build: fix -Wconversion/-Wsign-conversion warnings

Fix remaining warnings in examples and tests which are not suppressed
by the pragma in `lib/curl_setup.h`.

Silence a toolchain issue causing warnings in `FD_SET()` calls with
older Cygwin/MSYS2 builds. Likely fixed on 2020-08-03 by:
https://cygwin.com/git/?p=newlib-cygwin.git;a=commitdiff;h=5717262b8ecfed0f7fab63e2c09c78991e36f9dd

Follow-up to 2dbe75bd7f #12492

Closes #12557

docs/examples/10-at-a-time.c

@@ -95,7 +95,7 @@ static size_t write_cb(char *data, size_t n, size_t l, void *userp)
   return n*l;
 }
 
-static void add_transfer(CURLM *cm, int i, int *left)
+static void add_transfer(CURLM *cm, unsigned int i, int *left)
 {
   CURL *eh = curl_easy_init();
   curl_easy_setopt(eh, CURLOPT_WRITEFUNCTION, write_cb);

docs/examples/anyauthput.c

@@ -69,17 +69,15 @@ static int my_seek(void *userp, curl_off_t offset, int origin)
 /* read callback function, fread() look alike */
 static size_t read_callback(char *ptr, size_t size, size_t nmemb, void *stream)
 {
-  ssize_t retcode;
-  unsigned long nread;
+  size_t nread;
 
-  retcode = fread(ptr, size, nmemb, stream);
+  nread = fread(ptr, size, nmemb, stream);
 
-  if(retcode > 0) {
-    nread = (unsigned long)retcode;
-    fprintf(stderr, "*** We read %lu bytes from file\n", nread);
+  if(nread > 0) {
+    fprintf(stderr, "*** We read %lu bytes from file\n", (unsigned long)nread);
   }
 
-  return retcode;
+  return nread;
 }
 
 int main(int argc, char **argv)

docs/examples/ftpget.c

@@ -42,7 +42,7 @@ static size_t my_fwrite(void *buffer, size_t size, size_t nmemb, void *stream)
     /* open file for writing */
     out->stream = fopen(out->filename, "wb");
     if(!out->stream)
-      return -1; /* failure, cannot open file to write */
+      return 0; /* failure, cannot open file to write */
   }
   return fwrite(buffer, size, nmemb, out->stream);
 }

docs/examples/ftpsget.c

@@ -44,7 +44,7 @@ static size_t my_fwrite(void *buffer, size_t size, size_t nmemb,
     /* open file for writing */
     out->stream = fopen(out->filename, "wb");
     if(!out->stream)
-      return -1; /* failure, cannot open file to write */
+      return 0; /* failure, cannot open file to write */
   }
   return fwrite(buffer, size, nmemb, out->stream);
 }

docs/examples/http2-download.c

@@ -54,7 +54,7 @@ struct transfer {
 #define NUM_HANDLES 1000
 
 static
-void dump(const char *text, int num, unsigned char *ptr, size_t size,
+void dump(const char *text, unsigned int num, unsigned char *ptr, size_t size,
           char nohex)
 {
   size_t i;
@@ -66,7 +66,7 @@ void dump(const char *text, int num, unsigned char *ptr, size_t size,
     /* without the hex output, we can fit more on screen */
     width = 0x40;
 
-  fprintf(stderr, "%d %s, %lu bytes (0x%lx)\n",
+  fprintf(stderr, "%u %s, %lu bytes (0x%lx)\n",
           num, text, (unsigned long)size, (unsigned long)size);
 
   for(i = 0; i<size; i += width) {

docs/examples/sendrecv.c

@@ -44,6 +44,13 @@ static int wait_on_socket(curl_socket_t sockfd, int for_recv, long timeout_ms)
   FD_ZERO(&outfd);
   FD_ZERO(&errfd);
 
+/* Avoid this warning with pre-2020 Cygwin/MSYS releases:
+ * warning: conversion to 'long unsigned int' from 'curl_socket_t' {aka 'int'} may change the sign of the result [-Wsign-conversion]
+ */
+#if defined(__GNUC__) && defined(__CYGWIN__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wsign-conversion"
+#endif
   FD_SET(sockfd, &errfd); /* always check for error */
 
   if(for_recv) {
@@ -52,6 +59,9 @@ static int wait_on_socket(curl_socket_t sockfd, int for_recv, long timeout_ms)
   else {
     FD_SET(sockfd, &outfd);
   }
+#if defined(__GNUC__) && defined(__CYGWIN__)
+#pragma GCC diagnostic pop
+#endif
 
   /* select() returns the number of signalled sockets or -1 */
   res = select((int)sockfd + 1, &infd, &outfd, &errfd, &tv);

docs/examples/sftpget.c

@@ -53,7 +53,7 @@ static size_t my_fwrite(void *buffer, size_t size, size_t nmemb,
     /* open file for writing */
     out->stream = fopen(out->filename, "wb");
     if(!out->stream)
-      return -1; /* failure, cannot open file to write */
+      return 0; /* failure, cannot open file to write */
   }
   return fwrite(buffer, size, nmemb, out->stream);
 }