Cross-compile error [-Werror=conversion]

[MAntoniak]

I am compiling libcurl on Windows using WSL on an ARM device. I have a problem with compiling libraries in version 7.78.0. Below I present the library configuration and a compilation error. Where could the problem be?

configure: Configured to build curl/libcurl:

  Host setup:       arm-unknown-linux-gnueabihf
  Install prefix:   /usr/local/tmp/lib/linux/arm
  Compiler:         arm-linux-gnueabihf-gcc
   CFLAGS:          -mthumb -mfpu=neon-vfpv4 -mfloat-abi=hard -mcpu=cortex-a7 -Werror-implicit-function-declaration -g -O2 -pedantic -Wall -W -Wpointer-arith -Wwrite-strings -Wunused -Wshadow -Winline -Wnested-externs -Wmissing-declarations -Wmissing-prototypes -Wno-long-long -Wbad-function-cast -Wfloat-equal -Wno-multichar -Wsign-compare -Wundef -Wno-format-nonliteral -Wendif-labels -Wstrict-prototypes -Wdeclaration-after-statement -Wold-style-definition -Wstrict-aliasing=3 -Wcast-align -Wtype-limits -Wold-style-declaration -Wmissing-parameter-type -Wempty-body -Wclobbered -Wignored-qualifiers -Wconversion -Wno-sign-conversion -Wvla -ftree-vrp -Wdouble-promotion -Wformat=2 -Warray-bounds=2 -Wshift-negative-value -Wshift-overflow=2 -Wnull-dereference -fdelete-null-pointer-checks -Wduplicated-cond -Wunused-const-variable -Wduplicated-branches -Wrestrict -Walloc-zero -Wformat-overflow=2 -Wformat-truncation=2 -Wimplicit-fallthrough=4 -Wno-system-headers -pthread
   CPPFLAGS:        -mthumb -mfpu=neon-vfpv4 -mfloat-abi=hard -mcpu=cortex-a7 -isystem /usr/local/tmp/lib/linux/arm/include
   LDFLAGS:         -mcpu=cortex-a7 -L/usr/local/armhf/lib -L/usr/local/tmp/lib/linux/arm/lib
   LIBS:            -lssl -lcrypto -ldl -lpthread

  curl version:     7.78.0
  SSL:              enabled (OpenSSL)
  SSH:              no      (--with-{libssh,libssh2})
  zlib:             no      (--with-zlib)
  brotli:           no      (--with-brotli)
  zstd:             no      (--with-zstd)
  GSS-API:          no      (--with-gssapi)
  GSASL:            no      (libgsasl not found)
  TLS-SRP:          no      (--enable-tls-srp)
  resolver:         POSIX threaded
  IPv6:             no      (--enable-ipv6)
  Unix sockets:     enabled
  IDN:              no      (--with-{libidn2,winidn})
  Build libcurl:    Shared=no, Static=yes
  Built-in manual:  no      (--enable-manual)
  --libcurl option: enabled (--disable-libcurl-option)
  Verbose errors:   enabled (--disable-verbose)
  Code coverage:    disabled
  SSPI:             no      (--enable-sspi)
  ca cert bundle:   no
  ca cert path:     no
  ca fallback:      no
  LDAP:             no      (--enable-ldap / --with-ldap-lib / --with-lber-lib)
  LDAPS:            no      (--enable-ldaps)
  RTSP:             no      (--enable-rtsp)
  RTMP:             no      (--with-librtmp)
  PSL:              no      (--with-libpsl)
  Alt-svc:          no
  HSTS:             enabled (--disable-hsts)
  HTTP1:            enabled (internal)
  HTTP2:            no      (--with-nghttp2, --with-hyper)
  HTTP3:            no      (--with-ngtcp2, --with-quiche)
  ECH:              no      (--enable-ech)
  Protocols:        HTTP HTTPS MQTT
  Features:         AsynchDNS HSTS Largefile SSL UnixSockets

