Can't understand this error while freeing memory in C

Question

I don't understand why I m am getting a segmentation fault inside the free_memory function. Here is the program:

#include <stdio.h> #include <stdlib.h> #include <string.h> void allocate_memory (char **cells) { int i; cells = (char **) malloc(9 * sizeof(char *)); if (cells == NULL) { perror("Couldn't allocate memory"); exit(1); } for (i = 0; i < 9; i++) { cells[i] = (char *) malloc(9 * sizeof(char)); if (cells[i] == NULL) { perror("Couldn't allocate memory"); exit(1); } memset(cells[i], 1, 9); } } void free_memory (char **cells) { int i; for (i = 0; i < 9; i++) { free(cells[i]); } free(cells); } int main (int argc, char *argv[]) { char **cells = NULL; allocate_memory(cells); printf("Allocated\n"); free_memory(cells); return 0; }

The debugger shows this message about the error:

Process 1433 launched: '/Users/Jaime/Documents/workspaceC/PruebasC/PruebasC/sk' (x86_64) Allocated Process 1433 stopped * thread #1: tid = 0x1058a, 0x0000000100000e95 sk`free_memory + 37, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0) frame #0: 0x0000000100000e95 sk`free_memory + 37 sk`free_memory: -> 0x100000e95 <+37>: movq (%rcx,%rax,8), %rdi 0x100000e99 <+41>: callq 0x100000f20 ; symbol stub for: free 0x100000e9e <+46>: movl -0xc(%rbp), %eax 0x100000ea1 <+49>: addl $0x1, %eax

I hope anyone can help me, I don't get why I am accessing a bad pointer.

Possible duplicate of Reading from stdin using fgets()

michael_wu
– michael_wu

2016-11-06 21:35:43 +00:00
Commented Nov 6, 2016 at 21:35 — michael_wu
– michael_wu, Commented Nov 6, 2016 at 21:35

michael_wu · Accepted Answer · 2016-11-06 21:31:12Z

You are not modifying your main's cells in allocate_memory. You are modifying a copy.

If you want to modify a pointer in a function, you have to pass a pointer-to-pointer to the function:

... void allocate_memory (char ***cells) { int i; *cells = (char **) malloc(9 * sizeof(char *)); if (*cells == NULL) { perror("Couldn't allocate memory"); exit(1); } for (i = 0; i < 9; i++) { (*cells)[i] = (char *) malloc(9 * sizeof(char)); if ((*cells)[i] == NULL) { perror("Couldn't allocate memory"); exit(1); } memset((*cells)[i], 1, 9); } } ... int main (int argc, char *argv[]) { char **cells = NULL; allocate_memory(&cells); ... }

Community · Accepted Answer · 2020-06-20 09:12:55Z

C uses pass-by-value for function argument passing. In case you want to allocate memory to cells itself, you need to either

pass a pointer to it, or
return the newly allocated pointer and store it back into the cells in main().

Otherwise, the cells in the allocate_memory() function is local to that any any change made to cells will be lost once you return from that function.

As a result, inside free_memory() function, the access cells[i] is invalid, as cells does not point to any valid memory, at all. Attempt to access invalid memory invokes undefined behavior.

Jonas Schäfer · Accepted Answer · 2017-04-01 20:16:56Z

The allocation function doesn't return a newly allocated block. allocate_memory(cells);

The external effect is as if cells was not set by the function (previously set to NULL)

Collectives™ on Stack Overflow

Can't understand this error while freeing memory in C

3 Answers 3

Comments

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Linked

Related