Getting error code when creating a socket in Linux

Question

I'm doing some socket programming in Linux and am wondering how to get the error code when the function socket(...); fails.

for example for the "getaddrinfo" function i can do this:

//Resolve the server address and port
    result = (struct addrinfo *) calloc(1, sizeof(struct addrinfo));
    iResult = getaddrinfo("google.com", DEFAULT_PORT, &hints, &result);
    if (iResult != 0){
        printf("%d\n", iResult);
        fprintf(stderr, "getaddrinfo failed: %s\n", gai_strerror(iResult));
        getchar();
        exit(EXIT_FAILURE);
    }

However I want to do a similar thing using socket(...) function.

According to this: http://linux.die.net/man/2/socket

the function returns -1 on failure, and sets errno to the appropriate error number. How do i access this "errno" though? This is my code so far:

int connectSocket = 0;
connectSocket = socket(AF_INET, SOCK_STREAM, 0);
printf("%d\n", connectSocket);

if (connectSocket == -1){
    printf("socket failed with error: %s\n", error_string); //TODO: HELP DECLARING error_string
    getchar();
    exit(EXIT_FAILURE);
}

DO NOT allocate memory for the addrinfo that you pass in the last parameter of getaddrinfo(). It will allocate the addrinfo for you, which you pass to freeaddrinfo() to free it: result = NULL; iResult = getaddrinfo(..., &result); if (iResult == 0) { ...; freeaddrinfo(result); } — Remy Lebeau
– Remy Lebeau, Commented Nov 24, 2014 at 6:29

Jonathon Reinhart · Accepted Answer · 2014-11-24 02:32:31Z

19

errno is a thread-local global variable, defined in <errno.h>. The man page for many library functions will indicate that they return -1 on error, and set errno.

You can convert an errno value to a useful string with the strerror function.

In general, you should code like this:

#include <stdio.h>
#include <errno.h>

int main(void) {
    int s;

    s = socket(...);
    if (s < 0) {
        fprintf(stderr, "socket() failed: %s\n", strerror(errno));
        exit(1);
    }
}

Alternatively, glibc's printf and friends support a %m format specifier, which is replaced with strerror(errno) (no argument is needed). So the above example could be replaced with:

    if (s < 0) {
        fprintf(stderr, "socket() failed: %m\n");
        exit(1);
    }

And to make it all simpler, there is the perror function, which prints out a message similar to above.

    if (s < 0) {
        perror("socket");
        exit(1);
    }

Wrapping it all up - error handling need not be complex and verbose. Putting the socket call and the test for < 0 in one statement, the above code could look like this, and you'll be a real UNIX pro:

#include <stdio.h>
#include <errno.h>

int main(void) {
    int s;

    if ((s = socket(...)) < 0) {
        perror("socket");
        exit(1);
    }
}

edited Nov 24, 2014 at 2:32

answered Nov 24, 2014 at 2:27

Jonathon Reinhart

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2 Comments

Yahya Uddin Over a year ago

Oddly enough: "fprintf(stderr, "socket() failed: %s\n", strerror(errno));" didn't seem to work! Instead it would just exit the program. But all the rest did! Many Thanks!

mafso Over a year ago

nit: #define errno (*__get_errno()) would be legal, it's not necessarily a variable, it's a macro "which expands to a modifiable lvalue that has type int and thread local storage".

Jon Lund Steffensen · Accepted Answer · 2014-11-24 02:29:20Z

5

Add #include <errno.h> and you'll be able to read the global errno variable.

connectSocket = socket(AF_INET, SOCK_STREAM, 0);
if (connectSocket < 0) {
    if (errno == EACCESS) ...

You can use perror in stdio.h to print an error message based on the value of errno or you can use strerror in string.h to access a string describing the error code

connectSocket = socket(AF_INET, SOCK_STREAM, 0);
if (connectSocket < 0) {
    perror("socket");
    exit(EXIT_FAILURE);
}

answered Nov 24, 2014 at 2:29

Jon Lund Steffensen

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