Build warnings (-Wunused-result) fixes

[rms95]

See patch below to fix some -Wunused-result warnings during build of net-snmp.

It may be wise to return an error in some cases instead of just ignoring the result.

diff --git a/agent/mibgroup/if-mib/data_access/interface_linux.c b/agent/mibgroup/if-mib/data_access/interface_linux.c
index c745132ef..a53d34c94 100644
--- a/agent/mibgroup/if-mib/data_access/interface_linux.c
+++ b/agent/mibgroup/if-mib/data_access/interface_linux.c
@@ -659,8 +659,8 @@ netsnmp_arch_interface_container_load(netsnmp_container* container,
      * to detect the position of individual fields directly,
      * but I suspect this is probably more trouble than it's worth.
      */
-    fgets(line, sizeof(line), devin);
-    fgets(line, sizeof(line), devin);
+    (void)! fgets(line, sizeof(line), devin);
+    (void)! fgets(line, sizeof(line), devin);
 
     if( 0 == scan_expected ) {
         if (strstr(line, "compressed")) {
diff --git a/agent/mibgroup/ip-forward-mib/data_access/route_linux.c b/agent/mibgroup/ip-forward-mib/data_access/route_linux.c
index 7e13084be..04ba7faed 100644
--- a/agent/mibgroup/ip-forward-mib/data_access/route_linux.c
+++ b/agent/mibgroup/ip-forward-mib/data_access/route_linux.c
@@ -69,7 +69,7 @@ _load_ipv4(netsnmp_container* container, u_long *index )
         return -2;
     }
 
-    fgets(line, sizeof(line), in); /* skip header */
+    (void)! fgets(line, sizeof(line), in); /* skip header */
 
     while (fgets(line, sizeof(line), in)) {
         char            rtent_name[32];
diff --git a/agent/mibgroup/ip-mib/data_access/systemstats_linux.c b/agent/mibgroup/ip-mib/data_access/systemstats_linux.c
index f68d12233..2cff5d51d 100644
--- a/agent/mibgroup/ip-mib/data_access/systemstats_linux.c
+++ b/agent/mibgroup/ip-mib/data_access/systemstats_linux.c
@@ -123,7 +123,7 @@ _systemstats_v4(netsnmp_container* container, u_int load_flags)
     /*
      * skip header, but make sure it's the length we expect...
      */
-    fgets(line, sizeof(line), devin);
+    (void)! fgets(line, sizeof(line), devin);
     len = strlen(line);
     if (224 != len) {
         fclose(devin);
diff --git a/agent/mibgroup/mibII/data_access/at_linux.c b/agent/mibgroup/mibII/data_access/at_linux.c
index 79eeef3ce..3293c08cc 100644
--- a/agent/mibgroup/mibII/data_access/at_linux.c
+++ b/agent/mibgroup/mibII/data_access/at_linux.c
@@ -74,7 +74,7 @@ ARP_Scan_Init(void)
     /*
      * Get rid of the header line
      */
-    fgets(line, sizeof(line), in);
+    (void)! fgets(line, sizeof(line), in);
 
     i = 0;
     while (fgets(line, sizeof(line), in)) {
diff --git a/agent/mibgroup/tcp-mib/data_access/tcpConn_linux.c b/agent/mibgroup/tcp-mib/data_access/tcpConn_linux.c
index 42121bbd8..79618085d 100644
--- a/agent/mibgroup/tcp-mib/data_access/tcpConn_linux.c
+++ b/agent/mibgroup/tcp-mib/data_access/tcpConn_linux.c
@@ -133,7 +133,7 @@ _load4(netsnmp_container *container, u_int load_flags)
     }
     
     setvbuf(in, rbuf, _IOFBF, rbufsize);
-    fgets(line, sizeof(line), in); /* skip header */
+    (void)! fgets(line, sizeof(line), in); /* skip header */
 
     /*
      *   sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode
@@ -278,7 +278,7 @@ _load6(netsnmp_container *container, u_int load_flags)
     }
 
     setvbuf(in, rbuf, _IOFBF, rbufsize);
-    fgets(line, sizeof(line), in); /* skip header */
+    (void)! fgets(line, sizeof(line), in); /* skip header */
 
