CODEWITHSUNDEEP
cmacossocketsgcc
taichino
taichino

Reputation: 1155

inet_ntoa() segmentation fault on OSX

I have encountered an issue that inet_ntoa results in segmentation fault on OSX, and this doesn't occur on Linux.

My test code is following.

// sample.c
#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
//#include <arpa/inet.h>  // adding this line prevents the segfault

int main() {
    struct sockaddr_in addr;
    struct sockaddr_in client;

    int sock0 = socket(AF_INET, SOCK_STREAM, 0);
    addr.sin_family = AF_INET;
    addr.sin_port = htons(12345);
    addr.sin_addr.s_addr = INADDR_ANY;

    bind(sock0, (struct sockaddr *)&addr, sizeof(addr));
    listen(sock0, 5);

    while (1) {
        int len = sizeof(client);
        int sock = accept(sock0, (struct sockaddr *)&client, (socklen_t *)&len);
        printf("accepted connection from %s, port=%d\n",
               inet_ntoa(client.sin_addr), ntohs(client.sin_port));
        send(sock, "HELLO", 5, 0);
        close(sock);
    }

    close(sock0);

    return 0;
}

I searched for a while and found a solution which is adding an include "arpa/inet.h". I confirmed inet_ntoa is surely declared in inet.h and I'm sure this is correct way.

But I can't understand why this code doesn't work without it. What happens when I build and link this code without the include line?

I know building this code shows warnings "implicit declaration of function", but link error doesn't occur. This means linker found the implementation of inet_ntoa somehow, I think. Is this wrong?

Thanks! Any advice is highly appreciated.

Upvotes: 2

Views: 1317

Answers (2)

DoxyLover
DoxyLover

Reputation: 3484

I'm going to guess that your OSX system is 64-bit while your Linux system is 32-bit. An undeclared function is assumed to return an integer. If the 64-bit pointer decays to a 32-bit integer which is dereferenced as a pointer, you will likely get a segmentation fault.

Upvotes: 6

sashang
sashang

Reputation: 12194

C permits implicit declarations of functions (Implicit function declarations in C). This is why you don't get a compiler error. Also inet_ntoa is in glibc which is linked by default by gcc, so the linker finds the definition of the function, so you don't get a linker error. The segfault happens because 1) C assumes that, because it is implicitly declared, the function returns an int and 2) on the Mac inet_ntoa in glibc is returning a 64 bit pointer which gets converted to a 32 bit value and loses information.

Upvotes: 4

Related Questions