# V2541. MISRA. Function should not be declared implicitly.

This diagnostic rule is based on the software development guidelines developed by MISRA (Motor Industry Software Reliability Association).

This rule only applies to C. In the C language, it is valid to use a function without its prior declaration. However, this usage is dangerous because it can cause incorrect operation of the program.

Let's look at a simple example:

    char *CharMallocFoo(int length)
    {
      return (char*)malloc(length);
    }

Since the header file <stdlib.h> is not included, the C compiler will conclude that the 'malloc' function's return type is 'int'. Incorrect interpretation of return value by the compiler can cause problems when executing a program, including its crash.

If the program is 64-bit, then most likely 32 high bits of the returned address will be lost. So the program will work incorrectly for some time. However when free memory in 4 low gigabytes of address space is run out or significantly fragmented, a buffer outside 4 low gigabytes will be allocated. As the address's high bits are lost, the consequences will be extremely unpleasant and unpredictable. You may find more details about this case in the article "A nice 64-bit error in C".

Correct code:

    #include <stdlib.h>
    ....
    char *CharMallocFoo(int length)
    {
      return (char*)malloc(length);
    }

This diagnostic is classified as:

MISRA-C-2012-17.3 MISRA-C-2023-17.3