如何在记忆后释放分配的指针

Question

Solution

apparently, according to what is the difference between free(a) and memset(a, 0, malloced_size) (actual title is "Free function in c" but i feel that is not specific enough), i need to memset the data to 0 before/after i free it to actually free it as if it was never assigned any data

UPDATE: fixed

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

uintptr_t round_up(uintptr_t value, uintptr_t size)
{
    fprintf(stderr, "called round_up\nreturning %p\n", (void *) (value ? size * ((value + (size - 1)) / size) : size));
    return value ? size * ((value + (size - 1)) / size) : size;
}

int read_fast_verify(const char *src, int len_of_source, char **dest, char ** a, int requested_len) {
    *a = malloc(requested_len+4096);
    if (len_of_source < requested_len) memcpy(*a, src, len_of_source);
    else memcpy(*a, src, requested_len);
    *dest = memmove((void *)round_up((uintptr_t)*a, 4096), src, requested_len);
    return requested_len;
}

void __lseek_string__(char **src, int len, int offset) {
    memmove(*src, *src+offset, len);
}

char * string1 = "hello";
char * string2;
char * s;
int main(void) {
    read_fast_verify(string1, strlen(string1), &string2, &s, (strlen(string1) + 5));
    __lseek_string__(&string2, strlen(string1), 5);
    free(s);
}

Problem

How do I free a malloced pointer after I memmove it, as memmove seems to seg fault if I use a NULL dest as its return (eg char * dest = memmove(...);)

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

uintptr_t round_up(uintptr_t value, uintptr_t size)
{
    fprintf(stderr, "called round_up\nreturning %p\n", (void *) (value ? size * ((value + (size - 1)) / size) : size));
    return value ? size * ((value + (size - 1)) / size) : size;
}

int read_fast_verify(const char *src, int len_of_source, char **dest, char ** a, int requested_len) {
    *a = malloc(requested_len+4096);
    if (len_of_source < requested_len) memcpy(*a, src, len_of_source);
    else memcpy(*a, src, requested_len);
    *dest = memmove((void *)round_up((uintptr_t)*a, 4096), *dest, requested_len);
    return requested_len;
}

void __lseek_string__(char **src, int len, int offset) {
    memmove(*src, *src+offset, len);
}

char * string1 = "hello";
char * string2;
char * s;
int main(void) {
    read_fast_verify(string1, strlen(string1), &string2, &s, (strlen(string1) + 5));
    __lseek_string__(&string2, strlen(string1), 5);
free(s);
}

the output

    Starting program: /home/arch/universal-dynamic-loader/loader/test_case 
    called round_up
    returning 0x55555555a000

    Program received signal SIGSEGV, Segmentation fault.
    0x00007ffff7f2d3b0 in __memmove_ssse3_back () from /usr/lib/libc.so.6

However if I just do it as normal

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

uintptr_t round_up(uintptr_t value, uintptr_t size)
{
    fprintf(stderr, "called round_up\nreturning %p\n", (void *) (value ? size * ((value + (size - 1)) / size) : size));
    return value ? size * ((value + (size - 1)) / size) : size;
}

int read_fast_verify(const char *src, int len_of_source, char **dest, char ** a, int requested_len) {
    *dest = malloc(requested_len+4096);
    if (len_of_source < requested_len) memcpy(*dest, src, len_of_source);
    else memcpy(*dest, src, requested_len);
    *dest = memmove((void *)round_up((uintptr_t)*dest, 4096), *dest, requested_len);
    return requested_len;
}

void __lseek_string__(char **src, int len, int offset) {
    memmove(*src, *src+offset, len);
}

char * string1 = "hello";
char * string2;
char * s;
int main(void) {
    read_fast_verify(string1, strlen(string1), &string2, &s, (strlen(string1) + 5));
    __lseek_string__(&string2, strlen(string1), 5);
    free(string2);
}

then I get this

called round_up
returning 0x564c86f54000
free(): invalid pointer
Aborted (core dumped)

and from valgrind

==19175== HEAP SUMMARY:
==19175==     in use at exit: 4,106 bytes in 1 blocks
==19175==   total heap usage: 1 allocs, 1 frees, 4,106 bytes allocated
==19175== 
==19175== Searching for pointers to 1 not-freed blocks
==19175== Checked 68,008 bytes
==19175== 
==19175== 4,106 bytes in 1 blocks are possibly lost in loss record 1 of 1
==19175==    at 0x4837757: malloc (vg_replace_malloc.c:299)
==19175==    by 0x10923C: read_fast_verify (test_case.c:13)
==19175==    by 0x109357: main (test_case.c:28)
==19175== 
==19175== LEAK SUMMARY:
==19175==    definitely lost: 0 bytes in 0 blocks
==19175==    indirectly lost: 0 bytes in 0 blocks
==19175==      possibly lost: 4,106 bytes in 1 blocks
==19175==    still reachable: 0 bytes in 0 blocks
==19175==         suppressed: 0 bytes in 0 blocks
==19175== 
==19175== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)
==19175== 
==19175== 1 errors in context 1 of 2:
==19175== Invalid free() / delete / delete[] / realloc()
==19175==    at 0x4838904: free (vg_replace_malloc.c:530)
==19175==    by 0x109388: main (test_case.c:30)
==19175==  Address 0x4a30000 is 4,032 bytes inside a block of size 4,106 alloc'd
==19175==    at 0x4837757: malloc (vg_replace_malloc.c:299)
==19175==    by 0x10923C: read_fast_verify (test_case.c:13)
==19175==    by 0x109357: main (test_case.c:28)
==19175== 
==19175== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)

as I am trying to make it memory safe as my dynamic linker is currently using over 2.7 gb (and over 4 GB shared memory) when reading a .so (such as libc 2.28) which it should not be at all.

Answer 1

You should be storing the previous value and freeing after the move completed successfully. Right now you just throw away the previous address.

Also not sure you need to move here in this example as you'll always point to a relative addrss within the first pointer and return that, there's no pointer to free but the origin pointer.

See also Does memmove actually "move" a chunk of memory and leave behind zeros at the source?