CURLOPT\_CHUNK\_BGN\_FUNCTION(3) Library Functions Manual CURLOPT\_CHUNK\_BGN\_FUNCTION(3)

# [NAME](#NAME)

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

# [SYNOPSIS](#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](#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)*](/man/os/curl/CURLOPT_WILDCARDMATCH.3) 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)*](/man/os/curl/CURLOPT_CHUNK_DATA.3). 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](#DEFAULT)

NULL

# [PROTOCOLS](#PROTOCOLS)

This functionality affects ftp only

# [EXAMPLE](#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](#AVAILABILITY)

Added in curl 7.21.0

# [RETURN VALUE](#RETURN_VALUE)

[*curl\_easy\_setopt(3)*](/man/os/curl/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)*](/man/os/curl/libcurl-errors.3).

# [SEE ALSO](#SEE_ALSO)

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

2026-10-31 libcurl

---

Document: CURLOPT_CHUNK_BGN_FUNCTION(3) from curl 8.23.0-r3 (Chainguard OS)
Source: https://github.com/curl/curl.git