How to correctly use the extern keyword in C

extern changes the linkage. With the keyword, the function / variable is assumed to be available somewhere else and the resolving is deferred to the linker.

There’s a difference between extern on functions and on variables.

For variables it doesn’t instantiate the variable itself, i.e. doesn’t allocate any memory. This needs to be done somewhere else. Thus it’s important if you want to import the variable from somewhere else.

For functions, this only tells the compiler that linkage is extern. As this is the default (you use the keyword static to indicate that a function is not bound using extern linkage) you don’t need to use it explicitly.

Related Posts:

  1. How big can a 64 bit unsigned integer be?
  2. How to use execvp()
  3. How to use execvp() to execute a command
  4. How does strtok() split the string into tokens in C?
  5. What is *(uint32_t*)?
  6. what is Segmentation fault (core dumped)? [duplicate]
  7. How to convert an int to string in C?
  8. How to print in C
  9. Why should we typedef a struct so often in C?
  10. What is a string of hexadecimal digits?
  11. What is newline character — ‘\n’
  12. Why does ENOENT mean “No such file or directory”?
  13. What does the question mark character (‘?’) mean?
  14. makefile:4: *** missing separator. Stop
  15. Stack smashing detected
  16. How do I determine the size of my array in C?
  17. Pointer Arithmetic
  18. Openssl : error “self signed certificate in certificate chain”
  19. What is the difference between C and embedded C?
  20. How do I create an array of strings in C?
  21. Undefined reference to pthread_create
  22. Difference between malloc and calloc?
  23. What is Bit Masking?
  24. How do function pointers in C work?
  25. segmentation fault : 11
  26. What are .a and .so files?
  27. So what does “return 0” actually mean?
  28. warning: assignment makes integer from pointer without a cast
  29. Why use bzero over memset?
  30. How do you make an array of structs in C?
  31. How do we check if a pointer is NULL pointer?
  32. how to use wait in C
  33. what is the meaning of == sign?
  34. How do I calculate MB/s & MiB/s?
  35. How to create my own header file in c++?
  36. ARM Assembler – How do I use CMP, BLT and BGT?
  37. In C programming, what is `undefined reference`error, when compiling?
  38. Invalid read of size 8 – Valgrind + C
  39. warning: incompatible implicit declaration of built-in function ‘xyz’
  40. strcpy vs strdup
  41. double free or corruption (fasttop)
  42. Warning comparison between pointer and integer in C language
  43. error: expected declaration or statement at end of input in c
  44. Why do I get clang: error: linker command failed with exit code 1?
  45. Difference between scanf() and fgets()
  46. Return char[]/string from a function [duplicate]
  47. How to convert integer to char in C?
  48. Is there a printf converter to print in binary format?
  49. Program received signal SIGPIPE, Broken pipe
  50. Difference between char* and const char*?
  51. How to read from stdin with fgets()?
  52. Output single character in C
  53. Does connect() block for TCP socket?
  54. Converting a C program to MIPS
  55. Why am I getting “array initializer must be an initializer list or string literal”?
  56. what is the unsigned datatype?
  57. C: linker command failed with exit code 1
  58. #31 expression must have integral type
  59. How do you pass a function as a parameter in C?
  60. What’s the equivalent of new/delete of C++ in C?
  61. sizeof float (3.0) vs (3.0f)
  62. How to trigger SIGUSR1 and SIGUSR2?
  63. What does “%.*s” mean in printf?
  64. Display value found at given address gdb
  65. Need more information about Aborted (core dumped)
  66. Does C have a “foreach” loop construct?
  67. What is a bus error? Is it different from a segmentation fault?
  68. Array definition – Expression must have a constant value
  69. Constant pointer vs Pointer to constant
  70. What is the cause of flexible array member not at end of struct error?
  71. Variable warning set but not used
  72. Reversing a string in C
  73. When a number is written as 0x00… what does the x mean
  74. Expected declaration specifier error in function
  75. How can I get argv[] as int?
  76. Why I do get “Cannot find bound of current function” when I overwrite the ret address of a vulnerable program?
  77. valgrind – Address —- is 0 bytes after a block of size 8 alloc’d
  78. strcmp not working
  79. Convert char array to string use C
  80. error: aggregate value used where an integer was expected
  81. char pointers: invalid conversion from ‘char*’ to ‘char’?
  82. How does one represent the empty char?
  83. Removing trailing newline character from fgets() input
  84. “Multiple definition”, “first defined here” errors
  85. Does C have a string type?
  86. how use EOF stdin in C
  87. Execution of printf() and Segmentation Fault
  88. How does the strtok function in C work? [duplicate]
  89. Allocating char array using malloc
  90. error: function returns address of local variable
  91. Implementation of strtok() function
  92. Return a `struct` from a function in C
  93. Reaching EOF with fgets
  94. C: scanf to array
  95. What are “prototypes” in a C program?
  96. What’s the difference between “mod” and “remainder”?
  97. undefined reference to `std::ios_base::Init::Init()’
  98. previous declaration of ‘function’ was here in C [duplicate]
  99. How to Compare 2 Character Arrays [duplicate]
  100. error: `itoa` was not declared in this scope

Leave a Comment