curl, new long option '--out-null'

Add a new commandline option --out-null that discards all
response bytes into the void. Replaces non-portable use of
'-o /dev/null' with more efficiency.

Feature earliest for 8.16.0

docs/cmdline-opts/Makefile.inc

@@ -181,6 +181,7 @@ DPAGES = \
   ntlm.md \
   oauth2-bearer.md \
   output-dir.md \
+  out-null.md \
   output.md \
   parallel-immediate.md \
   parallel-max.md \

docs/cmdline-opts/out-null.md

@@ -0,0 +1,26 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: out-null
+Help: Discard response data into the void
+Category: output
+Added: 8.16.0
+Multi: boolean
+See-also:
+  - output
+  - remote-name
+  - remote-name-all
+  - remote-header-name
+Example:
+  - "https://example.com" --out-null
+---
+
+# `--out-null`
+
+Discard all response output of a transfer silently. This is the more
+efficient and portable version of
+
+    curl https://host.example -o /dev/null
+
+The transfer is done in full, all data is received and checked, but
+the bytes are not written anywhere.

docs/cmdline-opts/output.md

@@ -9,6 +9,7 @@ Category: important output
 Added: 4.0
 Multi: per-URL
 See-also:
+  - out-null
   - remote-name
   - remote-name-all
   - remote-header-name
@@ -56,6 +57,10 @@ Or for Windows:
 
     curl example.com -o nul
 
+Or, even more efficient and portable, use
+
+    curl example.com --out-null
+
 Specify the filename as single minus to force the output to stdout, to
 override curl's internal binary output in terminal prevention:
 

docs/options-in-versions

@@ -145,6 +145,7 @@
 --ntlm                               7.10.6
 --ntlm-wb                            7.22.0
 --oauth2-bearer                      7.33.0
+--out-null                           8.16.0
 --output (-o)                        4.0
 --output-dir                         7.73.0
 --parallel (-Z)                      7.66.0

src/tool_cb_wrt.c

@@ -141,6 +141,9 @@ size_t tool_write_cb(char *buffer, size_t sz, size_t nmemb, void *userdata)
   intptr_t fhnd;
 #endif
 
+  if(config->out_null)
+    return bytes;
+
 #ifdef DEBUGBUILD
   {
     char *tty = curl_getenv("CURL_ISATTY");

src/tool_cfgable.h

@@ -340,6 +340,7 @@ struct OperationConfig {
   BIT(rm_partial);                /* on error, remove partially written output
                                      files */
   BIT(skip_existing);
+  BIT(out_null);                  /* WRITEFUNCTION into the void */
 };
 
 struct GlobalConfig {

src/tool_getparam.c

@@ -224,6 +224,7 @@ static const struct LongShort aliases[]= {
   {"ntlm",                       ARG_BOOL, ' ', C_NTLM},
   {"ntlm-wb",                    ARG_BOOL|ARG_DEPR, ' ', C_NTLM_WB},
   {"oauth2-bearer",              ARG_STRG|ARG_CLEAR, ' ', C_OAUTH2_BEARER},
+  {"out-null",                   ARG_BOOL, ' ', C_OUT_NULL},
   {"output",                     ARG_FILE, 'o', C_OUTPUT},
   {"output-dir",                 ARG_STRG, ' ', C_OUTPUT_DIR},
   {"parallel",                   ARG_BOOL, 'Z', C_PARALLEL},
@@ -1313,6 +1314,7 @@ static ParameterError parse_output(struct OperationConfig *config,
   err = getstr(&url->outfile, nextarg, DENY_BLANK);
   url->useremote = FALSE; /* switch off */
   url->outset = TRUE;
+  config->out_null = FALSE; /* switch off --out-null */
   return err;
 }
 
@@ -1833,6 +1835,9 @@ static ParameterError opt_bool(struct OperationConfig *config,
       return PARAM_LIBCURL_DOESNT_SUPPORT;
     togglebit(toggle, &config->authtype, CURLAUTH_NTLM);
     break;
+  case C_OUT_NULL: /* --out-null */
+    config->out_null = toggle;
+    break;
   case C_BASIC: /* --basic */
     togglebit(toggle, &config->authtype, CURLAUTH_BASIC);
     break;

src/tool_getparam.h

@@ -167,6 +167,7 @@ typedef enum {
   C_NTLM,
   C_NTLM_WB,
   C_OAUTH2_BEARER,
+  C_OUT_NULL,
   C_OUTPUT,
   C_OUTPUT_DIR,
   C_PARALLEL,

src/tool_listhelp.c

@@ -442,6 +442,9 @@ const struct helptxt helptext[] = {
    "OAuth 2 Bearer Token",
    CURLHELP_AUTH | CURLHELP_IMAP | CURLHELP_POP3 | CURLHELP_SMTP |
    CURLHELP_LDAP},
+  {"    --out-null",
+   "Discard response data into the void",
+   CURLHELP_OUTPUT},
   {"-o, --output <file>",
    "Write to file instead of stdout",
    CURLHELP_IMPORTANT | CURLHELP_OUTPUT},

tests/http/testenv/curl.py

@@ -587,7 +587,7 @@ class CurlClient:
             extra_args = []
         if no_save:
             extra_args.extend([
-                '-o', '/dev/null',
+                '--out-null',
             ])
         else:
             extra_args.extend([
@@ -636,7 +636,7 @@ class CurlClient:
         if extra_args is None:
             extra_args = []
         extra_args.extend([
-            '-X', 'DELETE', '-o', '/dev/null',
+            '-X', 'DELETE', '--out-null',
         ])
         if with_stats:
             extra_args.extend([
@@ -702,7 +702,7 @@ class CurlClient:
             extra_args = []
         if no_save:
             extra_args.extend([
-                '-o', '/dev/null',
+                '--out-null',
             ])
         else:
             extra_args.extend([