> diff --git a/src/legacy/error.c b/src/legacy/error.c
> new file mode 100644
> index 00000000..c5000fa4
> --- /dev/null
> +++ b/src/legacy/error.c
> @@ -0,0 +1,69 @@
> +#include <error.h>
> +#include <stdarg.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include "libc.h"
> +
> +void (*error_print_progname) (void) = 0;
> +unsigned int error_message_count = 0;
> +int error_one_per_line = 0;
> +
> +static unsigned int saved_linenum = 0;
> +static const char *saved_file = 0;
> +
> +static void errorv(int status, int errnum, const char *file, unsigned int linenum, const char *fmt, va_list ap)
> +{
> +     ++error_message_count;
> +
> +     fflush(stdout);
> +     flockfile(stderr);
> +
> +     if (error_print_progname)
> +             error_print_progname();
> +     else {
> +             fprintf(stderr, "%s:", __progname_full);
> +             if (!file)
> +                     fputc(' ', stderr);
> +     }
> +
> +     if (file)
> +             fprintf(stderr, "%s:%u: ", file, linenum);
> +
> +     vfprintf(stderr, fmt, ap);
> +     if (errnum)
> +             fprintf(stderr, ": %s", strerror(errnum));
> +     fputc('\n', stderr);
> +
> +     funlockfile(stderr);
> +
> +     if (status)
> +             exit(status);
> +}
> +
> +void error(int status, int errnum, const char *fmt, ...)
> +{
> +     va_list ap;
> +     va_start(ap, fmt);
> +     errorv(status, errnum, NULL, 0, fmt, ap);
> +     va_end(ap);
> +}
> +
> +void error_at_line(int status, int errnum, const char *file, unsigned int linenum, const char *fmt, ...)
> +{
> +     if (error_one_per_line) {
> +             if(saved_linenum == linenum && file != NULL &&
> +                saved_file != NULL && !strcmp(file, saved_file))
> +                     return;
> +             saved_linenum = linenum;
> +             // Assuming that the lifetime of the passed in file name extends
> +             // until the next call is rather questionable, but appears to be
> +             // the expected semantics.
> +             saved_file = file;
> +     }
> +
> +     va_list ap;
> +     va_start(ap, fmt);
> +     errorv(status, errnum, file, linenum, fmt, ap);
> +     va_end(ap);
> +}
> --
> 2.22.0.709.g102302147b-goog

One other question: what is the thread-safety of these interfaces
supposed to be? The globals are accessed without any synchronization,
but flockfile is used to synchronize with concurrent access to stderr.
This is plausibly correct (if the error.h functions themselves can't
be used from multiple threads, but can be used in a multithreaded
application), but at least questionable.

The error_at_line stuff is probably not possible to make thread-safe
without thread-local state for saved_file/linenum, which would be a
disproportionate cost for these interfaces.

I'm not objecting to anything with these comments, just wanting to
confirm that you think it's reasonable as-is.