c++ - incompatible implicit declaration of built-in function 'malloc' -

my program is

#include <iostream>  char * grabnumber ( char * begin ) {     // interpret *begin start of double , add characters     // string retstr      char * begincpy = begin;     int founddot = 0;     while ((*begin >= '0' && *begin  <= '9') || *begin == '.')     {         if (*begin == '.')         {             if (founddot == 0) founddot = 1;             else break;         }         ++begin;     }     long n = begin - begincpy; // # of characters parsed     char * retstr = malloc(sizeof(char) * (n + 1)); // string returned     (long k = 0; k < n; ++k) retstr[k] = *begincpy++;     retstr[n] = '\0';     return retstr; }  int main() {    char str [] = "abc3.14def";    std::cout << grabnumber(str+3); // should print "3.14"    return 0; } 

and errors i'm getting are

line 20: warning: incompatible implicit declaration of built-in function 'malloc' line 21: error: 'for' loop initial declaration used outside c99 mode

corresponding 2 lines

    char * retstr = malloc(sizeof(char) * (n + 1)); // string returned     (long k = 0; k < n; ++k) retstr[k] = *begincpy++; 

see: http://codepad.org/c2tngfeo

also, there way can cut down on redundancy of algorithm, because it's checking . twice in each iteration of while loop, , yet can't think of cleaner way handle fact need stop loop if we've run second .

i'm guessing trying write c++ have included iostream , used std::cout. error message shows using c compiler. guess wrote gcc myprogram.c. c++ compilation either need write g++ instead of gcc, or rename file have .cc extension. (preferably both).

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to use malloc need #include <cstdlib>.

also may need using namespace std; or using std::malloc; after that; , need cast value returned malloc because c++ not implicitly convert void * other pointer types.

however malloc used in c++ not initialize non-trivial objects properly. consider changing code to:

char * retstr = new char[n+1]; 

then won't need includes.

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but still weak design relying on caller free memory. in fact main function has memory leak not free memory.

in c++ better style have memory managed container class knows memory management; programmer can't make mistakes. (incase wondering, doesn't introduce inefficiency , may speed things up).

a better approach #include <string>, make function return std::string, , change last 5 lines of function to:

return { begincpy, begin }; 

or if using pre-c++11 compiler,

return std::string(begincpy, begin);