# V1031. Function is not declared. The passing of data to or from this function may be affected.

The analyzer has detected a function used in a file without being declared first. It is a legitimate practice in the C language, but it is potentially dangerous as it may lead to incorrect program behavior.

Consider the following simple example:

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

Since the header file <stdlib.h> has not been included, the C compiler assumes that the 'malloc' function will return a value of type 'int'. Incorrect interpretation of the return value may cause run-time errors, including a crash.

If your program is 64-bit, it is very likely that the 32 most significant bits of the return address will be lost. This means that the program will execute correctly for some time, but when the 4 less significant gigabytes of memory are used up or become heavily fragmented, the system will allocate a buffer beyond those 4 gigabytes. Since the most significant bits of the pointer have been lost, the implications will be quite unpleasant and unpredictable. This issue is discussed in detail in the article "A nice 64-bit error in C".

Fixed code:

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

This diagnostic is classified as:

CERT-DCL31-C