What is the difference between “” and <> when a header file is included in a program? [duplicate]

Answer 1

Use <whatever> for system headers, and "whatever" for your own headers.

The difference is that when it's enclosed in quotes, the compiler will look in the local directory, but with <>, it won't. If you want to get technical, the C standard doesn't guarantee that, but it's how essentially all compilers work.

Answer 2

#include <file>

This variant is used for system header files. It searches for a file named file in a standard list of system directories. You can prepend directories to this list with the `-I' option.

#include "file"

This variant is used for header files of your own program. It searches for a file named file first in the directory containing the current file, then in the same directories used for .

The argument of #include, whether delimited with quote marks or angle brackets, behaves like a string constant in that comments are not recognized, and macro names are not expanded. Thus, #include specifies inclusion of a system header file named `x/*y'.

However, if backslashes occur within filename, they are considered ordinary text characters, not escape characters. None of the character escape sequences appropriate to string constants in C are processed.

Reference:

Answer 3

"" searches in current file's path. <> searches in global include paths.

Edit: You asked for absolute path and relative path.

Assume you have a file structure as follows:

folderX
 -fileX.a
 -fileX.b
 -folderX.Y
  -fileX.Y.a
  -fileX.Y.b
 -folderX.Z
  -fileX.Z.a

Then, the absolute path of fileX.Z.a would be folderX/folderX.Z/fileX.Z.a, assuming that folderX is the topmost directory available. The relative path of fileX.Z.a relative to e.g. fileX.a is just the part folderX.Z/fileX.Z.a, i.e. you start the path in the directory where fileX.a lies.