Making all in lib
  CC       libcurl_la-altsvc.lo
  CC       libcurl_la-amigaos.lo
  CC       libcurl_la-asyn-ares.lo
  CC       libcurl_la-asyn-thread.lo
  CC       libcurl_la-base64.lo
  CC       libcurl_la-bufref.lo
  CC       libcurl_la-c-hyper.lo
  CC       libcurl_la-conncache.lo
  CC       libcurl_la-connect.lo
  CC       libcurl_la-content_encoding.lo
  CC       libcurl_la-cookie.lo
  CC       libcurl_la-curl_addrinfo.lo
  CC       libcurl_la-curl_ctype.lo
  CC       libcurl_la-curl_des.lo
  CC       libcurl_la-curl_endian.lo
  CC       libcurl_la-curl_fnmatch.lo
  CC       libcurl_la-curl_get_line.lo
  CC       libcurl_la-curl_gethostname.lo
  CC       libcurl_la-curl_gssapi.lo
  CC       libcurl_la-curl_memrchr.lo
  CC       libcurl_la-curl_multibyte.lo
  CC       libcurl_la-curl_ntlm_core.lo
  CC       libcurl_la-curl_ntlm_wb.lo
  CC       libcurl_la-curl_path.lo
  CC       libcurl_la-curl_range.lo
  CC       libcurl_la-curl_rtmp.lo
  CC       libcurl_la-curl_sasl.lo
  CC       libcurl_la-curl_sspi.lo
  CC       libcurl_la-curl_threads.lo
  CC       libcurl_la-dict.lo
  CC       libcurl_la-doh.lo
  CC       libcurl_la-dotdot.lo
  CC       libcurl_la-dynbuf.lo
  CC       libcurl_la-easy.lo
  CC       libcurl_la-easygetopt.lo
  CC       libcurl_la-easyoptions.lo
  CC       libcurl_la-escape.lo
  CC       libcurl_la-file.lo
  CC       libcurl_la-fileinfo.lo
  CC       libcurl_la-formdata.lo
  CC       libcurl_la-ftp.lo
  CC       libcurl_la-ftplistparser.lo
  CC       libcurl_la-getenv.lo
  CC       libcurl_la-getinfo.lo
  CC       libcurl_la-gopher.lo
  CC       libcurl_la-hash.lo
  CC       libcurl_la-hmac.lo
  CC       libcurl_la-hostasyn.lo
  CC       libcurl_la-hostcheck.lo
  CC       libcurl_la-hostip.lo
  CC       libcurl_la-hostip4.lo
  CC       libcurl_la-hostip6.lo
  CC       libcurl_la-hostsyn.lo
  CC       libcurl_la-hsts.lo
  CC       libcurl_la-http.lo
  CC       libcurl_la-http2.lo
  CC       libcurl_la-http_chunks.lo
  CC       libcurl_la-http_digest.lo
  CC       libcurl_la-http_negotiate.lo
  CC       libcurl_la-http_ntlm.lo
  CC       libcurl_la-http_proxy.lo
  CC       libcurl_la-http_aws_sigv4.lo
  CC       libcurl_la-idn_win32.lo
  CC       libcurl_la-if2ip.lo
  CC       libcurl_la-imap.lo
  CC       libcurl_la-inet_ntop.lo
  CC       libcurl_la-inet_pton.lo
  CC       libcurl_la-krb5.lo
  CC       libcurl_la-ldap.lo
  CC       libcurl_la-llist.lo
  CC       libcurl_la-md4.lo
  CC       libcurl_la-md5.lo
  CC       libcurl_la-memdebug.lo
  CC       libcurl_la-mime.lo
  CC       libcurl_la-mprintf.lo
  CC       libcurl_la-mqtt.lo
  CC       libcurl_la-multi.lo
  CC       libcurl_la-netrc.lo
  CC       libcurl_la-non-ascii.lo
  CC       libcurl_la-nonblock.lo
  CC       libcurl_la-openldap.lo
  CC       libcurl_la-parsedate.lo
  CC       libcurl_la-pingpong.lo
  CC       libcurl_la-pop3.lo
  CC       libcurl_la-progress.lo
  CC       libcurl_la-psl.lo
  CC       libcurl_la-rand.lo
  CC       libcurl_la-rename.lo
  CC       libcurl_la-rtsp.lo
  CC       libcurl_la-select.lo
progress.c: In function ‘trspeed’:
progress.c:380:40: error: conversion from ‘curl_off_t’ {aka ‘long long int’} to ‘long double’ may change value [-Werror=conversion]
  380 |   return (curl_off_t)((long double)size/us * 1000000);
      |                                        ^
  CC       libcurl_la-sendf.lo
  CC       libcurl_la-setopt.lo
  CC       libcurl_la-sha256.lo
cc1: all warnings being treated as errors
make[2]: *** [Makefile:2569: libcurl_la-progress.lo] Error 1
make[2]: *** Waiting for unfinished jobs....
  CC       libcurl_la-share.lo
make[1]: *** [Makefile:1359: all] Error 2
make: *** [Makefile:1262: all-recursive] Error 1
Making install in lib
make[1]: Entering directory '/usr/local/src/curl-7.78.0/lib'
  CC       libcurl_la-progress.lo
progress.c: In function ‘trspeed’:
progress.c:380:40: error: conversion from ‘curl_off_t’ {aka ‘long long int’} to ‘long double’ may change value [-Werror=conversion]
  380 |   return (curl_off_t)((long double)size/us * 1000000);
      |                                        ^
cc1: all warnings being treated as errors
make[1]: *** [Makefile:2569: libcurl_la-progress.lo] Error 1
make[1]: Leaving directory '/usr/local/src/curl-7.78.0/lib'
make: *** [Makefile:1262: install-recursive] Error 1

