docs: use the correct CURLOPT_WRITEFUNCTION signature

In order to not encourage users to use incompatible function pointers,
change the callback function definitions to use `char *` instead of
`void *` for the first argument.

Triggered by https://stackoverflow.com/questions/79921871/curl-c-c-library-based-application-produces-erronious-response-for-http-post-r#comment141032037_79921871 :

"The code was mostly modified from
 [this example code](https://curl.se/libcurl/c/postinmemory.html),
 honestly I never knew this is wrong. Thanks for pointing it out."

Signed-off-by: Ted Lyngmo <ted@lyncon.se>
Closes #21265

docs/FAQ.md

@@ -1187,7 +1187,7 @@ function that is passed a pointer to the class:
 
 ~~~c++
 // f is the pointer to your object.
-static size_t YourClass::func(void *buffer, size_t sz, size_t n, void *f)
+static size_t YourClass::func(char *buffer, size_t sz, size_t n, void *f)
 {
   // Call non-static member function.
   static_cast<YourClass*>(f)->nonStaticFunction();

docs/examples/cacertinmem.c

@@ -40,7 +40,7 @@ typedef size_t ossl_valsize_t;
 typedef int ossl_valsize_t;
 #endif
 
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   fwrite(ptr, size, nmemb, (FILE *)stream);
   return nmemb * size;

docs/examples/certinfo.c

@@ -29,7 +29,7 @@
 
 #include <curl/curl.h>
 
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   (void)stream;
   (void)ptr;

docs/examples/chkspeed.c

@@ -57,7 +57,7 @@
 
 #define CHKSPEED_VERSION "1.0"
 
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *data)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *data)
 {
   /* we are not interested in the downloaded bytes itself,
      so we only return the size we would have saved ... */

docs/examples/crawler.c

@@ -62,7 +62,7 @@ struct memory {
   size_t size;
 };
 
-static size_t write_cb(void *contents, size_t sz, size_t nmemb, void *ctx)
+static size_t write_cb(char *contents, size_t sz, size_t nmemb, void *ctx)
 {
   size_t realsize = sz * nmemb;
   struct memory *mem = (struct memory *)ctx;

docs/examples/ephiperfifo.c

@@ -325,7 +325,7 @@ static int sock_cb(CURL *e, curl_socket_t s, int what, void *cbp, void *sockp)
 }
 
 /* CURLOPT_WRITEFUNCTION */
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *data)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *data)
 {
   (void)ptr;
   (void)data;

docs/examples/evhiperfifo.c

@@ -292,7 +292,7 @@ static int sock_cb(CURL *e, curl_socket_t s, int what, void *cbp, void *sockp)
 }
 
 /* CURLOPT_WRITEFUNCTION */
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *data)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *data)
 {
   size_t realsize = size * nmemb;
   struct ConnInfo *conn = (struct ConnInfo *)data;

docs/examples/externalsocket.c

@@ -60,7 +60,7 @@
 #define INADDR_NONE 0xffffffff
 #endif
 
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   size_t written = fwrite(ptr, size, nmemb, (FILE *)stream);
   return written;

docs/examples/ftp-delete.c

@@ -29,7 +29,7 @@
 
 #include <curl/curl.h>
 
-static size_t write_cb(void *buffer, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *buffer, size_t size, size_t nmemb, void *stream)
 {
   (void)buffer;
   (void)stream;

docs/examples/ftpget.c

@@ -40,7 +40,7 @@ struct FtpFile {
   FILE *stream;
 };
 
-static size_t write_cb(void *buffer, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *buffer, size_t size, size_t nmemb, void *stream)
 {
   struct FtpFile *out = (struct FtpFile *)stream;
   if(!out->stream) {

docs/examples/ftpsget.c

@@ -40,7 +40,7 @@ struct FtpFile {
   FILE *stream;
 };
 
-static size_t write_cb(void *buffer, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *buffer, size_t size, size_t nmemb, void *stream)
 {
   struct FtpFile *out = (struct FtpFile *)stream;
   if(!out->stream) {

docs/examples/ftpuploadresume.c

@@ -51,7 +51,7 @@ static size_t getcontentlengthfunc(void *ptr, size_t size, size_t nmemb,
 }
 
 /* discard downloaded data */
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   (void)ptr;
   (void)stream;

docs/examples/getinmemory.c

@@ -37,7 +37,7 @@ struct MemoryStruct {
   size_t size;
 };
 
-static size_t write_cb(void *contents, size_t size, size_t nmemb, void *userp)
+static size_t write_cb(char *contents, size_t size, size_t nmemb, void *userp)
 {
   size_t realsize = size * nmemb;
