curl: add 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 added in 8.16.0

Closes #17800

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(outs->out_null)
+    return bytes;
+
 #ifdef DEBUGBUILD
   {
     char *tty = curl_getenv("CURL_ISATTY");

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},
@@ -1310,9 +1311,13 @@ static ParameterError parse_output(struct OperationConfig *config,
     return PARAM_NO_MEM;
 
   /* fill in the outfile */
-  err = getstr(&url->outfile, nextarg, DENY_BLANK);
+  if(nextarg)
+    err = getstr(&url->outfile, nextarg, DENY_BLANK);
+  else
+    url->outfile = NULL;
   url->useremote = FALSE; /* switch off */
   url->outset = TRUE;
+  url->out_null = !nextarg;
   return err;
 }
 
@@ -1351,6 +1356,7 @@ static ParameterError parse_remote_name(struct OperationConfig *config,
   url->outfile = NULL; /* leave it */
   url->useremote = toggle;
   url->outset = TRUE;
+  url->out_null = FALSE;
   return PARAM_OK;
 }
 
@@ -1833,6 +1839,8 @@ 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 */
+    return parse_output(config, NULL);
   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},

src/tool_operate.c

@@ -1321,7 +1321,8 @@ static CURLcode single_transfer(struct OperationConfig *config,
         }
       }
 
-      if((urlnode->useremote ||
+      outs->out_null = urlnode->out_null;
+      if(!outs->out_null && (urlnode->useremote ||
           (per->outfile && strcmp("-", per->outfile)))) {
         result = setup_outfile(config, per, outs, skipped);
         if(result)

src/tool_sdecls.h

@@ -74,6 +74,7 @@ struct OutStruct {
   BIT(is_cd_filename);
   BIT(s_isreg);
   BIT(fopened);
+  BIT(out_null);
 };
 
 /*
@@ -95,6 +96,7 @@ struct getout {
   BIT(useremote); /* use remote filename locally */
   BIT(noupload);  /* if set, -T "" has been used */
   BIT(noglob);    /* disable globbing for this URL */
+  BIT(out_null);  /* discard output for this URL */
 };
 /*
  * 'trace' enumeration represents curl's output look'n feel possibilities.