Choosing the Right Functions in C Programming: Strcpy vs. Strncpy

C continues to be one of the most relevant programming languages despite the presence of more modern options. One of the reasons for this sustained popularity is C's functions. From 'fgets' to 'toupper,' C provides a variety of ways for string manipulation and other common operations, which make coding easier.

However, with the threat of more advanced and relentless cyber-attacks, developers need to be careful with these C functions. They can create vulnerabilities in apps or systems that cybercriminals are itching to find and exploit. These vulnerabilities can be difficult to spot (for the owners) and rectify, especially if they are in disparate devices and shadow IT. 

Two important and commonly used C functions that may contribute to the emergence of security vulnerabilities are strcpy and strncpy. Having a thorough understanding of these functions will allow organizations to maximize their application while reducing the risks.

Understanding Strcpy and Strncpy

Before going into the strcpy vs. strncpy comparison, here's an overview of what each function does.

• strcpy - An abbreviation for "string copy," this standard library function copies strings to a new memory location. Its main purpose is to replicate into a destination buffer the contents of a source string while ensuring that all of the strings concerned are null-terminated.
• strncpy - This is strcpy with an "n," which denotes a maximum number of characters that can be copied from the source string. Strncpy prevents the copying of characters that exceed the capacity of the destination buffer. The drawback with it is that it may not append a null terminator or an indicator for the end of a string.

Developers employ strcpy in a number of situations. One of which is configuration parsing, wherein key-value pairs may need to be copied to separate buffers. The strcpy function makes it easy to replicate the values to the intended buffers for further processing.

Another use case of this C function is the establishment of file paths. It can replicate the directory path, file name, and extension into the designated buffer. Developers usually do this to build dynamic file paths according to user input.

Additionally, strcpy is useful in command-line argument processing. This function enables the copying of arguments into different buffers set for further manipulation or validation. Command-line processing is usually undertaken to receive instructions, inputs, or configuration details.

Strcpy also facilitates string manipulation in concert with other manipulation functions like strcat. In particular, it can modify a string by supplanting some parts of it or by introducing new content.

Moreover, developers can employ strcpy to store string constants that, ironically, may be modified in the future. This function can be used to copy the constant strings into different buffers, which simplifies the process of updating the constants.

When it comes to strncpy, the purpose and use cases are largely the same as those in strcpy. What makes it different is that a limit is imposed on the number of characters that can be copied from the source string.

Why the 'n' Matters

The restriction over the number of characters that can be copied is a security feature. Strncpy was created to address the inability of strcpy to set a limit on the character length to copy. The lack of this limitation may enable a buffer overflow, a vulnerability wherein a program writes data to a buffer beyond the allocated memory. This anomaly can result in the overwriting of adjacent memory locations, resulting in the freezing or malfunctioning of an application and the possibility of remote code execution.

Buffer overflow attacks are common cyber threats that can affect any organization that uses software, especially those written in C. The buffer overflow vulnerability can be found in IoT devices, desktop apps, network services, and web servers. Many apps and the software used in embedded or IoT devices are haphazardly built, failing to take good memory management into account. They may provide opportunities for threat actors to exploit buffer overflows.

The use of strncpy is one of the best ways to address the buffer overflow vulnerability. It ensures that the destination string does not go beyond what is allocated and overwrites adjacent memory. Additionally, this is the function to use in cases when an application is required to have a predetermined number of characters to copy into the destination string. It is also used in string truncation when dealing with display and storage constraints to copy only a part of the source string that is longer than the specified limit.

When Strcpy Is More Appropriate To Use

If strncpy is the more secure function, why is it that it has not fully supplanted strcpy? The answer is the former's limitation when it comes to the destination string's null-termination. When using strncpy, the destination string may not be null-terminated if the specified limit is shorter than the source string. This can result in crashing or the incorrect/unexpected execution of a program. As such, strcpy is still being used, especially when the following conditions exist:

• Simplicity and performance are prioritized. There are times when the security implications of using strcpy are minimal, and using strncpy can be considered unnecessary with its added steps, like having additional checks and the imposition of null terminators to the destination string. Strcpy is more straightforward and easier to use. Hence it is more efficient when there are no potential severe security problems to take into account.
• The string length is known. If a process only requires the copying of a string with a known character limit, the simpler strcpy function is more suitable. There is no risk of buffer overflow if it is clear that the source string is going to fit into the destination string.
• There is a null-termination guarantee. It also makes more sense to stick with strcpy if the null-terminator requirement for security is already addressed. If the source string is null-terminated and there is an assurance that the destination string will also be null-terminated, it would be unwise to ditch the simpler function.
• Convenience for all developers matters. When working on a collaborative development project, it can be preferable to use the base string copy function and let the participating developers address the need for null terminators. This can make code readability better and help simplify workflows and code maintenance.

In other words, it would be better to use strncpy if the conditions above are not present. There is no doubt that security is crucial, but strncpy is not the only way to achieve it. Developers can look at other factors, like the reconciliation of string lengths.

Also, there are other secure functions for certain cases, like strlcpy and strlcat. Strlcpy was specifically created to copy strings while preventing buffer overflows. Strlcat, on the other hand, is intended for string concatenation without buffer overflow risks.

No One-Size-Fits-All Option

The choice between strcpy and strncpy should be based on the need and conditions set for a project. One can only be better or more appropriate to use than the other based on the conditions they are used in. A good guide in deciding which function to use is to evaluate the risks of a buffer overflow. If this vulnerability is avoidable or if there are ways to address it without specifying character limits, the base string copy function is the better option. Otherwise, the more secure strncpy or strlcpy and strlcat (in some cases) should be considered.