---
name: "CURLOPT_CHUNK_BGN_FUNCTION"
section: "3"
platform: "linux"
package: "curl"
version: "8.23.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/CURLOPT_CHUNK_BGN_FUNCTION"
---

# CURLOPT\_CHUNK\_BGN\_FUNCTION(3)

callback before a transfer with FTP wildcard match

## NAME

CURLOPT\_CHUNK\_BGN\_FUNCTION - callback before a transfer with FTP wildcard match

## 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, only
                     available for CURLFILETYPE_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

Pass a pointer to your callback function, which should match the prototype shown above.

This callback function gets called by libcurl before a part of the stream is going to be transferred (if the transfer supports chunks).

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)](/CURLOPT_WILDCARDMATCH/3.md)* option for now.

The target of transfer\_info parameter is a "feature depended" structure. For the FTP wildcard download, the target is **curl\_fileinfo** structure (see *curl/curl.h*). The parameter *ptr* is a pointer given by *[CURLOPT\_CHUNK\_DATA(3)](/CURLOPT_CHUNK_DATA/3.md)*. The parameter remains contains number of chunks remaining per the transfer. If the feature is not available, the parameter has zero value.

Return *CURL\_CHUNK\_BGN\_FUNC\_OK* if everything is fine, *CURL\_CHUNK\_BGN\_FUNC\_SKIP* if you want to skip the concrete chunk or *CURL\_CHUNK\_BGN\_FUNC\_FAIL* to tell libcurl to stop if some error occurred.

## DEFAULT

NULL

## 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(void)
{
  /* 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)](/curl_easy_setopt/3.md)* returns a CURLcode indicating success or error.

CURLE\_OK (0) means everything was OK, non-zero means an error occurred, see *[libcurl-errors(3)](/libcurl-errors/3.md)*.

## SEE ALSO

[**CURLOPT\_CHUNK\_END\_FUNCTION**(3)](/CURLOPT_CHUNK_END_FUNCTION/3.md), [**CURLOPT\_WILDCARDMATCH**(3)](/CURLOPT_WILDCARDMATCH/3.md)