Returning string from C function

Question

I haven t used C in over 3 years  I m pretty rusty on a lot of things   I know this may seem stupid but I cannot return a string from a function at the moment  Please assume that  I cannot use string h for this    Here is my code     include  lt ncurses h gt   char   getStr int length           char word length        for  int i   0  i  lt  length  i                  word i    getch               word i      0       return word     int main         char wordd 10       initscr         wordd   getStr 10       printw  The string is  n        printw   s n   wordd       getch        endwin        return 0      I can capture the string  with my getStr function  but I cannot get it to display correctly  I get garbage    Help is appreciated

User · Answer

word is on the stack and goes out of scope as soon as getStr   returns  You are invoking undefined behavior

User · Answer

I came across this thread while working on my understanding of Cython   My extension to the original question might be of use to others working at the C   Cython interface   So this is the extension of the original question   how do I return a string from a C function  making it available to Cython  amp  thus to Python   For those not familiar with it  Cython allows you to statically type Python code that you need to speed up   So the process is  enjoy writing Python     find its a bit slow somewhere  profile it  calve off a function or two and cythonize them   Wow   Close to C speed  it compiles to C   Fixed   Yay    The other use is importing C functions or libraries into Python as done here   This will print a string and return the same or another string to Python   There are 3 files  the c file c hello c  the cython file sayhello pyx  and the cython setup file sayhello pyx    When they are compiled using python setup py build ext --inplace they generate a shared library file that can be imported into python or ipython and the function sayhello hello run   c hello c   include  lt stdio h gt   char  c hello       char  mystr    Hello World  n     return mystr       return  this string       alterative     sayhello pyx  cdef extern from  c hello c       cdef char  c hello    def hello        return c hello     setup py  from setuptools import setup from setuptools extension import Extension from Cython Distutils import build ext from Cython Build import cythonize   ext modules   cythonize  Extension  sayhello     sayhello pyx        setup  name    Hello world app   cmdclass     build ext   build ext   ext modules   ext modules

User · Answer

char word length   char  rtnPtr   word      return rtnPtr    This is not good  You are returning a pointer to an automatic  scoped  variable  which will be destroyed when the function returns  The pointer will be left pointing at a destroyed variable  which will almost certainly produce  strange  results  undefined behaviour    You should be allocating the string with malloc  e g  char  rtnPtr   malloc length    then freeing it later in main

User · Answer

You are allocating your string on the stack  and then returning a pointer to it  When your function returns  any stack allocations become invalid  the pointer now points to a region on the stack that is likely to be overwritten the next time a function is called   In order to do what you re trying to do  you need to do one of the following    Allocate memory on the heap using malloc or similar  then return that pointer  The caller will then need to call free when it is done with the memory  Allocate the string on the stack in the calling function  the one that will be using the string   and pass a pointer in to the function to put the string into  During the entire call to the calling function  data on its stack is valid  its only once you return that stack allocated space becomes used by something else

User · Answer

Your pointer is pointing to local variable of the function  So as soon as you return from the function  memory gets deallocated  You have to assign memory on heap in order to use it in other functions   Instead  char  rtnPtr   word   do this char  rtnPtr   malloc length    So that it is available in the main function  After it is used free the memory

User · Answer

Easier still  return a pointer to a string that s been malloc d with strdup    include  lt ncurses h gt   char   getStr int length           char word length        for  int i   0  i  lt  length  i                  word i    getch               word i      0       return strdup  amp word 0       int main         char wordd 10       initscr         wordd   getStr 10       printw  The string is  n        printw   s n   wordd       getch        endwin        return 0

User · Answer

Either allocate the string on the stack on the caller side and pass it to your function   void getStr char  wordd  int length               int main void        char wordd 10   1       getStr wordd  sizeof wordd  - 1               Or make the string static in getStr   char  getStr void        static char wordd 10   1               return wordd      Or allocate the string on the heap   char  getStr int length        char  wordd   malloc length   1               return wordd

[c] Returning string from C function

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