I am writing a program that reads a value from an .ini file, then passes the value into a function that accepts a PCSTR (i.e. const char *). The function is getaddrinfo()
.
So, I want to write PCSTR ReadFromIni()
. To return a constant string, I plan on allocating memory using malloc()
and casting the memory to a constant string. I will be able to get the exact number of characters that were read from the .ini file.
Is that technique okay? I don't really know what else to do.
The following example runs fine in Visual Studio 2013, and prints out "hello" as desired.
const char * m()
{
char * c = (char *)malloc(6 * sizeof(char));
c = "hello";
return (const char *)c;
}
int main(int argc, char * argv[])
{
const char * d = m();
std::cout << d; // use PCSTR
}
The second line is "horribly" wrong:
char* c = (char*)malloc(6*sizeof(char));
// 'c' is set to point to a piece of allocated memory (typically located in the heap)
c = "hello";
// 'c' is set to point to a constant string (typically located in the code-section or in the data-section)
You are assigning variable c
twice, so obviously, the first assignment has no meaning.
It's like writing:
int i = 5;
i = 6;
On top of that, you "lose" the address of the allocated memory, so you will not be able to release it later.
You can change this function as follows:
char* m()
{
const char* s = "hello";
char* c = (char*)malloc(strlen(s)+1);
strcpy(c,s);
return c;
}
Keep in mind that whoever calls char* p = m()
, will also have to call free(p)
at some later point...
One way is to return a local static pointer.
const char * m()
{
static char * c = NULL;
free(c);
c = malloc(6 * sizeof(char));
strcpy(c, "hello"); /* or fill in c by any other way */
return c;
}
This way, whenever the next time m()
is called, c
still points to the memory block allocated earlier. You could reallocate the memory on demand, fill in new content, and return the address.
NO. This is not OK. When you do
c = "hello";
the memory allocated by malloc
lost.
You can do as
const char * m()
{
char * c = (char *)malloc(6 * sizeof(char));
fgets(c, 6, stdin);
return (const char *)c;
}
来源:https://stackoverflow.com/questions/21579819/dynamically-allocating-memory-for-const-char-string-using-malloc