Скачать или смотреть Resolving load of null pointer of type 'char' Runtime Error in C: A Guide to Proper Pointer Handling

Struggling with a null pointer runtime error in your C code? This guide breaks down common pitfalls in pointer manipulation and provides solutions to ensure your program runs smoothly.
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This video is based on the question https://stackoverflow.com/q/77543016/ asked by the user 'Debuholden' ( https://stackoverflow.com/u/22881364/ ) and on the answer https://stackoverflow.com/a/77543126/ provided by the user 'Vlad from Moscow' ( https://stackoverflow.com/u/2877241/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving load of null pointer of type 'char' Runtime Error in C: A Guide to Proper Pointer Handling

When working with C, handling strings and memory can often lead to frustrating errors, particularly when it comes to pointers. One common issue developers encounter is the runtime error: load of null pointer of type 'char'. This guide aims to clarify the cause of this problem and provide a structured way to solve it.

Understanding the Problem

A user reported a runtime error while attempting to implement a trim function in C. The issue arose when they added a double pointer (char **str) to their function. The purpose of the function was to trim leading and trailing whitespace from a string. However, an error occurred during the execution of the trim function.

The code snippet below highlights the relevant part of the implementation where the error surfaced:

[[See Video to Reveal this Text or Code Snippet]]

Here, the variable len is derived from strlen(*str), yet the code attempts to access str[len - 1], resulting in a null pointer error.

Analyzing the Cause

Operator Precedence:

The root of the error lies in misunderstanding the precedence of operators in C. The postfix subscript operator [] has a higher priority than the dereference operator *. Therefore, interpreting the line:

[[See Video to Reveal this Text or Code Snippet]]

It actually evaluates as:

[[See Video to Reveal this Text or Code Snippet]]

This means it's attempting to dereference an index of the first string (i.e., str), which likely results in accessing memory that hasn't been initialized or is not pointing to a valid character.

Correcting the Code

To resolve the issue, you need to ensure that the dereferencing occurs correctly. Here's how you can adjust the problematic line:

Suggested Corrections

Proper Dereferencing:
Change the erroneous line to properly dereference and apply it to the pointer:

[[See Video to Reveal this Text or Code Snippet]]

This tells the compiler to first dereference *str, then index into the result.

Type Considerations:

It's crucial to use the appropriate types for the variable len. The return type of strlen() is size_t, which is an unsigned type used for sizes. Modify the variable declaration:

[[See Video to Reveal this Text or Code Snippet]]

Final Code Example:
Here's how the finished segment of the _JSON_Trim function would look with all the changes applied:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Working with pointers in C requires careful attention to detail, particularly with operator precedence and type handling. By following the adjustments outlined above, you can avoid the frustrating null pointer runtime errors and ensure that your trimming function works as intended. The next time you encounter a similar error, take a moment to evaluate the operator usage and data types to find a solution.

If you have further questions or need more clarification on pointer handling in C, feel free to share your thoughts in the comments below!