What is the connection between libraries and header files in Linux?

The relationship between libraries and header files in Linux: there are declarations of functions in header files, and library files implement the definition of functions; each function in the library needs to be declared in the header file. When writing a program, you need to use header files to provide definitions of constants and declarations of calls to system functions and library functions; and a library is a collection of compiled functions, that is, a collection of target files ".o". It is written according to the principle of reusability and generally consists of a set of interrelated functions to perform a common task.

#The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

In general application source code, there are function declarations in the header file, and library files implement the definition of the function; each function in the library needs to be declared in the header file.

1. Header file

When writing a program, we need to use header files to provide definitions of constants and system functions and libraries Declarations of function calls. These header files are located in /usr/include. These header files are located in directories that can be automatically searched by the compiler, such as /usr/include/X11 of the X window system and /usr/include /c of ​​GNU C.

When calling gcc, you can use the flag -I to include header files saved in subdirectories or other non-standard locations, such as gcc -I/usr/openwin/ include fred.c

2. Library file

The library is a collection of compiled functions, that is, the target file A collection of .o is written according to the principle of reusability and generally consists of a set of interrelated functions to perform a common task.

Standard Linux system library files are generally stored in the /lib and /usr/lib directories. By default, the GCC linker only searches the standard C language library, which is a historical issue. We must conform the library file name to a specific convention and specify it on the command line.

Library files always start with lib, and then indicate what library it is. The last part .a represents the static function library, and .so represents the shared function library (that is, the DLL dynamic link library under Windows).

Such as gcc -o fred fred.c /usr/lib/libm.a or gcc -o fred fred.c -lm
# The ##-L flag increases the library search path for the linker, such as gcc -o x11fred -L /usr/openwin/lib x11fred.c -1X11

##2.1 Static libraryThe static library ends with .a, also called an archive file. When generating an executable program, the static library file is linked in, and the generated executable The file is larger and can be run without the static library file.

2.1.1 Generation methodFirst create two source files fred.c and bill.c

Then use

to compile and generate two target files

Then for these two Write a header file for each function, make a function declaration

Then include the header file in the main function and call one of the library functions

Then compile the main function and link main.o with the previously generated bill.o, which can be executed correctly

Next we generate the library file and use ar to archive the program Create an archive file and add target files to it, which combines several individual files into one large file.

After that we need to use the ranlib command to generate a content table for the function library.

Then we link the library file with the previously generated program.o file and the execution is successful

Also Use

to specify function libraries, and -L to specify non-standard locations.

nm command to view which functions are included in the target file, function library or executable file, available For disassembly analysis. When a program is created, it only contains the functions it actually needs in the function library. Although there are declarations of all functions in the header file, the linker will not link them all in.

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