Passing pointer argument by reference under C?

#include <stdio.h>
#include <stdlib.h>

void
getstr(char *&retstr)
{
 char *tmp = (char *)malloc(25);
 strcpy(tmp, "hello,world");
 retstr = tmp;
}

int
main(void)
{
 char *retstr;

 getstr(retstr);
 printf("%s\n", retstr);

 return 0;
}

gcc would not compile this file, but after adding #include <cstring> I could use g++ to compile this source file.

The problem is: does the C programming language support passing pointer argument by reference? If not, why?

Thanks.

Solution 1:

No, C doesn't support references. It is by design. Instead of references you could use pointer to pointer in C. References are available only in C++ language.

Solution 2:

References are a feature of C++, while C supports only pointers. To have your function modify the value of the given pointer, pass pointer to the pointer:

void getstr(char ** retstr)
{
    char *tmp = (char *)malloc(25);
    strcpy(tmp, "hello,world");
    *retstr = tmp;
}

int main(void)
{
    char *retstr;

    getstr(&retstr);
    printf("%s\n", retstr);

    // Don't forget to free the malloc'd memory
    free(retstr);

    return 0;
}