Issue

I'm trying to use functions from mman.h in my c code. The code compiles fine with g++/clang++, but when using gcc/clang it says that memfd_create has not been declared, however the code still runs fine.

I tried compiling online with godbolt and it's the same as locally, so I doubt it's something wrong with my setup. Does anyone know why this is happening? I'm using gcc 11.3 and clang 14.

#include <stdint.h>
#include <stdio.h>

#define _GNU_SOURCE
#include <sys/mman.h>

int main()
{
    int32_t fd = memfd_create("", 0);
    
    if (fd == -1) 
    {
        printf("Error creating fd\n");
        return -1;  
    }   
    
    return 0;
}

Compile warning:

main.c:9:15: warning: implicit declaration of function 'memfd_create' is invalid in C99 [-Wimplicit-function-declaration]
        int32_t fd = memfd_create("", 0);

Solution

_GNU_SOURCE has to be before any #include. See man feature_test_macros.

#define _GNU_SOURCE
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>

Answered By - KamilCuk

Answer Checked By - Terry (WPSolving Volunteer)

Sunday, October 30, 2022

[SOLVED] Implicit declaration of 'memfd_create'

Issue

Solution