Resolve the gcc: undefined reference to function Error by Using -pthread Flag

Learn how to fix the `undefined reference to function` error in GCC by properly linking your program with the `-pthread` flag to enable threads in C.
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Understanding the GCC undefined reference to function Error

If you’re working with C and using GCC (GNU Compiler Collection) to compile your programs, you might have stumbled upon the following error messages:

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

This is a common issue developers face, especially when their programs rely on multi-threading. The error indicates that certain functions, which you are trying to use in your program, are not found during the linking phase of the compilation process.

What Causes This Error?

These error messages can arise from a few key reasons:

Missing Libraries: The functions you are trying to use, such as pthread_create, pthread_cancel, etc., belong to the pthread library, which handles threading in C. If this library is not linked during compilation, you will see these undefined references.

Incorrect Compilation Command: The way your command is structured can impact whether the correct libraries are linked. In the provided command, GCC is not informed to include the pthread library.

The Solution: Use the -pthread Flag

The solution to the undefined reference to function error is simple: you need to tell GCC to link against the pthread library. This can be done by adding the -pthread flag to your compilation command. Here’s how to do it:

Updated Compilation Command

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

Explanation of the Command

gcc: This is the command for invoking the GNU Compiler.

myprogram.c csapp.c: These are the source files you are compiling. Here, csapp.c contains definitions for functions used in myprogram.c.

-pthread: This flag indicates that the program will use multi-threading capabilities, linking the necessary libraries automatically.

Summary

In summary, if you encounter the undefined reference to function error when using functions that require threading, remember the following points:

Always check if you're linking against the correct libraries.

Use the -pthread flag when compiling programs that utilize threading functions from the pthread library.

By making this small adjustment, you can avoid these errors and get your program running smoothly. Happy coding!