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);
  ^

or

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

#include <stdio.h>

at the top of the C file will solve the issue.