What is a bus error

Question

What does the  bus error  message mean  and how does it differ from a segfault

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A segfault is accessing memory that you re not allowed to access   It s read-only  you don t have permission  etc     A bus error is trying to access memory that can t possibly be there   You ve used an address that s meaningless to the system  or the wrong kind of address for that operation

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I agree with all the answers above  Here are my 2 cents regarding the BUS error  A BUS error need not arise from the instructions within the program s code  This can happen when you are running a binary and during the execution  the binary is modified  overwritten by a build or deleted  etc    Verifying if this is the case A simple way to check if this is the cause is by launching a couple of instances of the same binary form a build output directory  and running a build after they start  Both the running instances would crash with a SIGBUS error shortly after the build has finished and replaced the binary  the one that both the instances are currently running   Underlying Reason This is because OS swaps memory pages and in some cases  the binary might not be entirely loaded in memory  These crashes would occur when the OS tries to fetch the next page from the same binary  but the binary has changed since the last time it was read

User · Answer

I believe the kernel raises SIGBUS   when an application exhibits data   misalignment on the data bus  I think   that since most    modern compilers   for most processors pad   align the   data for the programmers  the   alignment troubles of yore  at least    mitigated  and hence one does not see   SIGBUS too often these days  AFAIK     From  Here

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I believe the kernel raises SIGBUS   when an application exhibits data   misalignment on the data bus  I think   that since most    modern compilers   for most processors pad   align the   data for the programmers  the   alignment troubles of yore  at least    mitigated  and hence one does not see   SIGBUS too often these days  AFAIK     From  Here

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A typical buffer overflow which results in Bus error is         char buf 255       sprintf buf   s  s n   ifname  message       Here if size of the string in double quotes      is more than buf size it gives bus error

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It depends on your OS  CPU  Compiler  and possibly other factors   In general it means the CPU bus could not complete a command  or suffered a conflict  but that could mean a whole range of things depending on the environment and code being run   -Adam

User · Answer

A segfault is accessing memory that you re not allowed to access   It s read-only  you don t have permission  etc     A bus error is trying to access memory that can t possibly be there   You ve used an address that s meaningless to the system  or the wrong kind of address for that operation

User · Answer

It depends on your OS  CPU  Compiler  and possibly other factors   In general it means the CPU bus could not complete a command  or suffered a conflict  but that could mean a whole range of things depending on the environment and code being run   -Adam

User · Answer

A typical buffer overflow which results in Bus error is         char buf 255       sprintf buf   s  s n   ifname  message       Here if size of the string in double quotes      is more than buf size it gives bus error

User · Answer

Bus errors are rare nowadays on x86 and occur when your processor cannot even attempt the memory access requested  typically    using a processor instruction with an address that does not satisfy its alignment requirements    Segmentation faults occur when accessing memory which does not belong to your process  they are very common and are typically the result of    using a pointer to something that was deallocated  using an uninitialized hence bogus pointer  using a null pointer  overflowing a buffer    PS  To be more precise this is not manipulating the pointer itself that will cause issues  it s accessing the memory it points to  dereferencing

User · Answer

A specific example of a bus error I just encountered while programming C on OS X    include  lt string h gt   include  lt stdio h gt   int main void        char buffer 120       fgets buffer  sizeof buffer  stdin       strcat  foo   buffer       return 0      In case you don t remember the docs strcat appends the second argument to the first by changing the first argument flip the arguments and it works fine   On linux this gives a segmentation fault as expected   but on OS X it gives a bus error  Why  I really don t know

User · Answer

You can also get SIGBUS when a code page cannot be paged in for some reason

User · Answer

My reason for bus error on Mac OS X was that I tried to allocate about 1Mb on the stack  This worked well in one thread  but when using openMP this drives to bus error  because Mac OS X has very limited stack size for non-main threads

User · Answer

It depends on your OS  CPU  Compiler  and possibly other factors   In general it means the CPU bus could not complete a command  or suffered a conflict  but that could mean a whole range of things depending on the environment and code being run   -Adam