[bagder]

Does it make any difference if you change the long doubles to just double on that line?

[MAntoniak]

I'll try to answer that question next week.

[MAntoniak]

Unfortunately it doesn't help:

  CC       libcurl_la-smtp.lo
  CC       libcurl_la-socketpair.lo
  CC       libcurl_la-socks.lo
progress.c: In function ‘trspeed’:
progress.c:380:35: error: conversion from ‘curl_off_t’ {aka ‘long long int’} to ‘double’ may change value [-Werror=conversion]
  380 |   return (curl_off_t)((double)size/us * 1000000);
      |                                   ^
  CC       libcurl_la-socks_gssapi.lo
  CC       libcurl_la-socks_sspi.lo
  CC       libcurl_la-speedcheck.lo
  CC       libcurl_la-splay.lo
  CC       libcurl_la-strcase.lo
  CC       libcurl_la-strdup.lo
  CC       libcurl_la-strerror.lo
cc1: all warnings being treated as errors
make[2]: *** [Makefile:2569: libcurl_la-progress.lo] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [Makefile:1359: all] Error 2
make: *** [Makefile:1262: all-recursive] Error 1

I forgot to write that this error does not occur when compiling version 7.74.0

[bagder]

I forgot to write that this error does not occur when compiling version 7.74.0

The trspeed() function (in which this warning occurs) didn't exist back then.

This is probably due to some limitation in the floating point handling your compiler/platform impose.

[bagder]

You provide the -Wconversion option that apparently causes this error. I don't see how curl itself adds this warning. This looks like a case of you breaking the build for yourself.

Wrong. We also use that in debug builds without problems on my native x86-64 builds.

[MAntoniak]

I made this change in the code (similar to what was before):

static curl_off_t trspeed(curl_off_t size, /* number of bytes */
                          curl_off_t us)   /* microseconds */
{
  curl_off_t timespent;

  if(us < 1)
    return size * 1000000;

  if(size < CURL_OFF_T_MAX/1000) {
    timespent = (curl_off_t)(us+500)/1000;
    return (size * 1000 / (timespent>0?timespent:1));
  }
  else {
    timespent = (curl_off_t)(us+500000)/1000000;
    return (size / (timespent>0?timespent:1));
  }
}

After it, the compilation was successful.

[bagder]

This particular suggestion makes it never use microseconds at all in the calculations so it would then probably make more sense to pass in milliseconds to begin with.

Or perhaps we should divide by microseconds if the size is "large" ?

[bagder]

Something like this:

static curl_off_t trspeed(curl_off_t size, /* number of bytes */
                          curl_off_t us)   /* microseconds */
{
  if(us < 1)
    return size * 1000000;

  if(size < CURL_OFF_T_MAX/1000000)
    return (size * 1000000) / us;
  else if(size < CURL_OFF_T_MAX/1000)
    return (size * 1000) / (us / 1000);
  else
    return size / (us / 1000000);
}

[MAntoniak]

static curl_off_t trspeed(curl_off_t size, /* number of bytes */
                          curl_off_t us)   /* microseconds */
{
  if(us < 1)
    return size * 1000000;

  if(size < CURL_OFF_T_MAX/1000000)
    return (size * 1000000) / us;
  else if(size < CURL_OFF_T_MAX/1000)
    return (size * 1000) / (us / 1000);
  else
    return size / (us / 1000000);
}

It works for me. Thanks for PR.

[bagder]

I think we can probably also optimize away the middle check (/1000) without any particular downside.

[MAntoniak]

I think we can probably also optimize away the middle check (/1000) without any particular downside.

Indeed, I've checked the idea and also it works fine.