在 strcat() 中编程段错误

Question

我正在尝试用 C 语言编写一个 Web 服务器，但我的代码目前出现了段错误，我不知道为什么。它似乎与我的 strcats 有关，但据我所知。我已经发布了代码和 gdb 输出。任何帮助是极大的赞赏。

*** 代码***

    /*  web-server.c    */

    #include <arpa/inet.h>
    #include <netdb.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <unistd.h>

    #define BUFLEN 1500
    #define BACKLOG 10

    static int server_socket(int port) {
int fd;
struct sockaddr_in servaddr;

if ((fd = socket(AF_INET, SOCK_STREAM, 0)) == -1){
    perror("Unable to create socket");
    return -1;
}
printf("created socket\n");
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = INADDR_ANY;
servaddr.sin_port = htons(port);

if (bind(fd, (struct sockaddr *)&servaddr, sizeof(servaddr)) == -1) {
    perror("Unable to bind to port");
    return -1;
}
printf("binding completed\n");
if (listen(fd, BACKLOG) == -1){
    perror("Unable to listen for connections");
    return -1;
}
printf("socket setup\n");
return fd;

    }

    static char *html_request(char *request) {
char request_code[3], url[BUFLEN], http_code[BUFLEN];
printf(request);
printf("\n");
sscanf(request, "%3s %s %s", request_code, url, http_code);
return url;
    }


    int main() {
int connfd, servfd, rlen, i, eof;
int content_length = 0;
char buf[BUFLEN];
struct sockaddr_in saddr;
char *response;
char *read_buffer = malloc(1);
    char *response200 = "HTTP/1.1 200 OK\nContent-Type: text/html\nConnection: close\nConnection-Length: ";
char *response404 = "HTTP/1.1 404 Not Found\nContent-Type: text/html\nConnection: close\nConnection-Length: ";
FILE *in;
char buffer[1500], content_buffer[5];
char *headers = malloc(BUFLEN);
char *output = malloc(BUFLEN);
socklen_t saddr_len = sizeof(saddr);
servfd = server_socket(8080);
printf("before connection\n");
if ((connfd = accept(servfd, (struct sockaddr *)&saddr, &saddr_len)) == -1) {
    perror("Unable to accept connection");
    return -1;
}
else {
printf("Accept connection\n");

}
if ((rlen = read(connfd, buf, BUFLEN)) > 0) {
    realloc(read_buffer,rlen);
    for (i = 0; i < rlen; i++){
        printf("%c", buf[i]);
        read_buffer[i] = buf[i];
    }
    printf("\n");

}
printf("IMMA BEAST!\n");
response = html_request(read_buffer);
printf(response);

if (strcmp(response, "/index.html") == 0){
    printf("\nMatches!");
    in = fopen("index.txt", "r");
    if (in == NULL){
        printf("Read fail!");
    }
    while ((eof = fread(buffer, 1, 296, in )) > 0){
        content_length = content_length + 1;
    }
    snprintf(content_buffer, 5, "%d" ,content_length);
    headers = strcat(response200, content_buffer);
}
else{
    printf("\nFails!");
    in = fopen("index-error.txt", "r");
    if (in == NULL){
        printf("Read fail!");
    }
    while ((eof = fread(buffer, 1, 300, in )) > 0){
        content_length = content_length + 1;
    }
    snprintf(content_buffer, 5, "%d" ,content_length);
    headers = strcat(response404, content_buffer);
}
fclose(in);
output = strcat(headers, buffer);
write(connfd, output, BUFLEN);
close(connfd);
free(read_buffer);
free(headers);
free(output);

return 0;

}

* GDB 输出 *

Program received signal SIGSEGV, Segmentation fault.
0x0000003d4647eff0 in strcat () from /lib64/libc.so.6
(gdb) where
#0  0x0000003d4647eff0 in strcat () from /lib64/libc.so.6
#1  0x0000000000400ecd in main ()
(gdb) n
Single stepping until exit from function strcat,
which has no line number information.

Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.

Answer 1

这根本不行：

strcat("HTTP/1.1 404 Not Found\nContent-Type: text/html\nConnection: close\nConnection-Length: ", content_buffer);

strcat() 将第二个参数附加到第一个参数。第一个参数是一个文字字符串，绝对不能修改。 C 允许它编译的唯一原因是不幸的事实，即旧代码依赖于类型为 char[] 而不是 const char[] 的文字字符串。

Answer 2

http://www.manpagez.com/man/3/strcat/

您必须将第一个字符串分配到正确的大小（第一个字符串的长度 + 第二个字符串的长度 + 1。

http://www.manpagez.com/man/3/malloc/

类似：

char *str;
int len = (strlen("HTTP/1.1 200 OK\nContent-Type: text/html\nConnection: close\nConnection-Length: ") + strlen(content_buffer));

str = malloc((len + 1) * sizeof(char));
bzero(str, len);
strcpy(str, "HTTP/1.1 200 OK\nContent-Type: text/html\nConnection: close\nConnection-Length: ");
strcat(str, content_buffer);
str[len] = 0;