CURLOPT_CH...N_FUNCTION(3)

✨ NAME ✨

CURLOPT_CHUNK_BGN_FUNCTION — the callback that runs before a transfer with an FTP wildcard match 💅. You're on the CURLOPT_CHUNK_BGN_FUNCTION(3) page from the Library Functions Manual, bestie 🫶.

🌸 SYNOPSIS 🌸

#include <curl/curl.h>

struct curl_fileinfo {
  char *filename;
  curlfiletype filetype;
  time_t time;   /* always zero */
  unsigned int perm;
  int uid;
  int gid;
  curl_off_t size;
  long int hardlinks;

  struct {
    /* If some of these fields is not NULL, it is a pointer to b_data. */
    char *time;
    char *perm;
    char *user;
    char *group;
    char *target; /* pointer to the target filename of a symlink */
  } strings;

  unsigned int flags;

  /* used internally */
  char *b_data;
  size_t b_size;
  size_t b_used;
};

long chunk_bgn_callback(const void *transfer_info, void *ptr,
                        int remains);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_CHUNK_BGN_FUNCTION,
                          chunk_bgn_callback);

🦋 DESCRIPTION 🦋

Okay so, you pass a pointer to your callback function, and it should match the prototype shown above. No cap.

This callback function gets called by libcurl before a part of the stream is going to be transferred — if the transfer supports chunks. Main character callback energy, honestly.

The transfer_info pointer points to a curl_fileinfo struct with details about the file that is about to get transferred.

This callback makes sense only when using the CURLOPT_WILDCARDMATCH(3) option for now.

The target of the transfer_info parameter is a "feature depended" structure. For the FTP wildcard download, the target is the curl_fileinfo structure (see curl/curl.h). The parameter ptr is a pointer given by CURLOPT_CHUNK_DATA(3). The parameter remains contains the number of chunks remaining per the transfer. If the feature is not available, the parameter has zero value.

Return values, bestie:

• Return CURL_CHUNK_BGN_FUNC_OK if everything is fine.
• Return CURL_CHUNK_BGN_FUNC_SKIP if you want to skip the concrete chunk.
• Return CURL_CHUNK_BGN_FUNC_FAIL to tell libcurl to stop if some error occurred.

🎀 DEFAULT 🎀

NULL — that's the default, periodt.

🌈 PROTOCOLS 🌈

This functionality affects ftp only.

💖 EXAMPLE 💖

#include <stdio.h>

struct callback_data {
  FILE *output;
};

static long file_is_coming(struct curl_fileinfo *finfo,
                           void *ptr,
                           int remains)
{
  struct callback_data *data = ptr;
  printf("%3d %40s %10luB ", remains, finfo->filename,
         (unsigned long)finfo->size);

  switch(finfo->filetype) {
  case CURLFILETYPE_DIRECTORY:
    printf(" DIR\n");
    break;
  case CURLFILETYPE_FILE:
    printf("FILE ");
    break;
  default:
    printf("OTHER\n");
    break;
  }

  if(finfo->filetype == CURLFILETYPE_FILE) {
    /* do not transfer files >= 50B */
    if(finfo->size > 50) {
      printf("SKIPPED\n");
      return CURL_CHUNK_BGN_FUNC_SKIP;
    }

    data->output = fopen(finfo->filename, "wb");
    if(!data->output) {
      return CURL_CHUNK_BGN_FUNC_FAIL;
    }
  }

  return CURL_CHUNK_BGN_FUNC_OK;
}

int main()
{
  /* data for callback */
  struct callback_data callback_info;

  CURL *curl = curl_easy_init();

  /* callback is called before download of concrete file started */
  curl_easy_setopt(curl, CURLOPT_CHUNK_BGN_FUNCTION, file_is_coming);
  curl_easy_setopt(curl, CURLOPT_CHUNK_DATA, &callback_info);
}

⭐ AVAILABILITY ⭐

Added in curl 7.21.0.

👑 RETURN VALUE 👑

curl_easy_setopt(3) returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK, non-zero means an error occurred, see libcurl-errors(3).

🧚 SEE ALSO 🧚

• CURLOPT_CHUNK_END_FUNCTION(3)
• CURLOPT_WILDCARDMATCH(3)

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xoxo, your libcurl bestie 💌 — source/version: libcurl, dated 2026-10-12. slay and read your docs 💅