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Why is the pointer to my allocated memory for my type not being realloc'd properly?

July 9, 2022

I am working on a file system of sorts in my free time and I have ran into an issue with reallocating memory for a pointer to a typedef struct.
file_t:

typedef struct {
    char *fileName;
    FILE *filePointer;
    char *fileContents;
    char *filePermissions;
    size_t fileSize;
    int numFilesUsed;
    char *fileOwner;
    char *fileGroup;
    char *fileCreationDate;
} file_t;

dir_t:

typedef struct {
    char **fileNames;
    int numFiles;
    int *fileSizes;
    int dirSize;
    file_t *files;
    char *path;
} dir_t;

Here is the part that is causing the issue with allocation:

dir_t *currentDir = (dir_t*)malloc(sizeof(dir_t));
for(int i = 0; fileLines[i] != NULL; i++){
    currentDir->files = (file_t *)realloc(currentDir->files, sizeof(file_t) * (i+2)); // I am using i+2 so that I have space for a null
}

When I compile and run, the output is:

EXEC(33314,0x1042f4580) malloc: *** error for object 0x13cf0414a: pointer being realloc’d was not allocated
EXEC(33314,0x1042f4580) malloc: *** set a breakpoint in malloc_error_break to debug Abort trap: 6

I originally thought that this could be an issue with fileLines[i] not having a proper NULL terminator, causing some sort of infinite loop, but it is working fine. Examining the variables on the stack it shows:
Before for loop:

(char **) fileLines = 0x0000600002900000

(dir_t *) currentDir = 0x0000600000c04000

During loop:
i = 0:

(char **) fileLines = 0x0000600002900000

(dir_t *) currentDir = 0x0000600000c04000

Then the abort signal arrives.
The assembly for malloc_error_break:

    libsystem_malloc.dylib`malloc_error_break:
->  0x184876278 <+0>:  pacibsp 
    0x18487627c <+4>:  stp    x29, x30, [sp, #-0x10]!
    0x184876280 <+8>:  mov    x29, sp
    0x184876284 <+12>: nop    
    0x184876288 <+16>: ldp    x29, x30, [sp], #0x10
    0x18487628c <+20>: retab

Current contents of registers:

General Purpose Registers:
        x0 = 0x0000000000000000
        x1 = 0x000000010020c000
        x2 = 0x0000000000004000
        x3 = 0x0000000184a597aa  libsystem_platform.dylib`___lldb_unnamed_symbol3$$libsystem_platform.dylib + 10
        x4 = 0x0000000000000000
        x5 = 0x0000000000000000
        x6 = 0x0000000000000001
        x7 = 0x0000000000000000
        x8 = 0x0000000010000003
        x9 = 0x000000010020c07c
       x10 = 0xcccccccccccccccd
       x11 = 0x000000000000000a
       x12 = 0x0000000000000000
       x13 = 0x0000000000000033
       x14 = 0x0000000000600000
       x15 = 0x0000000000000002
       x16 = 0xfffffffffffffff4
       x17 = 0x00000001dec1c0f8  (void *)0x0000000184a0bd40: vm_deallocate
       x18 = 0x0000000000000000
       x19 = 0x0000000000000050
       x20 = 0x0000000000000000
       x21 = 0x0000000100208028
       x22 = 0x000000016fdff5b0
       x23 = 0x0000000100208000
       x24 = 0x0000000000000000
       x25 = 0x0000000000000000
       x26 = 0x000000016fdff9df
       x27 = 0x000000010007c580  dyld`_main_thread
       x28 = 0x0000000000000000
        fp = 0x000000016fdff580
        lr = 0x0000000184867d50  libsystem_malloc.dylib`malloc_vreport + 428
        sp = 0x000000016fdff510
        pc = 0x0000000184876278  libsystem_malloc.dylib`malloc_error_break
      cpsr = 0x80000000

What am I not seeing here? Thanks.

>Solution :

While you allocated currentDir with malloc, you did not allocate any memory for currentDir->files to point to. As such, you cannot realloc it.

Allocating memory for a struct that contains pointer members does allocate memory for the pointers, but not memory for them to point to.