Which functions from the standard library must (should) be avoided?

I've read on Stack Overflow that some C functions are "obsolete" or "should be avoided". Can you please give me some examples of this kind of function and the reason why?

What alternatives to those functions exist?

Can we use them safely - any good practices?

Deprecated Functions
Insecure
A perfect example of such a function is http://opengroup.org/onlinepubs/007908775/xsh/gets.html">gets()</a>;, because there is no way to tell it how big the destination buffer is. Consequently, any program that reads input using gets() has a http://en.wikipedia.org/wiki/Buffer_overflow">buffer overflow vulnerability. For similar reasons, one should use http://opengroup.org/onlinepubs/007908775/xsh/strncpy.html">strncpy()</a> in place of http://www.opengroup.org/onlinepubs/000095399/functions/strcpy.html">strcpy()</a> and http://www.opengroup.org/onlinepubs/009695399/functions/strncat.html">strncat()</a> in place of http://www.opengroup.org/onlinepubs/009695399/functions/strcat.html">strcat()</a>;.

Yet some more examples include the http://www.opengroup.org/onlinepubs/000095399/functions/tmpfile.html">tmpfile()</a> and http://www.opengroup.org/onlinepubs/000095399/functions/mktemp.html">mktemp()</a> function due to https://www.securecoding.cert.org/confluence/display/seccode/FIO43-C.+Do+not+create+temporary+files+in+shared+directories">potential security issues with overwriting temporary files and which are superseded by the more secure http://www.opengroup.org/onlinepubs/000095399/functions/mkstemp.html">mkstemp()</a> function.

Non-Reentrant
Other examples include http://www.opengroup.org/onlinepubs/009695399/functions/gethostbyaddr.html">gethostbyaddr()</a> and http://www.opengroup.org/onlinepubs/009695399/functions/gethostbyaddr.html">gethostbyname()</a> which are non-reentrant (and, therefore, not guaranteed to be threadsafe) and have been superseded by the reentrant http://www.opengroup.org/onlinepubs/009695399/functions/getaddrinfo.html">getaddrinfo()</a> and http://www.opengroup.org/onlinepubs/009695399/functions/getaddrinfo.html">freeaddrinfo()</a>;.

You may be noticing a pattern here... either lack of security (possibly by failing to include enough information in the signature to possibly implement it securely) or non-reentrance are common sources of deprecation.

Outdated, Non-Portable
Some other functions simply become deprecated because they duplicate functionality and are not as portable as other variants. For example, http://www.opengroup.org/onlinepubs/000095399/functions/bzero.html">bzero()</a> is deprecated in favor of http://opengroup.org/onlinepubs/007908799/xsh/memset.html">memset()</a>;.

Thread Safety and Reentrance
You asked, in your post, about thread safety and reentrance. There is a slight difference. A function is reentrant if it does not use any shared, mutable state. So, for example, if all the information it needs is passed into the function, and any buffers needed are also passed into the function (rather than shared by all calls to the function), then it is reentrant. That means that different threads, by using independent parameters, do not risk accidentally sharing state. Reentrancy is a stronger guarantee than thread safety. A function is thread safe if it can be used by multiple threads concurrently. A function is thread safe if:

• It is reentrant (i.e. it does not share any state between calls), or:
• It is non-reentrant, but it uses synchronization/locking as needed for shared state.

In general, in the http://www.opengroup.org/onlinepubs/009695399/">Single UNIX Specification and http://www.unix.org/version3/ieee_std.html">IEEE 1003.1 (i.e. "POSIX"), any function which is not guaranteed to be reentrant is not guaranteed to be thread safe. So, in other words, only functions which are guaranteed to be reentrant may be portably used in multithreaded applications (without external locking). That does not mean, however, that implementations of these standards cannot choose to make a non-reentrant function threadsafe. For example, Linux frequently adds synchronization to non-reentrant functions in order to add a guarantee (beyond that of the Single UNIX Specification) of threadsafety.

