C libcurl get output into a string

You can set a callback function to receive incoming data chunks using curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, myfunc);

The callback will take a user defined argument that you can set using curl_easy_setopt(curl, CURLOPT_WRITEDATA, p)

Here’s a snippet of code that passes a buffer struct string {*ptr; len} to the callback function and grows that buffer on each call using realloc().

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

struct string {
  char *ptr;
  size_t len;
};

void init_string(struct string *s) {
  s->len = 0;
  s->ptr = malloc(s->len+1);
  if (s->ptr == NULL) {
    fprintf(stderr, "malloc() failed\n");
    exit(EXIT_FAILURE);
  }
  s->ptr[0] = '\0';
}

size_t writefunc(void *ptr, size_t size, size_t nmemb, struct string *s)
{
  size_t new_len = s->len + size*nmemb;
  s->ptr = realloc(s->ptr, new_len+1);
  if (s->ptr == NULL) {
    fprintf(stderr, "realloc() failed\n");
    exit(EXIT_FAILURE);
  }
  memcpy(s->ptr+s->len, ptr, size*nmemb);
  s->ptr[new_len] = '\0';
  s->len = new_len;

  return size*nmemb;
}

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

  curl = curl_easy_init();
  if(curl) {
    struct string s;
    init_string(&s);

    curl_easy_setopt(curl, CURLOPT_URL, "curl.haxx.se");
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writefunc);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &s);
    res = curl_easy_perform(curl);

    printf("%s\n", s.ptr);
    free(s.ptr);

    /* always cleanup */
    curl_easy_cleanup(curl);
  }
  return 0;
}