Qt for embedded: Cannot open framebuffer device

Question

I have built a small linux using Buildroot. I have included QT for embedded and support for framebuffer devices, but I am unable to run the Qt examples from it. I want to run GUI without X server.

When running /usr/share/qt/examples/qws/framebuffer/framebuffer -qws I am getting the following error:

Error: cannot open framebuffer device: No such file or directory.

The output of ls /dev/fb* is:

/dev/fb0 /dev/fb1 /dev/fb2 /dev/fb3

strace output:

execve("./framebuffer", ["./framebuffer", "-qws"], [/* 16 vars */]) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_UNINITIALIZED, -1, 0) = 0xb7701000
open("/lib/libstdc++.so.6", O_RDONLY)   = -1 ENOENT (No such file or directory)
open("/lib/libstdc++.so.6", O_RDONLY)   = -1 ENOENT (No such file or directory)
open("/usr/lib/libstdc++.so.6", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=751308, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_UNINITIALIZED, -1, 0) = 0xb7700000
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0004o\4\0004\0\0\0"..., 4096) = 4096
mmap2(NULL, 778240, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7642000
mmap2(0xb7642000, 729051, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0) = 0xb7642000
mmap2(0xb76f4000, 20864, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0xb2000) = 0xb76f4000
mmap2(0xb76fa000, 22428, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb76fa000
close(3)                                = 0
munmap(0xb7700000, 4096)                = 0
open("/lib/libm.so.0", O_RDONLY)        = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=45864, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_UNINITIALIZED, -1, 0) = 0xb7700000
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0 \33\0\0004\0\0\0"..., 4096) = 4096
mmap2(NULL, 53248, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7635000
mmap2(0xb7635000, 43784, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0) = 0xb7635000
mmap2(0xb7640000, 4100, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0xa000) = 0xb7640000
close(3)                                = 0
munmap(0xb7700000, 4096)                = 0
open("/lib/libgcc_s.so.1", O_RDONLY)    = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=86600, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_UNINITIALIZED, -1, 0) = 0xb7700000
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0t\36\0\0004\0\0\0"..., 4096) = 4096
mmap2(NULL, 90112, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb761f000
mmap2(0xb761f000, 85004, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0) = 0xb761f000
mmap2(0xb7634000, 3536, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x14000) = 0xb7634000
close(3)                                = 0
munmap(0xb7700000, 4096)                = 0
open("/lib/libc.so.0", O_RDONLY)        = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=392428, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_UNINITIALIZED, -1, 0) = 0xb7700000
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\0\256\0\0004\0\0\0"..., 4096) = 4096
mmap2(NULL, 413696, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb75ba000
mmap2(0xb75ba000, 385628, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0) = 0xb75ba000
mmap2(0xb7619000, 4868, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x5e000) = 0xb7619000
mmap2(0xb761b000, 15928, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb761b000
close(3)                                = 0
munmap(0xb7700000, 4096)                = 0
open("/lib/libm.so.0", O_RDONLY)        = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=45864, ...}) = 0
close(3)                                = 0
open("/lib/libc.so.0", O_RDONLY)        = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=392428, ...}) = 0
close(3)                                = 0
stat("/lib/ld-uClibc.so.0", {st_mode=S_IFREG|0755, st_size=25296, ...}) = 0
open("/lib/libgcc_s.so.1", O_RDONLY)    = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=86600, ...}) = 0
close(3)                                = 0
open("/lib/libc.so.0", O_RDONLY)        = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=392428, ...}) = 0
close(3)                                = 0
open("/lib/libc.so.0", O_RDONLY)        = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=392428, ...}) = 0
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_UNINITIALIZED, -1, 0) = 0xb7700000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb77006b0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0
mprotect(0xb76f4000, 16384, PROT_READ)  = 0
mprotect(0xb7640000, 4096, PROT_READ)   = 0
mprotect(0xb7619000, 4096, PROT_READ)   = 0
mprotect(0xb7708000, 4096, PROT_READ)   = 0
ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
brk(0)                                  = 0x9093000
brk(0x9094000)                          = 0x9094000
open("-qws", O_RDWR)                    = -1 ENOENT (No such file or directory)
write(2, "Error: cannot open framebuffer d"..., 37Error: cannot open framebuffer device) = 37
write(2, ": ", 2: )                       = 2
write(2, "No such file or directory", 25No such file or directory) = 25
write(2, "\n", 1
)                       = 1
exit_group(1)                           = ?
+++ exited with 1 +++

(Don't know what this means)

If I run the calculator example with:

usr/share/qt/examples/widgets/calculator -qws

QLock::QLock: Cannot create semaphore /tmp/qtembedded-0/QtEmbedded-0 'd' (38, Function not implemented)
Cannot get display lock
Aborted

Answer 1

It seems that I didn't include the System V IPC option in the kernel. I found the asnwer here.

Answer 2

Thanks to the output of strace I've found the issue: you have wrongly put a space between - and qws:

 - qws

should be

 -qws

Answer 3

The strace shows that the program tries to open the '-qws' framebuffer device, which obviously is not valid:

open("-qws", O_RDWR)                    = -1 ENOENT (No such file or directory)
write(2, "Error: cannot open framebuffer d"..., 37Error: cannot open framebuffer device) = 37

Why do you run the program like that? Have you tried running without parameters? Briefly looking at the source code shows that it should work like that. The default framebuffer is /dev/fb0 in that program.