Strings (and Memory Buffers, in General)
You also asked if there is some fundamental flaw with strings/arrays. Some might argue that this is the case, but I would argue that no, there is no fundamental flaw in the language. C and C++ require you to pass the length/capacity of an array separately (it is not a ".length" property as in some other languages). This is not a flaw, per-se. Any C and C++ developer can write correct code simply by passing the length as a parameter where needed. The problem is that several APIs that required this information failed to specify it as a parameter. Or assumed that some MAX_BUFFER_SIZE constant would be used. Such APIs have now been deprecated and replaced by alternative APIs that allow the array/buffer/string sizes to be specified.

Scanf (In Answer to Your Last Question)
Personally, I use the C++ iostreams library (std::cin, std::cout, the << and >> operators, std::getline, std::istringstream, std::ostringstream, etc.), so I do not typically deal with that. If I were forced to use pure C, though, I would personally just use http://www.opengroup.org/onlinepubs/009695399/functions/fgetc.html">fgetc()</a> or http://www.opengroup.org/onlinepubs/000095399/functions/getchar.html">getchar()</a> in combination with http://opengroup.org/onlinepubs/007908775/xsh/strtol.html">strtol()</a>;, http://opengroup.org/onlinepubs/007908775/xsh/strtoul.html">strtoul()</a>;, etc. and parse things manually, since I'm not a huge fan of varargs or format strings. That said, to the best of my knowledge, there is no problem with http://www.opengroup.org/onlinepubs/000095399/functions/fscanf.html">[f]scanf()</a>;, http://www.opengroup.org/onlinepubs/000095399/functions/printf.html">[f]printf()</a>;, etc. so long as you craft the format strings yourself, you never pass arbitrary format strings or allow user input to be used as format strings, and you use the formatting macros defined in http://www.opengroup.org/onlinepubs/000095399/basedefs/inttypes.h.html">&lt;inttypes.h&gt;</a> where appropriate. (Note, http://www.opengroup.org/onlinepubs/000095399/functions/printf.html">snprintf()</a> should be used in place of http://www.opengroup.org/onlinepubs/000095399/functions/printf.html">sprintf()</a>;, but that has to do with failing to specify the size of the destination buffer and not the use of format strings). I should also point out that, in C++, http://www.boost.org/doc/libs/1_42_0/libs/format/doc/format.html">boost::format</a> provides printf-like formatting without varargs.

Once again people are repeating, mantra-like, the ludicrous assertion that the "n" version of str functions are safe versions.

If that was what they were intended for then they would always null terminate the strings.

The "n" versions of the functions were written for use with fixed length fields (such as directory entries in early file systems) where the nul terminator is only required if the string does not fill the field. This is also the reason why the functions have strange side effects that are pointlessly inefficient if just used as replacements - take strncpy() for example:

> If the array pointed to by s2 is a > string that is shorter than n bytes, > null bytes are appended to the copy in > the array pointed to by s1, until n > bytes in all are written.

As buffers allocated to handle filenames are typically 4kbytes this can lead to a massive deterioration in performance.

If you want "supposedly" safe versions then obtain - or write your own - strl routines (strlcpy, strlcat etc) which always nul terminate the strings and don't have side effects. Please note though that these aren't really safe as they can silently truncate the string - this is rarely the best course of action in any real-world program. There are occasions where this is OK but there are also many circumstances where it could lead to catastrophic results (e.g. printing out medical prescriptions).

Several answers here suggest using strncat() over strcat(); I'd suggest that strncat() (and strncpy()) should also be avoided. It has problems that make it difficult to use correctly and lead to bugs:

• the length parameter to strncat() is related to (but not quite exactly - see the 3rd point) the maximum number of characters that can be copied to the destination rather than the size of the destination buffer. This makes strncat() more difficult to use than it should be, particularly if multiple items will be concatenated to the destination.
• it can be difficult to determine if the result was truncated (which may or may not be important)
• it's easy to have an off-by-one error. As the C99 standard notes, "Thus, the maximum number of characters that can end up in the array pointed to by s1 is strlen(s1)+n+1" for a call that looks like strncat( s1, s2, n)

strncpy() also has an issue that can result in bugs you try to use it in an intuitive way - it doesn't guarantee that the destination is null terminated. To ensure that you have to make sure you specifically handle that corner case by dropping a '\0' in the buffer's last location yourself (at least in certain situations).

