将IP地址char转换为long

Question

I wrote a small program to convert a char to a long, however when I try to print the char form, it has mangled output.

int main()
{
    long l;
    char *buf = "127.0.0.1";
    memcpy(&l, buf, sizeof(long));
    printf("%s\n", (char *)&l);

    return 0;
}

The output looks like this:

127.0.0.t 턾U

I know I'm missing something simple but I'm drawing a blank.

Answer 1

Seems as if there are some basic misunderstandings :-)

Copying between char * and long data type does not convert a "number" represented by a sequence of digits into a "binary" number like long; neither does a cast from long to char * do the opposite in your printf -statement.

Converting char* to long is done using atol , whereas printing a long to console is done through printf("%ld",l)

Good luck and patience on your way to get an experienced C programmer, and don't hesitate to ask :-)

Answer 2

Here is a program that takes an IP address in the form of an input string, and converts it to a long value. The filter_ip() function is used to first remove all non-numeric characters from the string. Then strtol() is used to convert the string to a long value.

It is better to use strtol() than atol() because strtol() detects integer overflows. errno is declared in the error.h header file, and strtol() sets the value of errno if there is an overflow. strtol() converts as many characters as it can, starting from the beginning of the string, and sets tailptr to point the the remaining characters. If no conversion can be performed, tailptr will point to the beginning of the string.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>

void filter_ip(char *str);

int main(void)
{
    char ip_addr[] = "127.0.0.1";
    long res;
    char *tailptr;

    printf("IP Address: %s\n", ip_addr);
    filter_ip(ip_addr);

    errno = 0;
    res = strtol(ip_addr, &tailptr, 10);
    if (errno) {
        perror("Unable to convert IP address");
    } else if (tailptr == ip_addr) {
        fprintf(stderr, "No conversion performed\n");
    } else {
        printf("%ld\n", res);
    }

    return 0;
}

void filter_ip(char *str)
{
    size_t i, j;

    for (i = 0, j = 0; str[i] != '\0'; i++) {
        if (isdigit(str[i])) {
            str[j++] = str[i];
        }
    }
    str[j] = '\0';
}

Program output:

IP Address: 127.0.0.1
127001