Question

I cant work out where my code is segfaulting?

Basically it connects to a server over an SSL (HTTPS) secured connection and makes a GET, supplying a MachineID (which the program takes as a parameter).

Some custom headers should also be set. I then save the body returned, and the header, as separate text files. (Basically it will be getting a shell script to run, so my remote systems can automagically grab ‘orders’ from my server)

But it segfaults and the only linux machine I have to develop this on is my VPS which, due to thier strange virtualisation, always crashes loading GDB… 😮

Can anyone tell me where the problem is? – I think its almost certainly in my string concatenation – where i build up the header, and request URL.

EDIT: Duh, forgot the code !

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <curl/curl.h>
#include <string.h>

 static size_t write_data(void *ptr, size_t size, size_t nmemb, void *stream)
{
 int written = fwrite(ptr, size, nmemb, (FILE *)stream);
 return written;
}

int main(int argc, char *argv[])
{
static const char *headerfilename = "head.out";
static const char *bodyfilename = "body.out";

char *url = "https://fakeserver.fakesite.com:8443/SystemManager/getOrders.jsp?machineID=";
char *customHeader = "MachineID:";
char *machineID = NULL;

struct curl_slist *chunk = NULL;

CURL *curl;
CURLcode res;
FILE *headerfile;
FILE *bodyfile;

    if (argc == 2)
{
    machineID = argv[1];
    strcat(url,machineID);
}
else
{
 printf("Usage: %s <MachineID>\n", argv[0]);
 return 1;
}

 curl_global_init(CURL_GLOBAL_SSL);

 // init the curl session 
 curl = curl_easy_init();

 if(curl) {
    // set URL to get 

 curl_easy_setopt(curl, CURLOPT_URL, url);

    // no progress meter please 
    curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 1L);

    // send all data to this function 
 curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);

 // some servers don't like requests that are made without a user-agent field, so we provide one 
 curl_easy_setopt(curl, CURLOPT_USERAGENT, "libcurl-agent/1.0");

 // Also add a custom MachineID header 
 strcat(customHeader, machineID);
 chunk = curl_slist_append(chunk, customHeader);
 res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, chunk);

    // These tweaks must be enabled for my dodgy self-signed certificate.

    // DONT bother verifying our certificate is signed by a trusted CA.
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L);
    // DONT check the hostname on the certificate matcheds the remote system.
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0L);


    // open the files 
 headerfile = fopen(headerfilename,"w");
 if (headerfile == NULL) {
 curl_easy_cleanup(curl);
 return -1;
 }
 else
 {
 // we want the headers to this file handle 
 curl_easy_setopt(curl, CURLOPT_WRITEHEADER, headerfile);
 }

 bodyfile = fopen(bodyfilename,"w");
 if (bodyfile == NULL) {
 curl_easy_cleanup(curl);
 return -1;
 }
 else
 {
 // we want the body to this file handle 
 curl_easy_setopt(curl, CURLOPT_WRITEDATA, bodyfile);
 }
     // get it! 
 res = curl_easy_perform(curl);

    // close the files 
 fclose(headerfile);
 fclose(bodyfile);

 // always cleanup curl stuff 
 curl_easy_cleanup(curl);
 }
 curl_global_cleanup();
 return 0;
}


												

Answer 1

You are using string literals as destination for strcat. You need to allocate memory to the pointers and then use them as destination in strcpy and strcat functions

A string literal generally resides in RO region and writing to such regions might invoke undefined behavior

Change things like

char *url

to

char url[100];

Also consider using n versions of strcat, strcpy and appropriate buffer size to prevent inadvertent buffer overflows