There is no 'printf'(netmeister.org)

    return 1+2+3;

    return 6;

   int printf(char *);

   puts("daemons are running in your nose");

   puts("Hello World!");

  struct

  {

    unsigned SyncHeader:16;

    unsigned MessageID:8;

    unsigned short MessageData[msgDataSize];  // ID-unique data

    unsigned CRC:8;             // LSB of CCITT-16 for above data

  } __attribute__ ((packed)) Frame;

  unsigned char  b[16];         // Accessible as raw bytes as well

  unsigned short w[8];          // Accessible as raw words as well

  unsigned long  l[4];          // Accessible as raw long words as well

    // If frame was accepted into TX queue, prepare next frame for transmission

    IpcMessage.Frame.MessageID = IpcMessage.Frame.MessageID + 1; // Bump up to next message type

   int main() {
           printf("Hello World!\n");
   }

   cat <<eof >1.c
   int main() {
           printf("Hello World!\n");
   }
   eof
   cc -Wall -std=c89 -pedantic -ansi 1.c
   ./a.out

   1.c: In function 'main':
   1.c:4:10: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
       4 |          printf("Hello World!\n");
         |          ^~~~~~
   1.c:4:10: warning: incompatible implicit declaration of built-in function 'printf'
   1.c:1:1: note: include '<stdio.h>' or provide a declaration of 'printf'
     +++ |+#include <stdio.h>
       1 | 
   1.c:5:2: warning: control reaches end of non-void function [-Wreturn-type]
       5 |  }
         |  ^

    #!/bin/sh
    test $# = 1||exec echo usage: $0 function
    grep -r " $1(" /usr/include/* 2>/dev/null|sed 's/.*://;s/ *//' 

   sed \$r1.c <<eof >2.c
   int printf(const char *restrict, ...); 
   eof
   cc -Wall -std=c89 -pedantic -ansi 2.c
   ./a.out

   2.c: In function 'main':
   2.c:5:2: warning: control reaches end of non-void function [-Wreturn-type]
       5 |  }
         |  ^

   sed 4r2.c <<eof >3.c
           exit(0);
   eof
   cc -Wall -std=c89 -pedantic -ansi 3.c
   ./a.out

   3.c: In function 'main':
   3.c:5:35: warning: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
       5 |          printf("Hello World!\n");exit(0);
         |                                   ^~~~
   3.c:5:35: warning: incompatible implicit declaration of built-in function 'exit'
   3.c:1:1: note: include '<stdlib.h>' or provide a declaration of 'exit'
     +++ |+#include <stdlib.h>
       1 | 

   sed 2r3.c <<eof >4.c
   void exit(int);
   eof
   cc -Wall -std=c89 -pedantic -ansi 4.c
   ./a.out