I'd suggest using something like OpenBSD's strlcat() and strlcpy() (though I know that some people dislike those functions; I believe they're far easier to use safely than strncat()/strncpy()).

Here's a little of what Todd Miller and Theo de Raadt had to say about problems with strncat() and strncpy():

> There are several problems encountered when strncpy() and strncat() are used as safe versions of strcpy() and strcat(). Both functions deal with NUL-termination and the length parameter in different and non-intuitive ways that confuse even experienced programmers. They also provide no easy way to detect when truncation occurs. ... Of all these issues, the confusion caused by the length parameters and the related issue of NUL-termination are most important. When we audited the OpenBSD source tree for potential security holes we found rampant misuse of strncpy() and strncat(). While not all of these resulted in exploitable security holes, they made it clear that the rules for using strncpy() and strncat() in safe string operations are widely misunderstood.

OpenBSD's security audit found that bugs with these functions were "rampant". Unlike gets(), these functions can be used safely, but in practice there are a lot of problems because the interface is confusing, unintuitive and difficult to use correctly. I know that Microsoft has also done analysis (though I don't know how much of their data they may have published), and as a result have banned (or at least very strongly discouraged - the 'ban' might not be absolute) the use of strncat() and strncpy() (among other functions).

Some links with more information:

• http://www.usenix.org/events/usenix99/full_papers/millert/millert_html/
• http://en.wikipedia.org/wiki/Off-by-one_error#Security_implications
• http://blogs.msdn.com/michael_howard/archive/2004/10/29/249713.aspx
• http://blogs.msdn.com/michael_howard/archive/2004/11/02/251296.aspx
• http://blogs.msdn.com/michael_howard/archive/2004/12/10/279639.aspx
• http://blogs.msdn.com/michael_howard/archive/2006/10/30/something-else-to-look-out-for-when-reviewing-code.aspx

Standard library functions that should never be used:

setjmp.h

• setjmp(). Together with longjmp(), these functions are widely recogniced as incredibly dangerous to use: they lead to spaghetti programming, they come with numerous forms of undefined behavior, they can cause unintended side-effects in the program environment, such as affecting values stored on the stack. References: MISRA-C:2012 rule 21.4, CERT C MSC22-C.
• longjmp(). See setjmp().

stdio.h

• gets(). The function has been removed from the C language (as per C11), as it was unsafe as per design. The function was already flagged as obsolete in C99. Use fgets() instead. References: ISO 9899:2011 K.3.5.4.1, also see note 404.

stdlib.h

• atoi() family of functions. These have no error handling but invoke undefined behavior whenever errors occur. Completely superfluous functions that can be replaced with the strtol() family of functions. References: MISRA-C:2012 rule 21.7.

string.h

• strncat(). Has an awkward interface that are often misused. It is mostly a superfluous function. Also see remarks for strncpy().
• strncpy(). The intention of this function was never to be a safer version of strcpy(). Its sole purpose was always to handle an ancient string format on Unix systems, and that it got included in the standard library is a known mistake. This function is dangerous because it may leave the string without null termination and programmers are known to often use it incorrectly. References: Why are strlcpy and strlcat considered insecure?, with a more detailed explanation here: Is strcpy dangerous and what should be used instead?.

Standard library functions that should be used with caution:

assert.h

• assert(). Comes with overhead and should generally not be used in production code. It is better to use an application-specific error handler which displays errors but does not necessarily close down the whole program.

signal.h

• signal(). References: MISRA-C:2012 rule 21.5, CERT C SIG32-C.

stdarg.h

• va_arg() family of functions. The presence of variable-length functions in a C program is almost always an indication of poor program design. Should be avoided unless you have very specific requirements.

stdio.h
Generally, this whole library is not recommended for production code, as it comes with numerous cases of poorly-defined behavior and poor type safety.

