CODEWITHSUNDEEP
cmemory-managementsegmentation-faultmmap
Alexi Coard
Alexi Coard

Reputation: 7772

C : mmap initialize at 0xffffffff

I am currently doing a memory allocator for a C project and I am facing something that I don't understand

I'm calling mmap to reserve the heap for my program where i will put some headers.

I have

void * START_ADDRESS;
int MEMORY_INITIALIZED;

void* Mem_Init(int sizeOfRegion){
    START_ADDRESS = mmap(NULL, sizeOfRegion, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_FILE |MAP_PRIVATE, -1,0);

    header_t* startingHeader = START_ADDRESS;

    //rest of the fontion
}

void* Mem_Alloc(int size){
    if(MEMORY_INITIALIZED==0){
    START_ADDRESS= Mem_Init(size); 

    //rest of the funtion
}

When i use the errno on this code it returns me the invalid Argument error.

Why do mmap returns a valid adress when i call it using the Mem_Init method and why do it returns me invalidArgument when i call Mem_Init in the Mem_Alloc method ?

Thanks in advance

Upvotes: 1

Views: 4555

Answers (2)

Alexi Coard
Alexi Coard

Reputation: 7772

I finally found the error.

In the test program the teacher gave us, he tried to allocate a 0 size header, and mmap fails when you try to alocate a null pointer.

A simple correction was

void* Mem_Init(int sizeOfRegion){
    if(sizeOfRegion==0) sizeOfRegion = 1024; //1024 is an exemple
    START_ADDRESS = mmap(NULL, sizeOfRegion, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_FILE |MAP_PRIVATE, -1,0);

    if ((void *) -1 == START_ADDRESS) {
        printf("Could not map memory: %s\n", strerror(errno));
    }

Thanks for the support guys

Upvotes: 1

user3657941
user3657941

Reputation:

With some minor changes, your code appears to work:

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/mman.h>

void * START_ADDRESS;
int MEMORY_INITIALIZED = 0;

void* Mem_Init(int sizeOfRegion){
    START_ADDRESS = mmap(NULL, sizeOfRegion, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_FILE |MAP_PRIVATE, -1,0);

    if ((void *) -1 == START_ADDRESS) {
        printf("Could not map memory: %s\n", strerror(errno));
    }

    printf("%p\n", START_ADDRESS);

    void* startingHeader = START_ADDRESS;

    //rest of the fontion
    return startingHeader;
}

void* Mem_Alloc(int size){
    if(MEMORY_INITIALIZED==0){
    START_ADDRESS= Mem_Init(size); 
    }

    //rest of the funtion
    return START_ADDRESS;
}

int main(void) {
    Mem_Alloc(1024);
    return 0;
}

Compiled using

gcc -Wall -Werror -o code code.c

I thought the most important change is that MEMORY_INITIALIZED is set to 0, but the language lawyers assure me that this is already done by compilers that follow the C89 standard.

Upvotes: 2

Related Questions