c - using a method from another file without including it -

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• why #include <stdio.h> not required use printf()? 3 answers

i have 2 .c files compile on makefile.

foo.c:

void foo() {     printf("this foo"); } 

main.c:

#include  <stdio.h>  int main() {     printf("this main\n");     foo(); } 

the makefile looks that:

all: main.o foo.o     gcc -o prog foo.o main.o  main.o: main.c     gcc -c main.c  foo.o: foo.c     gcc -c foo.c 

so question is: how can foo.c use printf() without me including stdio.h , how can main.c use method foo() without me including foo.c.

my guess/research makefile works linker. dont have prove , want understand how works excactly.

correct me if misunderstood something.

in compilation phase, compiler checks function calls against prototypes. function lacks prototype assumed return int , accept number of arguments.

if turn warning level, gcc warn if prototype missing. should add -wall , add -pedantic diagnostics on additional things compiler think suspicious.

if compilation step succeeds, compiler creates object file contains compiled code , 2 reference tables. first table export table. contains names of functions , variables exported object file. second table import table. contains list of functions , variables referenced, declaration missing.

in case have:

foo.o:

export:

foo 

import:

printf 

main.o

export:

main 

import:

printf foo 

in linker phase, linker take list of imports , exports , match them. in addition object files , libraries specify on command line, linker automatically link libc, contains functions defined c language.