
See here. Disadvantages to this approach is that it is a relatively recent addition to the POSIX specification and it is not specified in the C specification at all, so it remains rather unportable for now.
The advantage of scanf is once you learn how use the tool, as you should always do in C, it has immensely useful usecases. You can learn how to use scanf and friends by reading and understanding the manual. If you can't get through that manual without serious comprehension issues, this would probably indicate that you don't know C very well.
scanf and friends suffered from unfortunate design choices that rendered it difficult (and occasionally impossible) to use correctly without reading the documentation, as other answers have shown. This occurs throughout C, unfortunately, so if I were to advise against using scanf then I would probably advise against using C.
One of the biggest disadvantages seems to be purely the reputation it's earned amongst the uninitiated; as with many useful features of C we should be well informed before we use it. The key is to realise that as with the rest of C, it seems succinct and idiomatic, but that can be subtly misleading. This is pervasive in C; it's easy for beginners to write code that they think makes sense and might even work for them initially, but doesn't make sense and can fail catastrophically.
For example, the uninitiated commonly expect that the %s delegate would cause a line to be read, and while that might seem intuitive it isn't necessarily true. It's more appropriate to describe the field read as a word. Reading the manual is strongly advised for every function.
What would any response to this question be without mentioning its lack of safety and risk of buffer overflows? As we've already covered, C isn't a safe language, and will allow us to cut corners, possibly to apply an optimisation at the expense of correctness or more likely because we're lazy programmers. Thus, when we know the system will never receive a string larger than a fixed number of bytes, we're given the ability to declare an array that size and forego bounds checking. I don't really see this as a down-fall; it's an option. Again, reading the manual is strongly advised and would reveal this option to us.
Lazy programmers aren't the only ones stung by scanf. It's not uncommon to see people trying to read float or double values using %d, for example. They're usually mistaken in believing that the implementation will perform some kind of conversion behind the scenes, which would make sense because similar conversions happen throughout the rest of the language, but that's not the case here. As I said earlier, scanf and friends (and indeed the rest of C) are deceptive; they seem succinct and idiomatic but they aren't.
Inexperienced programmers aren't forced to consider the success of the operation. Suppose the user enters something entirely non-numeric when we've told scanf to read and convert a sequence of decimal digits using %d. The only way we can intercept such erroneous data is to check the return value, and how often do we bother checking the return value?
Much like fgets, when scanf and friends fail to read what they're told to read, the stream will be left in an unusual state;
In the case of fgets, if there isn't sufficient space to store a complete line, then the remainder of the line left unread might be erroneously treated as though it's a new line when it isn't.
In the case of scanf and friends, a conversion failed as documented above, the erroneous data is left unread on the stream and might be erroneously treated as though it's part of a different field.
It's no easier to use scanf and friends than to use fgets. If we check for success by looking for a '\n' when we're using fgets or by inspecting the return value when we use scanf and friends, and we find that we've read an incomplete line using fgets or failed to read a field using scanf, then we're faced with the same reality: We're likely to discard input (usually up until and including the next newline)! Yuuuuuuck!
Unfortunately, scanf both simultaneously makes it hard (non-intuitive) and easy (fewest keystrokes) to discard input in this way. Faced with this reality of discarding user input, some have tried scanf("%*[^\n]%*c");, not realising that the %*[^\n] delegate will fail when it encounters nothing but a newline, and hence the newline will still be left on the stream.
A slight adaptation, by separating the two format delegates and we see some success here: scanf("%*[^\n]"); getchar();. Try doing that with so few keystrokes using some other tool ;)
There is one big problem with scanf-like functions - the lack of any type safety. That is, you can code this:
int i;
scanf("%10s", &i);
Hell, even this is "fine":
scanf("%10s", i);
It's worse than printf-like functions, because scanf expects a pointer, so crashes are more likely.
Sure, there are some format-specifier checkers out there, but, those are not perfect and well, they are not part of the language or the standard library.

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