CODEWITHSUNDEEP
cmemorymalloc
yupe
yupe

Reputation: 127

Why the function malloc can be used like this?

I am reading the book "Expert C Programming", but here is the code that really confused me. I can't understand the use of the function malloc and the value it returns here, and why running this will return the memory allocated in the process? Thanks in advance.

#include<stdio.h>
#include<stdlib.h>
main()
{
    int MB = 0;
    while(malloc(1<<20))  ++MB;
    printf("Allocated %d MB total\n",MB);
}

Upvotes: 2

Views: 115

Answers (2)

ismail akın &#231;elik
ismail akın &#231;elik

Reputation: 343

unsigned int
A = 1 << 0, //1         byte        2^0     0x00000001  0b00000000000000000000000000000001
B = 1 << 8, //256       byte        2^8     0x00000100  0b00000000000000000000000100000000
C = 1 << 10,//1024      byte 1KB    2^10    0x00000400  0b00000000000000000000010000000000  
D = 1 << 20,//1048576   byte 1MB    2^20    0x00100000  0b00000000000100000000000000000000
E = 1 << 28;//256 e6    byte 1GB/4  2^28    0x10000000  0b00010000000000000000000000000000


const int TOP = 100;//25GB max
void* p[TOP] = {};
for (int i = 0; i < TOP; i++)           //32 bit system max 2Gb
{
    p[i] = malloc(0x10000000);          //1GB/4  or 256Mb
    //p[i] = malloc(1<<28);
    //p[i] = malloc(E);

    printf("%4d 0x%08x\n", i, p[i]);        //32 bit system
    //printf("%4d 0x%016llx\n", i, p[i]);   //64 bit system
}
for (int i = 0; i < TOP; i++)
    free(p[i]);

Upvotes: 1

Alex B
Alex B

Reputation: 56

The << operator does a bit-wise left shift. So this shifts binary 1 left by 20 positions. i.e. 0000 0000 0000 0000 0001 becomes 1000 0000 0000 0000 0000. This is equivalent to 1 MB since a MB is defined as 2^20.

Therefore each malloc() call tries to allocate 1 MB. malloc() will return NULL if it cannot allocate this much space. In C, NULL will evaluate to False.

Therefore, in each iteration of the loop, the program will allocate 1 MB. If it's successful, it increases the MB counter. If malloc() runs out of space, it will return NULL and end the loop. The printf statement will then print the amount of memory that was allocated.

Upvotes: 3

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