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I do not have much experience reading C code. Is the use of gotos and labels in the GNU code common?

Peaker 15y ago
For error handling it is considered fine. The way it is used in that code is pretty horrible, IMO, and splitting it into functions and using "return" in place of goto would have been far better.
sjs 15y ago
In C it's generally accepted that forward jumps using goto are okay because the version that avoids goto would be complicated and confusing. Often used for error handling and memory management.
- sliverstorm 15y ago
  Error handling because you want to make a beeline to the handler, and memory management because you need it to be quick and goto is cheap?
  - sparky 15y ago
    In both cases, to avoid duplicating code. If you malloc() something in your function and intend on free()ing it before returning, it's often considered best practices to write the return block (which includes the free()s) once, and use "goto returnblocklabel" if you need to return early. So rather than this: void test() { int *x = malloc(1000); int *y = malloc(1000); for(int i = 0; i < 999; i++) { if(badThing) { free(x); free(y); return; } doStuff() if(otherBadThing) { free(x); free(y); return; } } free(x); free(y); return; } You'd have: void test() { int *x = malloc(1000); int *y = malloc(1000); for(int i = 0; i < 999; i++) { if(badThing) goto ret; doStuff() if(otherBadThing) goto ret; } ret: free(x); free(y); return; }
    - Jach 15y ago
      I realize your example is contrived, but a simple 'break' statement (which in essence is a goto...) would work just as well. :) I somewhat vaguely recall a situation in my C class that I wanted to use goto to avoid duplicate code but the professor had previously threatened huge negative points if his scripts detected one. (That whole semester was just as much about conforming your code to his narrow specifics because "that's what happens in the real world." as learning C.)
- Peaker 15y ago
  Can you explain what you mean by use of gotos for memory management?
  - sjs 15y ago
    Here's an example: void do_stuff(char *name) { char *my_name = strdup(name); if (condition_signalling_no_work()) goto done; if (do_something_with(my_name)) goto done; ... done: free(my_name); return; } If it helps think of it as a finally block. The function does some things and no matter what it has to run that block at the end.
    - Peaker 15y ago
      Ah, so for resource cleanup in general, not memory management in particular.
hackermom 15y ago
A somewhat related by-the-way: if you grep for "goto " in the Linux kernel sources, you'll find several hundreds of occurrences.

For error handling it is considered fine. The way it is used in that code is pretty horrible, IMO, and splitting it into functions and using "return" in place of goto would have been far better.

sjs 15y ago

In C it's generally accepted that forward jumps using goto are okay because the version that avoids goto would be complicated and confusing. Often used for error handling and memory management.

sliverstorm 15y ago
Error handling because you want to make a beeline to the handler, and memory management because you need it to be quick and goto is cheap?
- sparky 15y ago
  In both cases, to avoid duplicating code. If you malloc() something in your function and intend on free()ing it before returning, it's often considered best practices to write the return block (which includes the free()s) once, and use "goto returnblocklabel" if you need to return early. So rather than this: void test() { int *x = malloc(1000); int *y = malloc(1000); for(int i = 0; i < 999; i++) { if(badThing) { free(x); free(y); return; } doStuff() if(otherBadThing) { free(x); free(y); return; } } free(x); free(y); return; } You'd have: void test() { int *x = malloc(1000); int *y = malloc(1000); for(int i = 0; i < 999; i++) { if(badThing) goto ret; doStuff() if(otherBadThing) goto ret; } ret: free(x); free(y); return; }
  - Jach 15y ago
    I realize your example is contrived, but a simple 'break' statement (which in essence is a goto...) would work just as well. :) I somewhat vaguely recall a situation in my C class that I wanted to use goto to avoid duplicate code but the professor had previously threatened huge negative points if his scripts detected one. (That whole semester was just as much about conforming your code to his narrow specifics because "that's what happens in the real world." as learning C.)
Peaker 15y ago
Can you explain what you mean by use of gotos for memory management?
- sjs 15y ago
  Here's an example: void do_stuff(char *name) { char *my_name = strdup(name); if (condition_signalling_no_work()) goto done; if (do_something_with(my_name)) goto done; ... done: free(my_name); return; } If it helps think of it as a finally block. The function does some things and no matter what it has to run that block at the end.
  - Peaker 15y ago
    Ah, so for resource cleanup in general, not memory management in particular.

hackermom 15y ago

A somewhat related by-the-way: if you grep for "goto " in the Linux kernel sources, you'll find several hundreds of occurrences.