Why isn't libcurl doing anything?

@[email protected] · 27 days ago

Why isn't libcurl doing anything?

@[email protected] · edit-2 27 days ago

As a sanity check, does this work?

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

size_t save_to_disk(char* ptr, size_t size, size_t nmemb, void* user_data)
{
    /* according to curl's docs size is always 1 */
    
    FILE* fp = (FILE*)user_data;
    fprintf(stderr, "got %lu bytes\n", nmemb);
    return fwrite(ptr, size, nmemb, fp);
}

int main(int argc, char* argv[])
{
    char errbuf[CURL_ERROR_SIZE];
    FILE* fp = NULL;
    CURLcode res;
    
    CURL* curl = curl_easy_init();
    
    if(!curl)
    {
        fprintf(stderr, "Failed to initialize curl\n");
        return EXIT_FAILURE;
    }
    
    fp = fopen("output.data", "wb");
    if(!fp)
    {
        fprintf(stderr, "Failed to open file for writing!");
        return EXIT_FAILURE;
    }
    
    curl_easy_setopt(curl, CURLOPT_URL, "https://www.wikipedia.org");
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, save_to_disk);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
    curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, errbuf);
    
    errbuf[0] = 0;  /* set error buffer to empty string */
    res = curl_easy_perform(curl);
    
    if(fp)
    {
        fclose(fp);
        fp = NULL;
    }
    
    if(res != CURLE_OK)
    {
        fprintf(stderr, "error code   : %d\n", res);
        fprintf(stderr, "error buffer : %s\n", errbuf);
        fprintf(stderr, "easy_strerror: %s\n", curl_easy_strerror(res));
        
        return EXIT_FAILURE;
    }
    else
    {
        fprintf(stderr, "\nDone\n");
        return EXIT_SUCCESS;
    }
}

That should write a file called output.data with the HTML from https://www.wikipedia.org and print out the number of bytes each time the write callback receives data for processing.

On my machine, it prints the following when it works successfully (byte counts may vary for you):

got 13716 bytes
got 16320 bytes
got 2732 bytes
got 16320 bytes
got 16320 bytes
got 128 bytes
got 16320 bytes
got 16320 bytes
got 1822 bytes

Done

If I change the URL to nonsense instead to make it fail, it prints text like this on my system:

error code   : 6
error buffer : Could not resolve host: nonsense
easy_strerror: Couldn't resolve host name

Edit: corrected missing line in source (i.e. added line with CURLOPT_ERRORBUFFER which is needed to get extra info in the error buffer on failure, of course)

Edit 2: tweaks to wording to try to be more clear

@[email protected] · 27 days ago

PS C:\Users\USERNAME\3ds> gcc test.c -o test.exe -IC:\Users\22.tom.carroll\scoop\apps\curl\current\include -LC:\Users\22.tom.carroll\scoop\apps\curl\current\lib -lcurl
PS C:\Users\USERNAMEl\3ds> ./test.exe
PS C:\Users\USERNAME3ds> ls


    Directory: C:\Users\USERNAME\3ds


Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
d-----        19/11/2024     20:48                c
-a----        29/11/2024     21:35           1880 file.c
-a----        29/11/2024     22:52           1409 test.c
-a----        29/11/2024     22:52         236221 test.exe


PS C:\Users\USERNAME\3ds>

Its not working unfortunatly.

@[email protected] · 27 days ago

Does hello world work? You should’ve gotten at least some console output.

#include <stdio.h>

int main()
{
    fprintf(stderr, "Hello world\n");
    return 0;
}

@[email protected] · 27 days ago

That works perfectly

PS C:\Users\username\3ds> gcc test.c -o test.exe 
PS C:\Users\username\3ds> ./test.exe
    Hello world

@[email protected] · 27 days ago

Try adding some prints to stderr through my earlier test program then and see if you can find where it stops giving you output. Does output work before curl_easy_init? After it? Somewhere later on?

Note that I did update the program to add the line with CURLOPT_ERRORBUFFER – that’s not strictly needed, but might provide more debug info if something goes wrong later in the program. (Forgot to add the setup line initially despite writing the rest of it… 🤦‍♂️️)

You could also try adding curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L); to get it to explain more details about what it’s doing internally if you can get it to print output at all.