Harmful C Functions and their replacements

There are quite a lot of functions in the printf and scanff amily. It might not be very clear when to use which of them.

Here is an explanation of the functions in the Standard C Library.

Most prefixed define where the function reads (scanf) or wirtes (printf) data from or to:

no prefix: use STDIN or STDOUT
f: use a FILE stream
s: use a char buffer (string)
sn: same as s but checks for buffer size (only printf)
v: takes a va_list instead ... (ellipsis), can be combined with the other prefixes

The C11 standard introduced addtional functions with the _s postfix which do some checks on the data.

So we get following printf-like functions:

The functions with _s postfix should be preferred if available (C11).

The sprintf and vsprintf functions should not be used since the can result in stack overflow. They are also suffer from string vulnerability.

Use snpritf or vsnprintf instead. If possible with _s postfix.

And force null-termination manually ¹.

And we get following scanf-like functions:

The functions with _s postfix should be preferred if available (C11).

And force null-termination manually ¹.

The OpenBSD kernel library defines some additional functions that are safer than their counterparts in the standard library.

	C standard library	OpenBSD kernellibrary
Copying string	`strcpy`	`strlcpy`
Applying (concatenating) string	`strcat`	`strlcat`

These C functions suffer buffer overflow problems:

Original	Replacement
`gets()`	`fgets()`
`cuserid()`	`getlogin()` or `getpwuid()`
`scanf()` family	See ² and ³, use functions with `_s`postfix (C11)
`sprintf()`	`snprintf()`, use functions with `_s`postfix (C11)
`vsprintf()`	`vsnprintf()`, use functions with `_s`postfix (C11)
`strcat()`	`strncat()`
`strcpy()`	`strncpy()`
`streadd()`, `strtrns()`, `strecpy()` …	Check lengths of buffers or use standard library functions
`getwd()`	`getcwd()`