User · Answer

A segfault is accessing memory that you re not allowed to access   It s read-only  you don t have permission  etc     A bus error is trying to access memory that can t possibly be there   You ve used an address that s meaningless to the system  or the wrong kind of address for that operation

User · Answer

It normally means an un-aligned access   An attempt to access memory that isn t physically present would also give a bus error  but you won t see this if you re using a processor with an MMU and an OS that s not buggy  because you won t have any non-existent memory mapped to your process s address space

User · Answer

Bus errors are rare nowadays on x86 and occur when your processor cannot even attempt the memory access requested  typically    using a processor instruction with an address that does not satisfy its alignment requirements    Segmentation faults occur when accessing memory which does not belong to your process  they are very common and are typically the result of    using a pointer to something that was deallocated  using an uninitialized hence bogus pointer  using a null pointer  overflowing a buffer    PS  To be more precise this is not manipulating the pointer itself that will cause issues  it s accessing the memory it points to  dereferencing

User · Answer

You can also get SIGBUS when a code page cannot be paged in for some reason

User · Answer

One classic instance of a bus error is on certain architecures  such as the SPARC  at least some SPARCs  maybe this has been changed   is when you do a mis-aligned access  For instance   unsigned char data 6    unsigned int     data   2    0xdeadf00d    This snippet tries to write the 32-bit integer value 0xdeadf00d to an address that is  most likely  not properly aligned  and will generate a bus error on architectures that are  picky  in this regard  The Intel x86 is  by the way  not such an architecture  it would allow the access  albeit execute it more slowly

User · Answer

It normally means an un-aligned access   An attempt to access memory that isn t physically present would also give a bus error  but you won t see this if you re using a processor with an MMU and an OS that s not buggy  because you won t have any non-existent memory mapped to your process s address space

User · Answer

mmap minimal POSIX 7 example   Bus error  happens when the kernel sends SIGBUS to a process   A minimal example that produces it because ftruncate was forgotten    include  lt fcntl h gt     O  constants     include  lt unistd h gt     ftruncate     include  lt sys mman h gt     mmap     int main         int fd      int  map      int size   sizeof int       char  name     a        shm unlink name       fd   shm open name  O RDWR   O CREAT   mode t 0600          THIS is the cause of the problem           ftruncate fd  size         map   mmap NULL  size  PROT READ   PROT WRITE  MAP SHARED  fd  0          This is what generates the SIGBUS          map   0      Run with   gcc -std c99 main c -lrt   a out   Tested in Ubuntu 14 04   POSIX describes SIGBUS as      Access to an undefined portion of a memory object    The mmap spec says that      References within the address range starting at pa and continuing for len bytes to whole pages following the end of an object shall result in delivery of a SIGBUS signal    And shm open says that it generates objects of size 0      The shared memory object has a size of zero    So at  map   0 we are touching past the end of the allocated object   Unaligned stack memory accesses in ARMv8 aarch64  This was mentioned at  What is a bus error  for SPARC  but here I will provide a more reproducible example   All you need is a freestanding aarch64 program    global  start  start  asm main after prologue         misalign the stack out of 16-bit boundary        add sp  sp   -4        access the stack        ldr w0   sp          exit syscall in case SIGBUS does not happen        mov x0  0     mov x8  93     svc 0   That program then raises SIGBUS on Ubuntu 18 04 aarch64  Linux kernel 4 15 0 in a ThunderX2 server machine   Unfortunately  I can t reproduce it on QEMU v4 0 0 user mode  I m not sure why   The fault appears to be optional and controlled by the SCTLR ELx SA and SCTLR EL1 SA0 fields  I have summarized the related docs a bit further here

User · Answer

I was getting a bus error when the root directory was at 100

User · Answer

I just found out the hard way that on an ARMv7 processor you can write some code that gives you a segmentation fault when unoptimized  but it gives you a bus error when compiled with -O2  optimize more    I am using the GCC ARM gnueabihf cross compiler from Ubuntu 64 bit

User · Answer

It depends on your OS  CPU  Compiler  and possibly other factors   In general it means the CPU bus could not complete a command  or suffered a conflict  but that could mean a whole range of things depending on the environment and code being run   -Adam

