malloc in a function doesn't work well

问题

I can't understand why the whole thing doesn't work.

I just want to do malloc in the function func, when I return from it, the malloc disappears... and I get

* glibc detected ./test: free(): invalid pointer: 0xb76ffff4 **

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <string.h>
#include <errno.h>

int func(char *p) {
    p=(char*)malloc(1);
    *p='a';
    printf("1. p= %c\n",*p);
    return 0;
}

int main()
{
    char *p;

    func(p);
    printf("2. p= %c\n",*p);
    free(p);

    return 0;
}

回答1:

char *p;

makes a pointer local to main(), you can pass it to another function, however your passing it by value, so any changes you make to it (like changing what its pointing at) outside of the scope.won't "stick".

the solution is to pass a pointer to a pointer, or simply the address of p:

func(&p);

but don't forget to change the parameter list of func()!

int func(char **p)

回答2:

While in func, p is a copy of the pointer you passed to it, so p is created inside func and deleted when func is finished.

You have two solutions :

Pass a pointer to the pointer, a char **p, and do *p = malloc(1), then func will accept a char **p
Return the alloced variable un func and assign it : p = func();

回答3:

pass &p in main means call func as func(&p) change the function sugnature of func(char *p) to func(char **p)

Thep in func is the local one to that func so when func exits it destroyed and also leads to memory leak.

And in main you are freeing a pointer which is not pointing to memory allocated by malloc, calloc so free(p) is Undefined behaviour in this case and it's not freeing that memory which you have allocated in func so it's memory leak

1st method:

void func(char **p)
{
 *p=malloc(1); 
//rest of your code here 
 }

int main()
{
 int *p;
 func(&p);
 // your code here 
 free(p); 
}

2nd method : It's easy no use of **

char * func(char *p)
{
p=malloc(1);
// rest of code here 
return p; 
}

int main()
{
char *p;
p=func(p);
//rest of code here 
free(p);
}

来源：https://stackoverflow.com/questions/13541990/malloc-in-a-function-doesnt-work-well

标签

Ubuntu

malloc