What size should I allow for strerror_r?

Question

The OpenGroup POSIX.1-2001 defines strerror_r, as does The Linux Standard Base Core Specification 3.1. But I can find no reference to the maximum size that could be reasonably expected for an error message. I expected some define somewhere that I could put in my code but there is none that I can find.

The code must be thread safe. Which is why strerror_r is used and not strerror.

Does any one know the symbol I can use? I should I create my own?

---

Example

int result = gethostname(p_buffy, size_buffy);
int errsv = errno;
if (result < 0)
{
    char buf[256];
    char const * str = strerror_r(errsv, buf, 256);
    syslog(LOG_ERR,
             "gethostname failed; errno=%d(%s), buf='%s'",
             errsv,
             str,
             p_buffy);
     return errsv;
}

---

From the documents:

The Open Group Base Specifications Issue 6:

ERRORS

The strerror_r() function may fail if:

• [ERANGE] Insufficient storage was supplied via strerrbuf and buflen to contain the generated message string.

---

From the source:

glibc-2.7/glibc-2.7/string/strerror.c:41:

    char *
    strerror (errnum)
         int errnum;
    {
        ...
        buf = malloc (1024);

Answer 1

I wouldn't worry about it - a buffer size of 256 is far more than sufficient, and 1024 is overkill. You could use strerror() instead of strerror_r(), and then optionally strdup() the result if you need to store the error string. This isn't thread-safe, though. If you really need to use strerror_r() instead of strerror() for thread safety, just use a size of 256. In glibc-2.7, the longest error message string is 50 characters ("Invalid or incomplete multibyte or wide character"). I wouldn't expect future error messages to be significantly longer (in the worst case, a few bytes longer).

Answer 2

Having a sufficiently large static limit is probably good enough for all situations. If you really need to get the entire error message, you can use the GNU version of strerror_r, or you can use the standard version and poll it with successively larger buffers until you get what you need. For example, you may use something like the code below.

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/* Call strerror_r and get the full error message. Allocate memory for the
 * entire string with malloc. Return string. Caller must free string.
 * If malloc fails, return NULL.
 */
char *all_strerror(int n)
{
    char *s;
    size_t size;

    size = 1024;
    s = malloc(size);
    if (s == NULL)
        return NULL;

    while (strerror_r(n, s, size) == -1 && errno == ERANGE) {
        size += 2;
        s = realloc(s, size);
        if (s == NULL)
            return NULL;
    }

    return s;
}

int main(int argc, char **argv)
{
    for (int i = 1; i < argc; ++i) {
        int n = atoi(argv[i]);
        char *s = all_strerror(n);
        printf("[%d]: %s\n", n, s);
        free(s);
    }

    return 0;
}