• fflush(). Perfectly fine to use for output streams. Invokes undefined behavior if used for input streams.
• gets_s(). Safe version of gets() included in C11 bounds-checking interface. It is preferred to use fgets() instead, as per C standard recommendation. References: ISO 9899:2011 K.3.5.4.1.
• printf() family of functions. Resource heavy functions that come with lots of undefined behavior and poor type safety. sprintf() also has vulnerabilities. These functions should be avoided in production code. References: MISRA-C:2012 rule 21.6.
• scanf() family of functions. See remarks about printf(). Also, - scanf() is vulnerable to buffer overruns if not used correctly. fgets() is preferred to use when possible. References: CERT C INT05-C, MISRA-C:2012 rule 21.6.
• tmpfile() family of functions. Comes with various vulnerability issues. References: CERT C FIO21-C.

stdlib.h

• malloc() family of functions. Perfectly fine to use in hosted systems, though be aware of well-known issues in C90 and therefore don't cast the result. The malloc() family of functions should never be used in freestanding applications. References: MISRA-C:2012 rule 21.3.

  Also note that realloc() is dangerous in case you overwrite the old pointer with the result of realloc(). In case the function fails, you create a leak.

• system(). Comes with lots of overhead and although portable, it is often better to use system-specific API functions instead. Comes with various poorly-defined behavior. References: CERT C ENV33-C.

string.h

• strcat(). See remarks for strcpy().
• strcpy(). Perfectly fine to use, unless the size of the data to be copied is unknown or larger than the destination buffer. If no check of the incoming data size is done, there may be buffer overruns. Which is no fault of strcpy() itself, but of the calling application - that strcpy() is unsafe is mostly a myth created by Microsoft.
• strtok(). Alters the caller string and uses internal state variables, which could make it unsafe in a multi-threaded environment.

Some people would claim that strcpy and strcat should be avoided, in favor of strncpy and strncat. This is somewhat subjective, in my opinion.

They should definitely be avoided when dealing with user input - no doubt here.

In code "far" from the user, when you just know the buffers are long enough, strcpy and strcat may be a bit more efficient because computing the n to pass to their cousins may be superfluous.

Avoid

• strtok for multithreaded programs as its not thread-safe.
• gets as it could cause buffer overflow

It is probably worth adding again that strncpy() is not the general-purpose replacement for strcpy() that it's name might suggest. It is designed for fixed-length fields that don't need a nul-terminator (it was originally designed for use with UNIX directory entries, but can be useful for things like encryption key fields).

It is easy, however, to use strncat() as a replacement for strcpy():

if (dest_size > 0)
{
    dest[0] = '\0';
    strncat(dest, source, dest_size - 1);
}

(The if test can obviously be dropped in the common case, where you know that dest_size is definitely nonzero).

Also check out Microsoft's list of banned APIs. These are APIs (including many already listed here) that are banned from Microsoft code because they are often misused and lead to security problems.

You may not agree with all of them, but they are all worth considering. They add an API to the list when its misuse has led to a number of security bugs.

It is very hard to use scanf safely. Good use of scanf can avoid buffer overflows, but you are still vulnerable to undefined behavior when reading numbers that don't fit in the requested type. In most cases, fgets followed by self-parsing (using sscanf, strchr, etc.) is a better option.

