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                     23: .TH CURLOPT_WRITEFUNCTION 3 "April 06, 2020" "libcurl 7.70.0" "curl_easy_setopt options"
                     24: 
                     25: .SH NAME
                     26: CURLOPT_WRITEFUNCTION \- set callback for writing received data
                     27: .SH SYNOPSIS
                     28: .nf
                     29: #include <curl/curl.h>
                     30: 
                     31: size_t write_callback(char *ptr, size_t size, size_t nmemb, void *userdata);
                     32: 
                     33: CURLcode curl_easy_setopt(CURL *handle, CURLOPT_WRITEFUNCTION, write_callback);
                     34: .SH DESCRIPTION
                     35: Pass a pointer to your callback function, which should match the prototype
                     36: shown above.
                     37: 
                     38: This callback function gets called by libcurl as soon as there is data
                     39: received that needs to be saved. For most transfers, this callback gets called
                     40: many times and each invoke delivers another chunk of data. \fIptr\fP points to
                     41: the delivered data, and the size of that data is \fInmemb\fP; \fIsize\fP is
                     42: always 1.
                     43: 
                     44: The callback function will be passed as much data as possible in all invokes,
                     45: but you must not make any assumptions. It may be one byte, it may be
                     46: thousands. The maximum amount of body data that will be passed to the write
                     47: callback is defined in the curl.h header file: \fICURL_MAX_WRITE_SIZE\fP (the
                     48: usual default is 16K). If \fICURLOPT_HEADER(3)\fP is enabled, which makes
                     49: header data get passed to the write callback, you can get up to
                     50: \fICURL_MAX_HTTP_HEADER\fP bytes of header data passed into it. This usually
                     51: means 100K.
                     52: 
                     53: This function may be called with zero bytes data if the transferred file is
                     54: empty.
                     55: 
                     56: The data passed to this function will not be zero terminated!
                     57: 
                     58: Set the \fIuserdata\fP argument with the \fICURLOPT_WRITEDATA(3)\fP option.
                     59: 
                     60: Your callback should return the number of bytes actually taken care of. If
                     61: that amount differs from the amount passed to your callback function, it'll
                     62: signal an error condition to the library. This will cause the transfer to get
                     63: aborted and the libcurl function used will return \fICURLE_WRITE_ERROR\fP.
                     64: 
                     65: If your callback function returns CURL_WRITEFUNC_PAUSE it will cause this
                     66: transfer to become paused.  See \fIcurl_easy_pause(3)\fP for further details.
                     67: 
                     68: Set this option to NULL to get the internal default function used instead of
                     69: your callback. The internal default function will write the data to the FILE *
                     70: given with \fICURLOPT_WRITEDATA(3)\fP.
                     71: .SH DEFAULT
                     72: libcurl will use 'fwrite' as a callback by default.
                     73: .SH PROTOCOLS
                     74: For all protocols
                     75: .SH AVAILABILITY
                     76: Support for the CURL_WRITEFUNC_PAUSE return code was added in version 7.18.0.
                     77: .SH RETURN VALUE
                     78: This will return CURLE_OK.
                     79: .SH EXAMPLE
                     80: .NF
                     81:  struct memory {
                     82:    char *response;
                     83:    size_t size;
                     84:  };
                     85: 
                     86:  static size_t cb(void *data, size_t size, size_t nmemb, void *userp)
                     87:  {
                     88:    size_t realsize = size * nmemb;
                     89:    struct memory *mem = (struct memory *)userp;
                     90: 
                     91:    char *ptr = realloc(mem->response, mem->size + realsize + 1);
                     92:    if(ptr == NULL)
                     93:      return 0;  /* out of memory! */
                     94: 
                     95:    mem->response = ptr;
                     96:    memcpy(&(mem->response[mem->size]), data, realsize);
                     97:    mem->size += realsize;
                     98:    mem->response[mem->size] = 0;
                     99: 
                    100:    return realsize;
                    101:  }
                    102: 
                    103:  struct memory chunk;
                    104: 
                    105:  /* send all data to this function  */
                    106:  curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, cb);
                    107: 
                    108:  /* we pass our 'chunk' struct to the callback function */
                    109:  curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, (void *)&chunk);
                    110: .FI
                    111: .SH "SEE ALSO"
                    112: .BR CURLOPT_WRITEDATA "(3), " CURLOPT_READFUNCTION "(3), "
                    113: .BR CURLOPT_HEADERFUNCTION "(3), "