Programming

extern char *strerror(int errno);

// Near top util.h
#include <string.h>

// Lower down
#ifndef strerror
extern char *strerror(int errno);
#endif/*strerror*/

// Start of file
#ifndef __STRING_H__
#define __STRING_H__

// file contents
...

#endif/*__STRING_H__*/

gcc -E find_rules.c | grep 'strerror'

cc -c file1.c -o file1.o
cc -c file2.c -o file2.o
...
cc -c file5.c -o file5.o
cc file1.o file2.o file3.o file4.o file5.o -o program

OBJS=file1.o file2.o file3.o file4.o file5.o

program:$(OBJS)
        $(CC) $(OBJS) $(LDFLAGS) -o $@

// main.c
#include <stdio.h>

int main(int argc, char *argv[])
{
  int n;
  fprintf(stderr,"main(%d,[",argc);
  for(n=0; n<argc; n++)
    fprintf(stderr," '%s'",argv[n]);
  fprintf(stderr," ]);\n");
  return(0);
}

# gcc main.c -o main
# ./main
main(1,[ './main' ]);
#

# gcc -c main.c -o main.o
# gcc main.o -o main
# ./main
main(1,[ './main' ]);
#

// library.c
#include <stdio.h>

int library_function(int c)
{
  fprintf(stderr,"library_function(%d)\n",c);
  return(c-3);
}

#include <stdio.h>

extern int library_function(int c);

int main(int argc, char *argv[])
{
  int n;
  fprintf(stderr,"main(%d,[",argc);
  for(n=0; n<argc; n++)
    fprintf(stderr," '%s'",argv[n]);
  fprintf(stderr," ]);\n");

  printf("library_function(%d) returns %d\n",32,library_function(32));

  return(0);
}

# gcc -c main.c -o main.o
# gcc main.o -o main
main.o: In function `main':
main.c:(.text+0x86): undefined reference to `library_function'
collect2: ld returned 1 exit status
#

# gcc -c main.c -o main.o
# gcc -c library.c -o library.o
# gcc main.o library.o -o main
# ./main
main(1,[ './main' ]);
library_function(32)
library_function(32) returns 29
#

main:main.o library.o
        $(CC) main.o library.o $(LDFLAGS) -o $@
clean:
        rm -f main main.o library.o

# make clean
rm -f main main.o library.o
# make main
cc    -c -o main.o main.c
cc    -c -o library.o library.c
cc main.o library.o  -o main
# ./main
main(1,[ './main' ]);
library_function(32)
library_function(32) returns 29
#