How do I read the value of all registers with gdb?

Question

I am debugging a c program in assembly to understand how the gcc complier works. I want to read my $fs segment register so I use x/x $fs, however it tells me it can't access the memory. How can I get a read out on any register which includes segment, general purpose, and control registers on the i386:86_64?

Answer 1

info registers prints out registers values for me which is what you are asking I think:

(gdb) info registers
rax            0x7ffff7731ec8   140737344904904
rbx            0x0  0
rcx            0x0  0
rdx            0x7fffffffd618   140737488344600
rsi            0x7fffffffd608   140737488344584
rdi            0x1  1
rbp            0x0  0x0
rsp            0x7fffffffd528   0x7fffffffd528
r8             0x7ffff7730300   140737344897792
r9             0x7ffff7dec250   140737351959120
r10            0x7fffffffd390   140737488343952
r11            0x7ffff73d0b50   140737341360976
r12            0x400be0 4197344
r13            0x7fffffffd600   140737488344576
r14            0x0  0
r15            0x0  0
rip            0x402330 0x402330 <main>
eflags         0x246    [ PF ZF IF ]
cs             0x33 51
ss             0x2b 43
ds             0x0  0
es             0x0  0
fs             0x0  0
gs             0x0  0
(gdb) 

Answer 2

info all-registers 

gives you all the register values including FPU register stack, xmm registers.

(gdb) i all-r
rax            0x2aaaaace62ce   46912498459342
rbx            0x2aab18e71290   46914345570960
rcx            0x2aaab2020d60   46912619285856
rdx            0xffffffffffd934ee       -2542354
rsi            0x2aab18ec7a40   46914345925184
rdi            0xa      10
rbp            0x2aab18e6f000   0x2aab18e6f000
rsp            0x2aab18e6f000   0x2aab18e6f000
r8             0xe      14
r9             0x2aab18eb1f08   46914345836296
r10            0x2aaab9085000   46912737136640
r11            0x0      0
r12            0x2aab18ec7170   46914345922928
r13            0x477f3280       1199518336
r14            0x7      7
r15            0x2aaada787000   46913298132992
rip            0x2aaaaae3b18e   0x2aaaaae3b18e <flt_fadd+4>
eflags         0x283    643
ds             0x0      0
es             0x0      0
fs             0x0      0
gs             0x0      0
st0            10       (raw 0x4002a000000000000000)
st1            3        (raw 0x4000c000000000000000)
st2            0        (raw 0x00000000000000000000)
st3            0        (raw 0x00000000000000000000)
st4            0        (raw 0x00000000000000000000)
st5            0        (raw 0x00000000000000000000)
st6            0        (raw 0x00000000000000000000)
st7            0        (raw 0x00000000000000000000)
fctrl          0x137f   4991
fstat          0x7000   28672
ftag           0xc0     192
fiseg          0xaae3afe5       -1427918875
fioff          0x2aaa   10922
foseg          0x18e6ee48       417787464
fooff          0x2aab   10923
fop            0x704    1796
xmm0           {f = {0x0, 0x6, 0x0, 0x0}}       {f = {0, 6.48876953, 0, 0}}
xmm1           {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm2           {f = {0x0, 0x1c0, 0x0, 0x0}}     {f = {0, 448, 0, 0}}
xmm3           {f = {0x0, 0x1, 0x0, 0x0}}       {f = {0, 1.75, 0, 0}}
xmm4           {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm5           {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm6           {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm7           {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm8           {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm9           {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm10          {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm11          {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm12          {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm13          {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm14          {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
xmm15          {f = {0x0, 0x0, 0x0, 0x0}}       {f = {0, 0, 0, 0}}
mxcsr          0x1fa0   8096