Question

What could possible by the problem when the errno value is not updated during successive calls to socket functions?

socket (AF_INET, -1, 0);
socket (AF_INET, SOCK_STREAM, -1);

The first should have errno = EINVAL
The second should have errno = EPROTONOSUPPORT

---

From the code provided below by @JonathanLeffler, output from cygwin is as follows:

*$ ./socket.exe
Error from socket(AF_INET, -1, 0): 124 (Socket type not supported)
-- errno = 124 (Socket type not supported)
Error from socket(AF_INET, SOCK_STREAM, -1): 123 (Protocol not supported)
-- errno = 123 (Protocol not supported)
socket(AF_INET, SOCK_STREAM, 0) succeeded (fd = 3)
-- errno = 123 (Protocol not supported)*

---

The said code below was edited to create a socket fd0 before creating the socket

int fd0 = socket(AF_INET, SOCK_STREAM, 0);
if (fd0 < 0)
    printf("Error from socket(AF_INET, SOCK_STREAM, 0): %d (%s)\n", errno, strerror(errno));
else
{
    printf("socket(AF_INET, SOCK_STREAM, 0) succeeded (fd = %d)\n", fd0);
    close(fd0);
}
printf("-- errno = %d (%s)\n", errno, strerror(errno));

int fd1 = socket(AF_INET, -1, 0);
.....

And the result is as follows:

*$ ./socket.exe
socket(AF_INET, SOCK_STREAM, 0) succeeded (fd = 3)
-- errno = 0 (No error)
Error from socket(AF_INET, -1, 0): 124 (Socket type not supported)
-- errno = 124 (Socket type not supported)
Error from socket(AF_INET, SOCK_STREAM, -1): 123 (Protocol not supported)
-- errno = 123 (Protocol not supported)
socket(AF_INET, SOCK_STREAM, 0) succeeded (fd = 3)
-- errno = 123 (Protocol not supported)*

---

It is expected that the first and the last socket creation should have the same errno values.

---

But how can this output be explained? 5th and 6th socket creation have the same errno value but the cause of the error is different.

$ ./test_select.exe
[1] ierr = 124, iSocket = -1 socket(AF_INET, -1, 0);
 : Socket type not supported
[2] ierr = 124, iSocket = -1 socket(AF_INET, -1, 0);
 : Socket type not supported
[3] ierr = 124, iSocket = 3 socket(AF_INET, SOCK_STREAM, 0);
 : Socket type not supported
[4] ierr = 124, iSocket = -1 socket (AF_INET, -1, 0);
 : Socket type not supported
[5] ierr = 123, iSocket = -1 socket (AF_INET, SOCK_STREAM, -1);
 : Protocol not supported
[6] ierr = 123, iSocket = -1 socket (AF_INET, -1, 0)
 : Protocol not supported
[7] ierr = 124, iSocket = -1 socket(AF_INET, SOCK_STREAM, -1)
 : Protocol not supported

Actual code is as follows:

   sockfd = socket(AF_INET, -1, 0);
    err = errno;
    printf("[1] err = %d, sockfd = %d socket(AF_INET, -1, 0);\n", err, sockfd);
    perror(" ");
    sockfd = socket(AF_INET, -1, 0);
     err = errno;
    printf("[2] err = %d, sockfd = %d socket(AF_INET, -1, 0);\n", err, sockfd);
    perror(" ");
     sockfd = socket(AF_INET, SOCK_STREAM, 0);
    err = errno;
    printf("[3] err = %d, sockfd = %d socket(AF_INET, SOCK_STREAM, 0);\n", err, sockfd);
    perror(" ");
    close(sockfd);
    sockfd = socket(AF_INET, -1, 0);
    err = errno;
    printf("[4] err = %d, sockfd = %d socket (AF_INET, -1, 0);\n", err, sockfd);
    perror(" ");
    sockfd = socket(AF_INET, SOCK_STREAM, -1);
    err = errno;
    printf("[5] err = %d, sockfd = %d socket (AF_INET, SOCK_STREAM, -1);\n", err, sockfd);
    perror(" ");
    sockfd = socket(AF_INET, -1, 0);
    err = errno;
    printf("[6] err = %d, sockfd = %d socket (AF_INET, -1, 0)\n", err, sockfd);
    perror(" ");
    err = errno;
    sockfd = socket(AF_INET, SOCK_STREAM, -1);
    printf("[7] err = %d, sockfd = %d socket(AF_INET, SOCK_STREAM, -1)\n", err, sockfd);
    perror(" ");

Answer 1

No library function sets errno to zero. If you want it zeroed, your program must do it.

You can only meaningfully test errno for information when a function says it has failed (and even then, only if its manual page indicates that it sets errno). You can find errno set to non-zero by a successful function. For example, on Solaris, you’ll often find ENOTTY in errno after a standard I/O call when writing to a file instead of a terminal.

In your example, there’s no guarantee about which of multiple possible error values will be set by a given erroneous call to a function. As long as one valid error condition is reported, it is up to the system to decide which error it reports. So, you’d need to give a compelling example of why the second call should return EINVAL instead of EBADF; your question as written doesn’t include enough information to let us pontificate.

Also note that the only safe way to declare errno is via the header: #include <errno.h>. In threaded environments in particular, it is often not simply extern int errno;. For example, on Mac OS X, it can be defined as:

extern int * __error(void);
#define errno (*__error())

That is, __error is a function returning a pointer to an integer, and errno is a macro that dereferences that pointer, becoming a modifiable lvalue.

---

Code Analysis

The given code is:

socket (AF_INET, -1, 0);
socket (AF_INET, SOCK_STREAM, -1);

Converted into runnable code, this might become:

#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

int main(void)
{
    int fd1 = socket(AF_INET, -1, 0);
    if (fd1 < 0)
        printf("Error from socket(AF_INET, -1, 0): %d (%s)\n", errno, strerror(errno));
    else
    {
        printf("socket(AF_INET, -1, 0) succeeded (fd = %d)\n", fd1);
        close(fd1);
    }
    printf("-- errno = %d (%s)\n", errno, strerror(errno));

    int fd2 = socket(AF_INET, SOCK_STREAM, -1);
    if (fd2 < 0)
        printf("Error from socket(AF_INET, SOCK_STREAM, -1): %d (%s)\n", errno, strerror(errno));
    else
    {
        printf("socket(AF_INET, SOCK_STREAM, -1) succeeded (fd = %d)\n", fd2);
        close(fd2);
    }
    printf("-- errno = %d (%s)\n", errno, strerror(errno));

    int fd3 = socket(AF_INET, SOCK_STREAM, 0);
    if (fd3 < 0)
        printf("Error from socket(AF_INET, SOCK_STREAM, 0): %d (%s)\n", errno, strerror(errno));
    else
    {
        printf("socket(AF_INET, SOCK_STREAM, 0) succeeded (fd = %d)\n", fd3);
        close(fd3);
    }
    printf("-- errno = %d (%s)\n", errno, strerror(errno));

    return(0);
}

Running on Mac OS X 10.7.3, this produces:

Error from socket(AF_INET, -1, 0): 43 (Protocol not supported)
-- errno = 43 (Protocol not supported)
Error from socket(AF_INET, SOCK_STREAM, -1): 43 (Protocol not supported)
-- errno = 43 (Protocol not supported)
socket(AF_INET, SOCK_STREAM, 0) succeeded (fd = 3)
-- errno = 43 (Protocol not supported)

As I said earlier, the error returned depends on the system.