When compiling a C program you might find that the compiler gives you a warning similar to
hello.c:6:3: warning: implicitly declaring library function 'printf' with type 'int (const char *, ...)' [-Wimplicit-function-declaration] printf("Name length: %u", length); ^
hello.c:5:16: warning: implicitly declaring library function 'strlen' with type 'unsigned long (const char *)' [-Wimplicit-function-declaration] int length = strlen(name); ^
This problem occurs because you used a function from the standard library without first including the appropriate header file.
The compiler will also give you a suggestion, like the following one:
hello.c:5:16: note: include the header <string.h> or explicitly provide a declaration for 'strlen'
which points you in the right direction.
In this case, adding
at the top of the C file will solve the issue.