User · Answer

One classic instance of a bus error is on certain architecures  such as the SPARC  at least some SPARCs  maybe this has been changed   is when you do a mis-aligned access  For instance   unsigned char data 6    unsigned int     data   2    0xdeadf00d    This snippet tries to write the 32-bit integer value 0xdeadf00d to an address that is  most likely  not properly aligned  and will generate a bus error on architectures that are  picky  in this regard  The Intel x86 is  by the way  not such an architecture  it would allow the access  albeit execute it more slowly

User · Answer

It normally means an un-aligned access   An attempt to access memory that isn t physically present would also give a bus error  but you won t see this if you re using a processor with an MMU and an OS that s not buggy  because you won t have any non-existent memory mapped to your process s address space

User · Answer

One classic instance of a bus error is on certain architecures  such as the SPARC  at least some SPARCs  maybe this has been changed   is when you do a mis-aligned access  For instance   unsigned char data 6    unsigned int     data   2    0xdeadf00d    This snippet tries to write the 32-bit integer value 0xdeadf00d to an address that is  most likely  not properly aligned  and will generate a bus error on architectures that are  picky  in this regard  The Intel x86 is  by the way  not such an architecture  it would allow the access  albeit execute it more slowly

User · Answer

I believe the kernel raises SIGBUS   when an application exhibits data   misalignment on the data bus  I think   that since most    modern compilers   for most processors pad   align the   data for the programmers  the   alignment troubles of yore  at least    mitigated  and hence one does not see   SIGBUS too often these days  AFAIK     From  Here

User · Answer

One classic instance of a bus error is on certain architecures  such as the SPARC  at least some SPARCs  maybe this has been changed   is when you do a mis-aligned access  For instance   unsigned char data 6    unsigned int     data   2    0xdeadf00d    This snippet tries to write the 32-bit integer value 0xdeadf00d to an address that is  most likely  not properly aligned  and will generate a bus error on architectures that are  picky  in this regard  The Intel x86 is  by the way  not such an architecture  it would allow the access  albeit execute it more slowly

User · Answer

A segfault is accessing memory that you re not allowed to access   It s read-only  you don t have permission  etc     A bus error is trying to access memory that can t possibly be there   You ve used an address that s meaningless to the system  or the wrong kind of address for that operation

User · Answer

To add to what blxtd answered above  bus errors also occur when your process cannot attempt to access the memory of a particular  variable    for  j   0  i  lt  n  j          for  i  0  i  lt  m  i              a n 1  j     a i  j             Notice the  inadvertent  usage of variable  i  in the first  for loop   That s what is causing the bus error in this case

User · Answer

I believe the kernel raises SIGBUS   when an application exhibits data   misalignment on the data bus  I think   that since most    modern compilers   for most processors pad   align the   data for the programmers  the   alignment troubles of yore  at least    mitigated  and hence one does not see   SIGBUS too often these days  AFAIK     From  Here

User · Answer

I just found out the hard way that on an ARMv7 processor you can write some code that gives you a segmentation fault when unoptimized  but it gives you a bus error when compiled with -O2  optimize more    I am using the GCC ARM gnueabihf cross compiler from Ubuntu 64 bit

User · Answer

You can also get SIGBUS when a code page cannot be paged in for some reason

User · Answer

Bus errors are rare nowadays on x86 and occur when your processor cannot even attempt the memory access requested  typically    using a processor instruction with an address that does not satisfy its alignment requirements    Segmentation faults occur when accessing memory which does not belong to your process  they are very common and are typically the result of    using a pointer to something that was deallocated  using an uninitialized hence bogus pointer  using a null pointer  overflowing a buffer    PS  To be more precise this is not manipulating the pointer itself that will cause issues  it s accessing the memory it points to  dereferencing

User · Answer

You can also get SIGBUS when a code page cannot be paged in for some reason

User · Answer

Bus errors are rare nowadays on x86 and occur when your processor cannot even attempt the memory access requested  typically    using a processor instruction with an address that does not satisfy its alignment requirements    Segmentation faults occur when accessing memory which does not belong to your process  they are very common and are typically the result of    using a pointer to something that was deallocated  using an uninitialized hence bogus pointer  using a null pointer  overflowing a buffer    PS  To be more precise this is not manipulating the pointer itself that will cause issues  it s accessing the memory it points to  dereferencing

