Cannot Write To Buffer In C

Question

I cannot see anything wrong with this.

#include <stdio.h> #include <stdlib.h> #include <stdint.h> #include <string.h> /* Reference: (0 offset) uint32_t size; (+4 offset) uint32_t elementSize; (+8 offset) uint32_t lastIndex; (+12 offset) void* data */ void* create_pinned_array(uint32_t elementSize, uint32_t numberOfElements, size_t sizeOfData, void* data) { uint32_t size = sizeOfData + 12; void* memory = malloc(size); memset(memory, size, 4); // size memset(memory + 4, elementSize, 4); // elementSize memset(memory + 8, numberOfElements-1, 4); // lastIndex memcpy(memory + 12, data, sizeOfData); // actual data return memory; } int main(void) { int randomList[5] = { 1, 6, 2, 9, 8 }; void* myIntegers = create_pinned_array(sizeof(int), 5, sizeof(randomList), randomList); FILE* testfp = fopen("testfile.bin", "wb"); fwrite(myIntegers, sizeof(myIntegers), 1, testfp); fclose(testfp); free(myIntegers); return 0; }

Each time I check with xxd, instead of an ordered binary file I am getting junk data which varies every time I execute the program.

sizeof(myIntegers) is the size of the pointer, not the mem block. — 001
– 001, Commented Jul 19, 2023 at 20:41
This makes wild assumptions about the endian form of those values that I hope are justified. — tadman
– tadman, Commented Jul 19, 2023 at 20:43
memset(memory, size, 4);?!?!?! Do you understand that memset() sets every byte of what is being set to the same byte value? — Andrew Henle
– Andrew Henle, Commented Jul 19, 2023 at 20:49
And please read both Flexible array member in C-structure and How to correctly use flexible array member? — Andrew Henle
– Andrew Henle, Commented Jul 19, 2023 at 20:53
@AndrewHenle i can't believe i made that mistake. Using memcpy instead fixed it. Thx — Hexa
– Hexa, Commented Jul 19, 2023 at 21:03

selbie · Accepted Answer · 2023-07-20 06:18:53Z

You are using memset when you mean to use memcpy.

memset(ptr, value, N)

Fills the bytes from ptr until ptr+n with value.

memcpy(ptr, src, N) copies N bytes from src into ptr.

Instead of this:

memset(memory, size, 4); // size memset(memory + 4, elementSize, 4); // elementSize memset(memory + 8, numberOfElements-1, 4); // lastIndex memcpy(memory + 12, data, sizeOfData); // actual data

This:

uitn32_t lastIndex = numberOfElements-1; memcpy(memory, &size, 4); // size memcpy(memory + 4, &elementSize, 4); memcpy(memory + 8, &lastIndex, 4); memcpy(memory + 12, data, sizeOfData);

Or you could try this, which should be ok since the malloc'd memory would be aligned. (Some non-intel platforms, alignment is important if the pointer isn't on a 32-bit boundary)

uint32_t* header = (uint32_t*)memory; header[0] = size; header[1] = elementSize; header[2] = nmberOfElements-1; memcpy(memory + 12, data, sizeOfData);

This might not be your only bug.

Chris · Accepted Answer · 2023-07-19 20:46:23Z

Looks like you;'ve made a mistake with sizeof.

sizeof(myIntegers)

Returns the size of the void pointer, while in this case it appears create_pinned_array has returned a block of memory 32 bytes in size (assuming int is 4 bytes). Assuming a 64-bit machine, this means you're writing the first 8 bytes of your data.

Your create_pinned_array function should also check the return of malloc for success before proceeding to work with the results.

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