     /*
      * Note: PPC (big endian)
diff --git a/agent/mibgroup/util_funcs.c b/agent/mibgroup/util_funcs.c
index e0f122958..cbe70ecd0 100644
--- a/agent/mibgroup/util_funcs.c
+++ b/agent/mibgroup/util_funcs.c
@@ -507,7 +507,7 @@ get_exec_pipes(char *cmd, int *fdIn, int *fdOut, netsnmp_pid_t *pid)
          * close all non-standard open file descriptors 
          */
         netsnmp_close_fds(1);
-        (void) dup(1);          /* stderr */
+        (void)! dup(1);          /* stderr */
 
         for (cnt = 1, cptr1 = cmd, cptr2 = argvs; *cptr1 != 0;
              cptr2++, cptr1++) {
diff --git a/agent/snmpd.c b/agent/snmpd.c
index 138d81136..c1ef5656b 100644
--- a/agent/snmpd.c
+++ b/agent/snmpd.c
@@ -987,7 +987,7 @@ main(int argc, char *argv[])
     
 #ifdef HAVE_CHOWN
     if ( uid != 0 || gid != 0 )
-        chown( persistent_dir, uid, gid );
+        (void)! chown( persistent_dir, uid, gid );
 #endif
 
 #ifdef HAVE_SETGID
diff --git a/snmplib/system.c b/snmplib/system.c
index c14208536..62dcfa508 100644
--- a/snmplib/system.c
+++ b/snmplib/system.c
@@ -228,7 +228,7 @@ _daemon_prep(int stderr_log)
     int fd;
 
     /* Avoid keeping any directory in use. */
-    chdir("/");
+    (void)! chdir("/");
 
     if (stderr_log)
         return;

[bvanassche]

I haven't seen any such warnings recently. On which Linux distro did you encounter these warnings?

Additionally, I doubt that this patch is sufficient for all gcc versions. Please introduce a macro for suppressing these warnings such that the implementation of that macro can be modified easily, e.g. something like this (untested):

#define IGNORE_RESULT(e) do { if ((e)) { } } while (0)

[rms95]

A macro would indeed be better, though I was not sure where to place the macro.
In my experience (void)! does the job for the gcc / clang versions I used. (Though your macro might be more thorough)

Currently I'm building on Debian 10, gcc 8.3. These of course only come up with a clean build / rebuild of those files, which is why I usually prefer building with -Werror.

If you're not seeing them, it may have something to do with the configuration I am building with.
Some details about the flags I send to the configure tool: (note I added -Werror).

NETSNMP_CFG     += --silent
NETSNMP_CFG     += --enable-static
NETSNMP_CFG     += --disable-shared
NETSNMP_CFG     += --disable-embedded-perl
NETSNMP_CFG     += --disable-perl-cc-checks
NETSNMP_CFG     += --disable-manuals
NETSNMP_CFG     += --without-root-access
NETSNMP_CFG     += --with-defaults
NETSNMP_CFG     += --with-mib-modules=snmp-usm-dh-objects-mib
MIBS_REMOVE     += notification-log-mib snmp-notification-mib
MIBS_REMOVE     += agentx ucd_snmp target utilities
MIBS_REMOVE     += disman/event disman/schedule host
NETSNMP_CFG     += --with-out-mib-modules="$(MIBS_REMOVE)"
NETSNMP_CFG     += --without-pcre
NETSNMP_CFG     += --without-nl

OPENSSL_DIR      = $(SDKTARGETSYSROOT)/usr
NETSNMP_CFG     += --with-openssl="$(OPENSSL_DIR)"
NETSNMP_CFG     += --with-cflags="-fPIC -fpic -g -I$(OPENSSL_DIR)/include -O2 -Werror"
NETSNMP_CFG     += --with-ldflags="-g -lcrypto -L$(OPENSSL_DIR) -ldl"

[bvanassche]

I have been able to reproduce the reported warnings by building the Net-SNMP code with -D_FORTIFY_SOURCE=1. A patch that fixes these warnings has been checked in. Please update your local copy of the Net-SNMP code and retest.

[rms95]

I just verified this to be working.