User · Answer

It normally means an un-aligned access   An attempt to access memory that isn t physically present would also give a bus error  but you won t see this if you re using a processor with an MMU and an OS that s not buggy  because you won t have any non-existent memory mapped to your process s address space

User · Answer

I agree with all the answers above  Here are my 2 cents regarding the BUS error  A BUS error need not arise from the instructions within the program s code  This can happen when you are running a binary and during the execution  the binary is modified  overwritten by a build or deleted  etc    Verifying if this is the case A simple way to check if this is the cause is by launching a couple of instances of the same binary form a build output directory  and running a build after they start  Both the running instances would crash with a SIGBUS error shortly after the build has finished and replaced the binary  the one that both the instances are currently running   Underlying Reason This is because OS swaps memory pages and in some cases  the binary might not be entirely loaded in memory  These crashes would occur when the OS tries to fetch the next page from the same binary  but the binary has changed since the last time it was read

User · Answer

mmap minimal POSIX 7 example   Bus error  happens when the kernel sends SIGBUS to a process   A minimal example that produces it because ftruncate was forgotten    include  lt fcntl h gt     O  constants     include  lt unistd h gt     ftruncate     include  lt sys mman h gt     mmap     int main         int fd      int  map      int size   sizeof int       char  name     a        shm unlink name       fd   shm open name  O RDWR   O CREAT   mode t 0600          THIS is the cause of the problem           ftruncate fd  size         map   mmap NULL  size  PROT READ   PROT WRITE  MAP SHARED  fd  0          This is what generates the SIGBUS          map   0      Run with   gcc -std c99 main c -lrt   a out   Tested in Ubuntu 14 04   POSIX describes SIGBUS as      Access to an undefined portion of a memory object    The mmap spec says that      References within the address range starting at pa and continuing for len bytes to whole pages following the end of an object shall result in delivery of a SIGBUS signal    And shm open says that it generates objects of size 0      The shared memory object has a size of zero    So at  map   0 we are touching past the end of the allocated object   Unaligned stack memory accesses in ARMv8 aarch64  This was mentioned at  What is a bus error  for SPARC  but here I will provide a more reproducible example   All you need is a freestanding aarch64 program    global  start  start  asm main after prologue         misalign the stack out of 16-bit boundary        add sp  sp   -4        access the stack        ldr w0   sp          exit syscall in case SIGBUS does not happen        mov x0  0     mov x8  93     svc 0   That program then raises SIGBUS on Ubuntu 18 04 aarch64  Linux kernel 4 15 0 in a ThunderX2 server machine   Unfortunately  I can t reproduce it on QEMU v4 0 0 user mode  I m not sure why   The fault appears to be optional and controlled by the SCTLR ELx SA and SCTLR EL1 SA0 fields  I have summarized the related docs a bit further here

User · Answer

I was getting a bus error when the root directory was at 100

User · Answer

A specific example of a bus error I just encountered while programming C on OS X    include  lt string h gt   include  lt stdio h gt   int main void        char buffer 120       fgets buffer  sizeof buffer  stdin       strcat  foo   buffer       return 0      In case you don t remember the docs strcat appends the second argument to the first by changing the first argument flip the arguments and it works fine   On linux this gives a segmentation fault as expected   but on OS X it gives a bus error  Why  I really don t know

User · Answer

To add to what blxtd answered above  bus errors also occur when your process cannot attempt to access the memory of a particular  variable    for  j   0  i  lt  n  j          for  i  0  i  lt  m  i              a n 1  j     a i  j             Notice the  inadvertent  usage of variable  i  in the first  for loop   That s what is causing the bus error in this case

User · Answer

My reason for bus error on Mac OS X was that I tried to allocate about 1Mb on the stack  This worked well in one thread  but when using openMP this drives to bus error  because Mac OS X has very limited stack size for non-main threads

[c] What is a bus error?

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