string - C compilation error for basic function -

i'm trying write own version of strcat (i call "append"). here's have:

#include <stdio.h>  int main() {    char *start = "start";   char *add = "add";   append(start, add);   printf(start);  }  void append(char *start, char *add) {   //get end of start word   char *temp = &start;   while (*temp != '\0') {     temp++;   }   *temp = *add;   while (*temp != '\0') {      *temp = *add;    } } 

when compile, 3 warnings , error:

1) warning: implicit declaration of function 'append' invalid in c99

2) warning: format string not string literal (potentially insecure)

3) error: conflicting types 'append'

i don't see how arguments pass append function within main conflict function definition below it.

4) warning: incompatible pointer types initializing 'char *' expression of type 'char **'; remove &

why want remove & here? thought declare , initialize char pointer proper memory address @ once.

any appreciated.

you have multiple problems short code. first have

warning: implicit declaration of function 'append' invalid in c99

the meaning of warning need declare functions before use them. if don't declare function before use it, compiler have guess arguments , return type, , guesses badly.

continuing next warning:

warning: format string not string literal (potentially insecure)

this because provide string variable printf, is, warning tells you, insecure. think example case read input user, , use input format string printf. stop user adding format codes in input string? , since don't pass arguments arguments formats come from?

and error:

error: conflicting types 'append'

this because of first problem, compiler guessed arguments or return type of function wrongly.

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now on major problem doesn't show compiler errors or warnings, namely undefined behavior.

the problem start , add variables points string literals. string literal read only (in fact, string literal pointer array of non-modifiable characters). first problem try modify contents of these arrays, second arrays big needed , writing outside of memory. both these problems causes undefined behavior.