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Comment by vortico | Hacker News Reader

Don't use `strncpy()` and `strncat()` at all, use

    snprintf(buffer, buffer_len, "%s Stuff %s", str1, str2);

It's safe (C99, C++11) and easily extendible. Format strings are fun! Not the fastest, but if the bottleneck of your program is concatinating strings, just do it manually.

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This is a safe, if slow, alternative for strncpy, but it does not safely replace strncat. The C standard does not define the behavior if the code "sprintf(buf, "%s some further text", buf);" is used.

#define strncat(a, b, maxlen) snprintf(a + strlen(a), maxlen - strlen(a), "%s", b)

Too many evaluations of a. If you have snprintf, you probably have inline functions.

One day someone will do something like strncat(a += previous_delta, ...).

OPTION 1 (c):

  char* mystrncat(char *a, const char *b, size_t maxlen){
    snprintf(a + strlen(a), maxlen - strlen(a), "%s", b);
    return a;
  }

OPTION 2 (gccism):

   #define strncat(_a, b, maxlen) ({char* a = _a; snprintf(a + strlen(a), maxlen - strlen(a), "%s", b); _a;})

Right, don't use `buf` as an input and output with `snprintf`. A lot of people use `str(n)cat` and `str(n)cpy` together like

    char *a;
    char *b;
    char *c;
    strcpy(c, a);
    strcat(c, b);

and I meant that `snprintf` can replace that. But if you actually only have two strings, one with a larger buffer than what it contains, and one to be concatinated, then you can't use `snprintf` like that.

Strncpy gives back the length copied which you can use to append more text: char buf[1024]; size_t pos = 0; int ret = snprintf(buf, sizeof(buf), src1); if(ret >= 0) { pos += ret } else { error() } ret = snprintf(buf + pos, sizeof(buf) - pos, src2); ...

It's a good solution for strncat for sure! I'd guess that the majority of strncpy calls look something like:

    char *x = calloc(n, 1);
    strncpy(x, y, n - 1);

Which can be replaced with:

    char *x = strdup(y);

Which is both one line shorter and 100% less error prone!

C99 also introduced swprintf for wide char strings, with a different return value convention. Just to add to the pain when you change char to wchar.

Under swprintf, and related functions, %s still takes a char pointer, not wchar_t! So when you make everything wide, you have to edit all the %s to %ls.

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