Troubleshooting Common Errors on the UMD Submit Server

Submitting your C code to the Umd Submit Server and encountering unexpected errors can be a frustrating experience. This guide aims to clarify some common issues you might face and provides insights into resolving them, ensuring smoother submissions and accurate results.

1. Incorrect Results in Grace, But Not in Submit Server

One perplexing situation is when your code runs flawlessly in your local Grace environment but produces incorrect results upon submission. This discrepancy often stems from subtle differences in environment behavior, particularly concerning uninitialized variables.

• Uninitialized Variables: Grace may initialize variables to 0 by default, masking potential issues in your code. The submit server, however, may not exhibit this behavior. Ensure all variables are explicitly initialized to avoid unpredictable outcomes.

• Missing Null Character in Strings: In C, strings must be null-terminated (). If you forget to add this crucial character at the end of your strings, especially when manually creating them, the submit server might misinterpret the string’s length, leading to errors.

• Insufficient Array Size: Declaring arrays that are too small to hold the intended data can cause buffer overflows. While Grace might tolerate this, the submit server is more likely to expose these memory violations, resulting in incorrect behavior or crashes.

• Array Bounds Errors: Iterating beyond the allocated memory of an array, even by a single element (off-by-one errors), can lead to memory corruption. The submit server’s environment might be more sensitive to these out-of-bounds accesses.

• Incorrect Pointer Type in Function Arguments: Passing a pointer to a single character when a function expects a pointer to an array of characters (string) is a type mismatch error. This can lead to the function reading memory incorrectly, causing unexpected results on the submit server.

2. Inconsistent Background Retests in Submit Server Error

The submit server periodically re-runs submissions in the background when system load is low. If subsequent retests yield different results from the initial submission, it indicates instability in your code. The “inconsistent background retests” column in your submission results will display a count of these discrepancies. This usually points to non-deterministic behavior in your code, often due to:

• Uninitialized Variables (again): As mentioned earlier, reliance on potentially uninitialized variables can introduce variability in program execution.
• Race Conditions (in multithreaded code, if applicable): If your code uses threads (which is less common in introductory C assignments but possible), race conditions can lead to unpredictable outcomes depending on thread scheduling.

3. Segmentation Fault (“expected … but output ended”)

A segmentation fault (segfault) occurs when your program attempts to access memory that it is not allowed to access. On the submit server, a segfault is often indicated by the message “expected … but output ended.” This strongly suggests a memory-related error in your code:

• Null Pointer Dereference: Trying to access memory through a pointer that is NULL (points to nothing) will cause a segfault.
• Accessing Freed Memory: Using memory that has already been deallocated using free() is another common cause.
• Stack Overflow: Excessive recursion or allocation of very large arrays on the stack can exhaust the stack space, leading to a segfault.
• Writing to Read-Only Memory: Attempting to modify memory that is designated as read-only will also trigger a segfault.

4. Linker Errors (“collect2: error: ld returned 1 exit status”)

If you encounter an error message like “collect2: error: ld returned 1 exit status,” it signifies a problem during the linking stage of compilation. The ld in the message refers to the GNU linker. This error usually arises from issues related to functions:

• Missing Required Function: If your code calls a function that is not defined or implemented, the linker will fail to find its definition, resulting in this error. Double-check that all necessary functions are present and correctly spelled.

• Typo in Function Name: A simple typo in a function name (e.g., “prontf” instead of “printf”) will also cause a linker error, as the linker won’t be able to locate the misspelled function.

5. Code Not Compiling in Submit Server

If your code compiles locally but fails to compile on the submit server, it might be due to unimplemented functions.

• Unimplemented Functions: If you have declared function prototypes for functions you haven’t yet implemented, the compiler might still compile your code locally. However, the submit server’s compilation environment might be stricter. To resolve this, provide empty function bodies (with a dummy return value if the function is not void) for functions that are not yet complete. This will allow your code to compile on the submit server, even with incomplete functionality.

6. Returning Pointer to Local Variable

Returning a pointer to a local variable from a function is a critical error. Local variables exist only within the scope of the function they are declared in. Once the function returns, the local variable’s memory is no longer guaranteed to be valid. The pointer you return will then point to invalid memory (“dangling pointer”), leading to unpredictable behavior when the caller tries to dereference it.

7. Uninitialized Pointer Variables

Similar to regular variables, pointer variables must also be initialized before use. An uninitialized pointer contains a garbage address, and dereferencing it can lead to segmentation faults or other memory corruption issues. Always initialize pointers to a valid memory location (allocated memory using malloc, address of another variable, or NULL if it’s not currently pointing to anything valid).

8. “ISO C90 forbids mixed declarations and code [-Wpedantic]”

The error message “ISO C90 forbids mixed declarations and code [-Wpedantic]” indicates that your code violates the older ISO C90 standard regarding variable declarations. In C90, variables must be declared at the beginning of a block (e.g., at the start of a function or within curly braces {}). Mixing declarations and code within a block is not allowed in C90.

 int main() {
     printf("heren");
     int x; // Declaration after code - Error in C90
     return 0;
 }

Correction: To fix this, move the variable declaration to the beginning of the block:

 int main() {
     int x; // Declaration at the beginning - Correct in C90
     printf("heren");
     return 0;
 }

9. Stack Smashing

A “stack smashing” error often points to a buffer overflow on the stack. This occurs when you write beyond the bounds of a local array or buffer, overwriting adjacent data on the stack. Stack smashing is a security vulnerability and can lead to program crashes and unpredictable behavior. It’s usually caused by:

• Array Out-of-Bounds Write: Writing past the end of an array declared on the stack.
• strcpy or similar unsafe string functions: Using functions like strcpy without proper bounds checking can lead to buffer overflows if the source string is longer than the destination buffer. Use safer alternatives like strncpy or snprintf.

10. Incorrect Use of sizeof()

The sizeof() operator in C returns the size in bytes of a variable or data type. A common mistake is to misuse sizeof() when dealing with array parameters in functions.

• sizeof() in Function Parameter: When you pass an array as a parameter to a function, it decays into a pointer to the first element of the array. Inside the function, sizeof(array_parameter) will return the size of the pointer itself (typically 4 or 8 bytes), not the size of the original array. You cannot determine the size of an array passed as a parameter using sizeof() within the function. You need to pass the array size as a separate argument if the function needs to know it.

For more comprehensive debugging techniques and strategies in C, refer to the detailed guide available at Debugging in C. This resource offers valuable insights and tools to help you effectively debug your C code and resolve issues encountered on the UMD submit server.