Let me be blunt: If you haven’t yet seen an undefined reference error, you truly aren’t a C programmer. In fact, the more of these messages you see, the longer you’ve been coding in C. Undefined reference errors are a badge of honor.
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\nThe short explanation of an undefined reference error is that the code contains a fictitious function. It could be a non-existent function, a typo, or it could be a function that requires a library that wasn’t linked in. Other mayhem ensues with this type of error as they often generate compiler warnings as a byproduct. Consider the following code:<\/p>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n char string[] = \"Judge me by my size, will you?\";\r\n int len;\r\n\r\n len = size(string);\r\n printf(\"The string \\\"%s\\\" is %d characters long.\\n\",\r\n string,\r\n len\r\n );\r\n\r\n return 0;\r\n}<\/pre>\nThis code contains the size()<\/em> function, which has no definition. When built, the code generates a warning from the compiler:<\/p>\n0830.c:19:8: warning: implicit declaration of function 'size' is invalid in C99 [-Wimplicit-function-declaration]
\n len = size(string);
\n ^
\n1 warning generated.<\/code><\/p>\nThis warning indicates a missing prototype; the compiler cannot confirm that the size()<\/em> function is of the correct data type or that the arguments are properly typed. It assumes that everything is an integer, which is why the code compiles.<\/p>\nNext comes the undefined reference error, generated by the linker:<\/p>\n
\/usr\/bin\/ld: \/tmp\/0830-3db658.o: in function `main':
\n0830.c:(.text+0x54): undefined reference to `size'
\nclang: error: linker command failed with exit code 1 (use -v to see invocation)<\/code><\/p>\nThis message is an error, meaning that though a program wasn’t built. The undefined reference is to “size,” a function that doesn’t exist.<\/p>\n
At this point, you need to address both the warning and the error. For the warning, the function must be prototyped: Add the proper header file or write the prototype directly. For example, if the size()<\/em> function is present in the source code file after the main()<\/em> function, add its protype before the function is called:<\/p>\nint size(char *);<\/code><\/p>\nThis step addresses the compiler warning.<\/p>\n
The second step is to tell the linker where to find the function. Or, when the compiler doesn’t generate a warning, and all you see is the linker’s undefined reference error, you must link in the proper library to make the function “funct.” I encounter this problem often in Linux where the math library must be linked in specifically. (This problem doesn’t exist in Windows.)<\/p>\n
Beyond forgetting to link in a library, undefined reference errors are generated by typos. In this case you see both the warning and error: The compiler doesn’t recognize the typo and neither does the linker. This happens often with me when I type print()<\/em> instead of printf(<\/em>).<\/p>\nFor multi-module programs, I set local function prototypes in a project header file, which is included for all source code files. Doing so ensures that the functions are recognized in whatever module they’re called.<\/p>\n
Undefined reference errors are common, but easily fixed. Don’t let them annoy you. Instead, use the references provided in the warning\/error messages, fix the problem, then build your program. And wear this error as a badge of honor!<\/p>\n","protected":false},"excerpt":{"rendered":"
It’s one of the most common and unpopular errors in C programming! Continue reading