4 ms·

C doesn't have exceptions...?

by dahfizz 4y ago

heywhatupboys 4y ago
kinda does though. floating point exception vectors are a thing and settable from most compiler impls
- dahfizz 4y ago
  fenv is available in standard C99. There is a GNU extension to trap and throw a signal when a fp error is hit (feenableexcept). I would definitely argue that signals != exceptions.
jacquesm 4y ago
The floating point implementation can signal SIGFPE, with a code FPE_INTOVF, it would seem to me that that is a suitable exception mechanism, it's just that the source isn't the floating point unit but the regular CPU. Signals (kill), signal, raise, sigsetjmp, siglongjmp etc are C's exception handling mechanism. It's not as well integrated into the language as say a try-catch construct but it works well enough for situations like these. See: signal.h and setjmp.h https://en.wikipedia.org/wiki/Signal_(IPC) https://en.wikipedia.org/wiki/Signal_(IPC)
- dahfizz 4y ago
  Signals are not exceptions. Languages with exceptions still have to handle signals separately. Signals are an OS construct, and exceptions are a language construct. To the question of "why not raise a signal on integer overflow in C?" - because signals are a terrible way of dealing with this. The signal handler doesn't know what code caused the overflow, and can't really do anything about it. Once the signal handler returns, the code itself has no idea it caused an overflow. Signals are a way for the OS to send signals to your program and not for control flow, after all. That's why `feenableexcept` is a niche extension that nobody uses. The standard way of checking for fp errors is by calling `fetestexcept`. Personally I prefer this strategy (doing operation, then checking for errors) vs the new proposal for ints (checking for potential errors before doing the operation). But that is a matter of taste.
  - jacquesm 4y ago
    Interesting, I've always considered signals to be an exception handling mechanism, as in the 'normal flow' of a program is interrupted and dealt with - or not - through some other mechanism. Learn a new thing every day, even after 40 years of programming in C :) Thanks! https://en.wikipedia.org/wiki/Exception_handling https://en.wikipedia.org/wiki/Exception_handling (which has this bit: "C does not have try-catch exception handling, but uses return codes for error checking. The setjmp and longjmp standard library functions can be used to implement try-catch handling via macros.")
    - dahfizz 4y ago
      Its all semantics, I guess. You could argue that signals can be used to handle exceptional conditions (dereffing NULL). But signals are significantly different than what other programming languages call "exceptions". Its the same thing as "run time". Pedantically, crt0 exists and therefore C has a "runtime". But it is nothing like what we refer to as a "runtime" today. The literal words are true but the meaning of the words doesn't match expectations.
      - jacquesm 4y ago
        > signals are significantly different than what other programming languages call "exceptions" C is likely considerably older than those 'other programming languages' and I'm still stuck in the past with my terminology.
        kevin_thibedeau 4y ago
        Ada had exceptions in 83 that are analogous to their modern incarnation. C was still just a baby then.
        jacquesm 4y ago
        Ada was unobtanium for a long time for mere mortals like myself. In 1983 I was 18 and had access to a C compiler, an Ada compiler would have cost me an arm, a leg and my still to be first born, and information about the language was pretty much limited to what you could get from magazine articles of people that had maybe at some point known someone who had seen an Ada compiler in the wild. The only other realistic options outside of government/enterprise were Pascal, BASIC and assembler, and within the bulk of the work was done in COBOL.

C doesn't have exceptions...?

heywhatupboys 4y ago

kinda does though. floating point exception vectors are a thing and settable from most compiler impls

dahfizz 4y ago
fenv is available in standard C99. There is a GNU extension to trap and throw a signal when a fp error is hit (feenableexcept). I would definitely argue that signals != exceptions.

jacquesm 4y ago

The floating point implementation can signal SIGFPE, with a code FPE_INTOVF, it would seem to me that that is a suitable exception mechanism, it's just that the source isn't the floating point unit but the regular CPU. Signals (kill), signal, raise, sigsetjmp, siglongjmp etc are C's exception handling mechanism. It's not as well integrated into the language as say a try-catch construct but it works well enough for situations like these. See: signal.h and setjmp.h https://en.wikipedia.org/wiki/Signal_(IPC) https://en.wikipedia.org/wiki/Signal_(IPC)

dahfizz 4y ago
Signals are not exceptions. Languages with exceptions still have to handle signals separately. Signals are an OS construct, and exceptions are a language construct. To the question of "why not raise a signal on integer overflow in C?" - because signals are a terrible way of dealing with this. The signal handler doesn't know what code caused the overflow, and can't really do anything about it. Once the signal handler returns, the code itself has no idea it caused an overflow. Signals are a way for the OS to send signals to your program and not for control flow, after all. That's why `feenableexcept` is a niche extension that nobody uses. The standard way of checking for fp errors is by calling `fetestexcept`. Personally I prefer this strategy (doing operation, then checking for errors) vs the new proposal for ints (checking for potential errors before doing the operation). But that is a matter of taste.
- jacquesm 4y ago
  Interesting, I've always considered signals to be an exception handling mechanism, as in the 'normal flow' of a program is interrupted and dealt with - or not - through some other mechanism. Learn a new thing every day, even after 40 years of programming in C :) Thanks! https://en.wikipedia.org/wiki/Exception_handling https://en.wikipedia.org/wiki/Exception_handling (which has this bit: "C does not have try-catch exception handling, but uses return codes for error checking. The setjmp and longjmp standard library functions can be used to implement try-catch handling via macros.")
  - dahfizz 4y ago
    Its all semantics, I guess. You could argue that signals can be used to handle exceptional conditions (dereffing NULL). But signals are significantly different than what other programming languages call "exceptions". Its the same thing as "run time". Pedantically, crt0 exists and therefore C has a "runtime". But it is nothing like what we refer to as a "runtime" today. The literal words are true but the meaning of the words doesn't match expectations.
    - jacquesm 4y ago
      > signals are significantly different than what other programming languages call "exceptions" C is likely considerably older than those 'other programming languages' and I'm still stuck in the past with my terminology.
      - kevin_thibedeau 4y ago
        Ada had exceptions in 83 that are analogous to their modern incarnation. C was still just a baby then.
        jacquesm 4y ago
        Ada was unobtanium for a long time for mere mortals like myself. In 1983 I was 18 and had access to a C compiler, an Ada compiler would have cost me an arm, a leg and my still to be first born, and information about the language was pretty much limited to what you could get from magazine articles of people that had maybe at some point known someone who had seen an Ada compiler in the wild. The only other realistic options outside of government/enterprise were Pascal, BASIC and assembler, and within the bulk of the work was done in COBOL.