Returning string from C function

问题

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 <ncurses.h>

char * getStr(int length)
{   
    char word[length];

    for (int i = 0; i < 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.

回答1:

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;
}

回答2:

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.

回答3:

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.

回答4:

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.

回答5:

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

回答6:

Easier still: return a pointer to a string that's been malloc'd with strdup.

#include <ncurses.h>

char * getStr(int length)
{   
    char word[length];

    for (int i = 0; i < length; i++)
    {
        word[i] = getch();
    }

    word[i] = '\0';
    return strdup(&word[0]);
}

int main()
{
    char wordd[10];
    initscr();
    *wordd = getStr(10);
    printw("The string is:\n");
    printw("%s\n",*wordd);
    getch();
    endwin();
    return 0;
}

回答7:

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 & 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 <stdio.h>

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
)

来源：https://stackoverflow.com/questions/25798977/returning-string-from-c-function

标签

string

return

local-variables