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Technical Information Database TI500C.txt Null Pointer Assignment Errors Explained Category :General Platform :All Product :Borland C++ All Description: 1. What is a Null Pointer Assignment error? The Null Pointer Assignment error is generated in programs that corrupt the bottom of the data segment in such a fashion as to indicate a high probability that an improperly-initialized pointer has been used. The error can only be generated in the small and medium memory models. 2. What causes a Null Pointer Assignment error? Borland places four zero bytes at the bottom of the data segment, followed by the Borland copyright notice. In the small and medium memory models, a null pointer points to DS:0000. Thus assigning a value to the memory referenced by this pointer will overwrite the first zero byte in the data segment. At program termination, the four zeros and the copyright banner are checked. If either has been modified, then the Null Pointer Assignment error is generated. Note that the pointer may not truly be null, but may be a wild pointer that references these key areas in the data segment. 3. How can I debug a Null Pointer Assignment error? In either the Integrated Development Environment or in Turbo Debugger, set two watches on these key memory locations. These watches, and what they should display in the watch window, are: (char*)4 char *ds:0004 "Borland C++ - Copyright ..." *(char*)0,4m 00 00 00 00 Step through your program and monitor theses values in the watch window. At the point where one of them changes, you have just executed a statement that uses a pointer that has not been properly initialized. The most common cause of this error is probably declaring a pointer and then using it before allocating memory for it. For example, compile either of the following programs in the small memory model and execute it: Program 1: #include #include #include int main(void) { char *ptr, *banner; banner = (char *) MK_FP(_DS, 4); printf("banner: %s\n", banner); strcpy(ptr, "Where will this text be copied?!?"); printf("&ptr = %Fp\n",(void far*) &ptr[0]); printf("banner: %s\n", banner); return 0; } Program 2: // The following code illustrates the null pointer // assignment error. Code using the "bad" pointer will // cause a null pointer assignment; code using the "good" // pointer will not. #include #include int main() { char good[81]; char *bad; strcpy (good, "This does not create a" " null pointer assignment.\n"); strcpy (bad, "This creates a null pointer assignment.\n"); printf("%s", good); printf("%s", bad); return 0; } One useful debugging technique is to turn on all warning compiler messages. If the above programs are compiled with warnings turned off, no warning messages will be generated. However, if all warnings are turned on, both the strcpy() and printf() calls will generate warnings. Be particularly suspicious of any warnings that a variable might be used before being initialized, or of a suspicious pointer assignment. With the command-line compiler, you may wish to use the -wdef switch to enable the "Possible use of identifier before definition" warning. In the Borland or Turbo C++ IDE, select "Options / Compiler / Messages / Display and set Display Warnings to "All". 4. Why is a Null Pointer Assignment error not generated in all models? In the compact, large and huge memory models, far pointers are used for data. Therefore, a null pointer will reference 0000:0000, or the base of system memory, and using it will not cause a corruption of the key values at the base of the data segment. Modifying the base of system memory usually causes a system crash, however. Although it would be possible that a wild pointer would overwrite the key values, it would not indicate a null pointer. In the tiny memory model, DS = CS = SS. Therefore, using a null pointer will overwrite the beginning of the code segment. 5. Can anything else generate a Null Pointer Assignment error? Using a wild pointer that happens to reference the base area of the data segment--thus causing a corruption of the zeros or the copyright banner--will generate this error. Since data corruption or stack corruption could cause an otherwise-valid pointer to be corrupted and point to the base of the data segment, any memory corruption could result in this error being generated. If the pointer used in the program statement which corrupts the key values appears to have been properly initialized, place a watch on that pointer. Step through your program again and watch for its value (address) to change. DISCLAIMER: You have the right to use this technical information subject to the terms of the No-Nonsense License Statement that you received with the Borland product to which this information pertains. Reference: 7/2/98 10:39:11 AM
Article originally contributed by Borland Staff
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