Invalid pointer error on invoking free() after malloc in C

Question

I am doing very basic dynamic allocation practice in C and I came across this issue: when I am trying to call free() with the pointer returned by malloc(), I am getting Invalid pointer error during run time.

When I remove free() from code, it works fine. The pointer that I am using is not modified anyhow after returned by malloc (except the casting). So that rules out some suggestion I found online. Below is my code:

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

int main() {
        int num = 0;
        //printf("Enter a number: ");
        //scanf("%d", &num);
        char* p = (char*)malloc(16*sizeof(char));
        printf("%p", p);
        if(p) {
                p = "mark";
                printf("\n%s\n", p);
                free(p); //when removed no error
        }
        return 0;
}

Following is the error message:

0xa26010
mark
*** Error in `./a.out': free(): invalid pointer: 0x00000000004006b7 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x777e5)[0x7fa968aed7e5]
/lib/x86_64-linux-gnu/libc.so.6(+0x8037a)[0x7fa968af637a]
/lib/x86_64-linux-gnu/libc.so.6(cfree+0x4c)[0x7fa968afa53c]
./a.out[0x40061a]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7fa968a96830]
./a.out[0x4004e9]
======= Memory map: ========
00400000-00401000 r-xp 00000000 08:03 6815746                            /home/kushal/Documents/GATE/prac_C/a.out
00600000-00601000 r--p 00000000 08:03 6815746                            /home/kushal/Documents/GATE/prac_C/a.out
00601000-00602000 rw-p 00001000 08:03 6815746                            /home/kushal/Documents/GATE/prac_C/a.out
00a26000-00a47000 rw-p 00000000 00:00 0                                  [heap]
7fa964000000-7fa964021000 rw-p 00000000 00:00 0 
7fa964021000-7fa968000000 ---p 00000000 00:00 0 
7fa968860000-7fa968876000 r-xp 00000000 08:03 39719295                   /lib/x86_64-linux-gnu/libgcc_s.so.1
7fa968876000-7fa968a75000 ---p 00016000 08:03 39719295                   /lib/x86_64-linux-gnu/libgcc_s.so.1
7fa968a75000-7fa968a76000 rw-p 00015000 08:03 39719295                   /lib/x86_64-linux-gnu/libgcc_s.so.1
7fa968a76000-7fa968c36000 r-xp 00000000 08:03 39714902                   /lib/x86_64-linux-gnu/libc-2.23.so
7fa968c36000-7fa968e36000 ---p 001c0000 08:03 39714902                   /lib/x86_64-linux-gnu/libc-2.23.so
7fa968e36000-7fa968e3a000 r--p 001c0000 08:03 39714902                   /lib/x86_64-linux-gnu/libc-2.23.so
7fa968e3a000-7fa968e3c000 rw-p 001c4000 08:03 39714902                   /lib/x86_64-linux-gnu/libc-2.23.so
7fa968e3c000-7fa968e40000 rw-p 00000000 00:00 0 
7fa968e40000-7fa968e66000 r-xp 00000000 08:03 39714844                   /lib/x86_64-linux-gnu/ld-2.23.so
7fa969046000-7fa969049000 rw-p 00000000 00:00 0 
7fa969064000-7fa969065000 rw-p 00000000 00:00 0 
7fa969065000-7fa969066000 r--p 00025000 08:03 39714844                   /lib/x86_64-linux-gnu/ld-2.23.so
7fa969066000-7fa969067000 rw-p 00026000 08:03 39714844                   /lib/x86_64-linux-gnu/ld-2.23.so
7fa969067000-7fa969068000 rw-p 00000000 00:00 0 
7ffcf8e4b000-7ffcf8e6c000 rw-p 00000000 00:00 0                          [stack]
7ffcf8f1d000-7ffcf8f20000 r--p 00000000 00:00 0                          [vvar]
7ffcf8f20000-7ffcf8f22000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Aborted (core dumped)

Answer 1

You don't need to allocate memory to a char I think, just declare it and when the compiler converts it to assembly it'll do its magic

I think you have some debug mode on

if you want to try malloc() try with ints first

int *ptr; 
ptr = malloc(sizeof(int));
ptr = 20;
printf("%d", ptr);
ptr = NULL;
free(ptr);
return 0;

with character arrays you just do char *description; and you can do description = "hello world" (without asterisk).

if you want to print a pointer to character array, just do printf("%s", description);

Answer 2

There is an error in your code:

 p = "mark";

is wrong, p is a pointer i.e. it is suppose to contain an address value. you cannot assign to it a string. You must use the functions provided by the standard library such as strcpy or strncpy (header: string.h).

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

int main() {
        int num = 0;
        //printf("Enter a number: ");
        //scanf("%d", &num);
        char* p = (char*)malloc(16*sizeof(char));
        printf("%p", p);
        if(p) {
                //*p = st"mark\0";
                strcpy(p, "mark");
                printf("\n%s\n", p);
                free(p); //when removed no error
        }
        return 0;
}

Answer 3

The problem is that p does not point to memory allocated using malloc(). Instead, it contains a pointer into constant "mark", so basically you are trying to delete a literal which is not allowed

int main() {
        int num = 0;
        //printf("Enter a number: ");
        //scanf("%d", &num);
        char* p = (char*)malloc(16*sizeof(char));
        printf("%p", p);
        if(p) {
                strcpy(p,"mark");
                printf("\n%s\n", p);
                free(p); //when removed no error
        }
        return 0;
}

usage of strcpy will solve the problem

Answer 4

You have a variable p that points to the memory returned by malloc. Then you change the variable p to point to the memory occupied by the string literal "mark". When you try to delete it, you are trying to delete the string literal.

In short, the line p = "mark" is not doing what you think it's doing. It isn't copying that value into the memory returned from malloc.

Try changing that line to strcpy(p, "mark") and I suspect things will start working better.

(That being said, I shouldn't even mention strcpy because it's so unsafe and there are alternatives that will prevent buffer overrun problems. But that's a separate question, and there are a lot of discussions about it here on SO.)