Cannot use mincore to check whether page allocated by malloc is in ram

Question

Question: Why do mincore failed with "Invalid argument" in the following code?

int main(int argc, char **argv) {
    char *ptr;
    unsigned int num_of_byte_allocated = 4;
    size_t page_size = getpagesize();
    size_t allocated_size = num_of_byte_allocated * page_size;
    unsigned char *vec = calloc(num_of_byte_allocated, sizeof(unsigned char));

    ptr = malloc(allocated_size);
    if (!ptr) {
       perror("malloc failed");
    }

    if (mincore(ptr, allocated_size, vec) == -1) {
        perror("mincore failed");
    } else {
        for (int i = 0; i < num_of_byte_allocated; i++) {
            printf("Page[%i]: %i\n", i, ptr[i] & 1);
        }
    }
}

However when I allocate with mmap then it works.

0___________ · Accepted Answer · 2025-02-18 01:26:07Z

The pointer has to be page size aligned.


void handleMincoreError(int errCode)
{
    switch (errCode)
    {
        case EAGAIN:
            fprintf(stderr, "mincore failed: Resources temporarily unavailable (EAGAIN). Try again later.\n");
            break;
        case EFAULT:
            fprintf(stderr, "mincore failed: The specified address range is not fully allocated (EFAULT).\n");
            break;
        case EINVAL:
            fprintf(stderr, "mincore failed: The address is not aligned or allocated_size is invalid (EINVAL).\n");
            break;
        case ENOMEM:
            fprintf(stderr, "mincore failed: Memory mapping failed, possibly due to lack of memory (ENOMEM).\n");
            break;
        default:
            fprintf(stderr, "mincore failed: %s (errno: %d)\n", strerror(errCode), errCode);
            break;
    }
}

void* alignToPage(void* addr)
{
    size_t pageSize = getpagesize();
    return (void*)((uintptr_t)addr & ~(pageSize - 1));
}


int main(int argc, char **argv) {
    char *ptr;
    unsigned int num_of_byte_allocated = 512;
    size_t page_size = getpagesize();
    size_t allocated_size = num_of_byte_allocated * page_size;
    unsigned char *vec = calloc(num_of_byte_allocated, sizeof(unsigned char));
    int err; 

    ptr = malloc(allocated_size);
    if (!ptr) {
       perror("malloc failed");
    }

    if ((err = mincore(alignToPage(ptr), allocated_size, vec)))
    {
        handleMincoreError(errno);
    } 

 /*....*/

Eric Postpischil · Accepted Answer · 2025-02-17 17:41:40Z

1

mincore requires the first address it is passed to be a multiple of the page size. malloc does not generally return an address that is page-aligned, even if the requested allocation amount is a multiple of the page size.

answered Feb 17 at 17:41

Eric Postpischil

233k15 gold badges199 silver badges379 bronze badges

Collectives™ on Stack Overflow

Cannot use mincore to check whether page allocated by malloc is in ram

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related