gdb/ddd Program received signal SIGILL

Question

I wrote a very simple program in Linux using c++, which downloads images from some website over http (basically developed a http client request), using cURL libraries. http://curl.haxx.se/libcurl/c/allfuncs.html

#define CURL_STATICLIB
#include <stdio.h>
#include <stdlib.h>
#include </usr/include/curl/curl.h>
#include </usr/include/curl/stdcheaders.h>
#include </usr/include/curl/easy.h>

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

int main(void) {
    CURL *curl;
    FILE *fp;
    CURLcode res;

    char *url = "http://www.example.com/test_img.png"; 
    char outfilename[FILENAME_MAX] = "/home/c++_proj/output/web_req_img.png";
    curl = curl_easy_init();
    if (curl) {
        fp = fopen(outfilename,"wb");
        curl_easy_setopt(curl, CURLOPT_URL, url);
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
        res = curl_easy_perform(curl);
        /* always cleanup */
        curl_easy_cleanup(curl);
        fclose(fp);
    }
    return 0;
}

I verified the code, and it works fine. I can see the image is downloaded and that I can view the image (with no errors or warnings). Since I plan on expanding my code, I tried to install ddd, and use the debugger, but the debugger doesn't work, and my program exits with some sort of Signal errors, when I try to run my program with ddd.

This is the error:

 (Threadd debugging using libthread_db enabled)
 Using host libthread_db library "/lib/arm-linux-gnueadihf/libthread_db.so.1"

 Program received signal SIGILL, illegal instruction.
 0xb6a5c4C0 in ?? () from /usr/lib/arm-linux-gnueadbihf/libcrypto.so.1.0.0

First I thought that I didn't properly install ddd, so I went back to gdb, but I get the exact same errors, when I run the program. (And I believe that I am using the latest version of gdb and ddd)

Then I tried to use ddd on another simple program, that doesn't involve cURL library, and it worked fine !!!

Does anyone know why this is the case, and what is the solution? Do I somehow need to point to cURL libraries while ddd is running? But, in the past, I don't recall doing this with different set of libraries! Maybe it is something abuot the cURL that ddd doesn't like? But the program runs fine itself without the debugger! I would appreciate some help.

Answer 1

I am guessing it may be part of some instruction set detection code. Just let the program continue and see if it handles the signal by itself (since it runs outside of gdb, it probably does). Alternatively, you can tell gdb to not bother you with SIGILL at all before you run the program: handle SIGILL pass nostop noprint.

It's only a problem if the program dies, which was not clear from your question.

Answer 2

 Program received signal SIGILL, illegal instruction.
 0xb6a5c4C0 in ?? () from /usr/lib/arm-linux-gnueadbihf/libcrypto.so.1.0.0

Does anyone know why this is the case, and what is the solution?

Jester gave you the solution. Here's the reason why it happens.

libcrypto.so is OpenSSL's crypto library. OpenSSL performs cpu feature probes by executing an instruction to see if its available. If a SIGILL is generated, then the feature is not available and an appropriate function is used instead.

The reason you see them on ARM and not IA-32 is, on Intel's IA-32 the cpuid instruction is non-privileged. Any program can execute cpuid to detect cpu features so there's no need for SIGILL-based feature program.

In contrast to IA-32, ARM's equivalent of cpuid is a privileged instruction. Your program needs Exception Level 1 (EL-1), but your program runs at EL-0. To side step the need for privileges on ARM programs setup a jmpbuf and install a SIGILL handler. They then try the instruction in question and the SIGILL handler indicates if the instruction or feature is available or not.

OpenSSL recently changed to SIGILL-free feature detection on some Apple platforms because Apple corrupts things. Also see PR 3108, SIGILL-free processor capabilities detection on MacOS X. Other libraries are doing similar. Also see How to determine ARMv8 features at runtime?

OpenSSL also documents the SIGILL behavior in their FAQ. See item 17 in the OpenSSL FAQ for more details: When debugging I observe SIGILL during OpenSSL initialization: why? Also see SSL_library_init cause SIGILL when running under gdb on Stack Overflow.