But I wouldn't say "avoid scanf all the time". scanf has its uses. As an example, let's say you want to read user input in a char array that's 10 bytes long. You want to remove the trailing newline, if any. If the user enters more than 9 characters before a newline, you want to store the first 9 characters in the buffer and discard everything until the next newline. You can do:

char buf[10];
scanf("%9[^\n]%*[^\n]", buf));
getchar();

Once you get used to this idiom, it's shorter and in some ways cleaner than:

char buf[10];
if (fgets(buf, sizeof buf, stdin) != NULL) {
    char *nl;
    if ((nl = strrchr(buf, '\n')) == NULL) {
        int c;
        while ((c = getchar()) != EOF && c != '\n') {
            ;
        }
    } else {
        *nl = 0;
    }
}

Almost any function that deals with NUL terminated strings is potentially unsafe. If you are receiving data from the outside world and manipulating it via the str*() functions then you set yourself up for catastrophe

Don't forget about sprintf - it is the cause of many problems. This is true because the alternative, snprintf has sometimes different implementations which can make you code unportable.

1. linux: http://linux.die.net/man/3/snprintf

2. windows: http://msdn.microsoft.com/en-us/library/2ts7cx93%28VS.71%29.aspx

In case 1 (linux) the return value is the amount of data needed to store the entire buffer (if it is smaller than the size of the given buffer then the output was truncated)

In case 2 (windows) the return value is a negative number in case the output is truncated.

Generally you should avoid functions that are not:

1. buffer overflow safe (a lot of functions are already mentioned in here)

2. thread safe/not reentrant (strtok for example)

In the manual of each functions you should search for keywords like: safe, sync, async, thread, buffer, bugs

In all the string-copy/move scenarios - strcat(), strncat(), strcpy(), strncpy(), etc. - things go much better (safer) if a couple simple heuristics are enforced:

   1. Always NUL-fill your buffer(s) before adding data.
   2. Declare character-buffers as [SIZE+1], with a macro-constant.

For example, given:

#define   BUFSIZE   10
char      Buffer[BUFSIZE+1] = { 0x00 };  /* The compiler NUL-fills the rest */

we can use code like:

memset(Buffer,0x00,sizeof(Buffer));
strncpy(Buffer,BUFSIZE,"12345678901234567890");

relatively safely. The memset() should appear before the strncpy(), even though we initialized Buffer at compile-time, because we don't know what garbage other code placed into it before our function was called. The strncpy() will truncate the copied data to "1234567890", and will not NUL-terminate it. However, since we have already NUL-filled the entire buffer - sizeof(Buffer), rather than BUFSIZE - there is guaranteed to be a final "out-of-scope" terminating NUL anyway, as long as we constrain our writes using the BUFSIZE constant, instead of sizeof(Buffer).

Buffer and BUFSIZE likewise work fine for snprintf():

memset(Buffer,0x00,sizeof(Buffer));
if(snprintf(Buffer,BUFIZE,"Data: %s","Too much data") > BUFSIZE) {
    /* Do some error-handling */
}   /* If using MFC, you need if(... < 0), instead */

Even though snprintf() specifically writes only BUFIZE-1 characters, followed by NUL, this works safely. So we "waste" an extraneous NUL byte at the end of Buffer...we prevent both buffer-overflow and unterminated string conditions, for a pretty small memory-cost.

My call on strcat() and strncat() is more hard-line: don't use them. It is difficult to use strcat() safely, and the API for strncat() is so counter-intuitive that the effort needed to use it properly negates any benefit. I propose the following drop-in:

#define strncat(target,source,bufsize) snprintf(target,source,"%s%s",target,source)

It is tempting to create a strcat() drop-in, but not a good idea:

#define strcat(target,source) snprintf(target,sizeof(target),"%s%s",target,source)

because target may be a pointer (thus sizeof() does not return the information we need). I don't have a good "universal" solution to instances of strcat() in your code.

A problem I frequently encounter from "strFunc()-aware" programmers is an attempt to protect against buffer-overflows by using strlen(). This is fine if the contents are guaranteed to be NUL-terminated. Otherwise, strlen() itself can cause a buffer-overrun error (usually leading to a segmentation violation or other core-dump situation), before you ever reach the "problematic" code you are trying to protect.

atoi is not thread safe. I use strtol instead, per recommendation from the man page.