// The c code shown here works:

int fd = open("/dev/gpiomem", O_FLAGS);
printf("fd = %d\n", fd);
long int* addr = mmap(0,
if (*addr == -1)
    printf("mmap() failed.");
printf("addr = %x\n", addr);

// But the assembly code shown here gives me EINVAL

mov     r0, #0
mov     r1, #0x1000
mov     r2, #PROT_RDWR
mov     r3, #MAP_SHARED
ldr     r4, =fileDescriptor
ldr     r4, [r4]
mov     r5, #0
bl      mmap   <== this fails with r0 = -1

When I step into mmap in gdb I see the following:

(gdb) s __GI___mmap (addr=0x0, len=4096, prot=3, flags=1, fd=0, offset=74548) at ../sysdeps/unix/sysv/linux/mmap.c:38 38 ../sysdeps/unix/sysv/linux/mmap.c: No such file or directory.

r0 through r5 were correct before issuing the call to mmap. It looks like fd and offset are getting clobbered.

Any suggestions would be greatly appreciated?

1 Answer 1


That was dumb! I forgot to push R5 and R4 onto the stack. The 32-bit Raspberry Pi only allows 4 parameters (R0 - R3) to be passed directly. Additional parameters must be passed on the stack.

  • Please accept your own answer with a click on the tick on its left side. Only this will finish the question and it will not pop up again year for year.
    – Ingo
    May 10, 2020 at 9:20

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