   struct MemoryStruct *mem = (struct MemoryStruct *)userp;

docs/examples/ghiper.c

@@ -281,7 +281,7 @@ static int sock_cb(CURL *e, curl_socket_t s, int what, void *cbp, void *sockp)
 }
 
 /* CURLOPT_WRITEFUNCTION */
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *data)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *data)
 {
   size_t realsize = size * nmemb;
   struct ConnInfo *conn = (struct ConnInfo *)data;

docs/examples/hiperfifo.c

@@ -292,7 +292,7 @@ static int sock_cb(CURL *e, curl_socket_t s, int what, void *cbp, void *sockp)
 }
 
 /* CURLOPT_WRITEFUNCTION */
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *data)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *data)
 {
   (void)ptr;
   (void)data;

docs/examples/http2-pushinmemory.c

@@ -36,7 +36,7 @@ struct Memory {
   size_t size;
 };
 
-static size_t write_cb(void *contents, size_t size, size_t nmemb, void *userp)
+static size_t write_cb(char *contents, size_t size, size_t nmemb, void *userp)
 {
   size_t realsize = size * nmemb;
   struct Memory *mem = (struct Memory *)userp;

docs/examples/postinmemory.c

@@ -36,7 +36,7 @@ struct MemoryStruct {
   size_t size;
 };
 
-static size_t write_cb(void *contents, size_t size, size_t nmemb, void *userp)
+static size_t write_cb(char *contents, size_t size, size_t nmemb, void *userp)
 {
   size_t realsize = size * nmemb;
   struct MemoryStruct *mem = (struct MemoryStruct *)userp;

docs/examples/sepheaders.c

@@ -36,7 +36,7 @@
 
 #include <curl/curl.h>
 
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   size_t written = fwrite(ptr, size, nmemb, (FILE *)stream);
   return written;

docs/examples/sessioninfo.c

@@ -44,7 +44,7 @@
 
 static CURL *curl;
 
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   const struct curl_tlssessioninfo *info;
   CURLcode result;

docs/examples/sftpget.c

@@ -50,7 +50,7 @@ struct FtpFile {
   FILE *stream;
 };
 
-static size_t write_cb(void *buffer, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *buffer, size_t size, size_t nmemb, void *stream)
 {
   struct FtpFile *out = (struct FtpFile *)stream;
   if(!out->stream) {

docs/examples/synctime.c

@@ -108,7 +108,7 @@ static const char *MthStr[] = {
 };
 #endif
 
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   fwrite(ptr, size, nmemb, stream);
   return nmemb * size;

docs/examples/url2file.c

@@ -36,7 +36,7 @@
 
 #include <curl/curl.h>
 
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   size_t written = fwrite(ptr, size, nmemb, (FILE *)stream);
   return written;

docs/examples/usercertinmem.c

@@ -40,7 +40,7 @@
 
 #include <curl/curl.h>
 
-static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t write_cb(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   fwrite(ptr, size, nmemb, (FILE *)stream);
   return nmemb * size;

docs/libcurl/libcurl-tutorial.md

@@ -203,7 +203,7 @@ that needs this transfer, I assume that you would like to get the data passed
 to you directly instead of getting it passed to stdout. You write your
 own function that matches this prototype:
 ~~~c
-    size_t write_data(void *buffer, size_t size, size_t nmemb, void *userp);
+    size_t write_data(char *buffer, size_t size, size_t nmemb, void *userp);
 ~~~
 
 You tell libcurl to pass all data to this function by issuing a function
@@ -807,7 +807,7 @@ Example C++ code:
 
 ~~~c
     class AClass {
-      static size_t write_data(void *ptr, size_t size, size_t nmemb,
+      static size_t write_data(char *ptr, size_t size, size_t nmemb,
                                void *ourpointer)
       {
         /* do what you want with the data */

docs/libcurl/opts/CURLINFO_TLS_SSL_PTR.md

@@ -128,7 +128,7 @@ https://github.com/curl/curl/issues/685
 #include <openssl/ssl.h>
 
 CURL *curl;
-static size_t wf(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t wf(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   const struct curl_tlssessioninfo *info = NULL;
   CURLcode result = curl_easy_getinfo(curl, CURLINFO_TLS_SSL_PTR, &info);

tests/libtest/lib3102.c

@@ -80,7 +80,7 @@ static bool is_chain_in_order(struct curl_certinfo *cert_info)
   return true;
 }
 
-static size_t wrfu(void *ptr, size_t size, size_t nmemb, void *stream)
+static size_t wrfu(char *ptr, size_t size, size_t nmemb, void *stream)
 {
   (void)stream;
   (void)ptr;