As others have said, you do not need the -1 in this case. If the array is fixed size, I would use strncpy instead. It was made for copying strings - sprintf was made for doing difficult formatting. However, if the size of the array is unknown or you are trying to determine how much storage is necessary for a formatted string. This is what I really like about the Standard specified version of snprintf:

char* get_error_message(char const *msg) {
    size_t needed = snprintf(NULL, 0, "%s: %s (%d)", msg, strerror(errno), errno);
    char  *buffer = malloc(needed+1);
    sprintf(buffer, "%s: %s (%d)", msg, strerror(errno), errno);
    return buffer;
}

Combine this feature with va_copy and you can create very safe formatted string operations.  

You don t need the -1, as the reference states:

The functions snprintf() and vsnprintf() do not write more than size bytes (including the trailing ).

Note the "including the trailing " part

No need for -1. C99 snprintf always zero-terminates. Size argument specifies the size of output buffer including zero terminator. The code, thus, becomes

char err_msg[32];
int ret = snprintf(err_msg, sizeof err_msg, "[ ST_ENGINE_FAILED ]");

ret contains actual number of characters printed (excluding zero terminator).

However, do not confuse with Microsoft s _snprintf (pre-C99), which does not null-terminate, and, for that matter, has completely different behaviour (e.g. returning -1 instead of would-be printed length in case if buffer is not big enough). If using _snprintf, you should be using the same code as in your question.

According to snprintf(3):

The functions snprintf() and vsnprintf() do not write more than size bytes (including the trailing ).

For the example given, you should be doing this instead:

char err_msg[32];
strncpy(err_msg, "[ ST_ENGINE_FAILED ]", sizeof(err_msg));
err_msg[sizeof(err_msg) - 1] =   ;

or even better:

char err_msg[32] = "[ ST_ENGINE_FAILED ]";

sizeof will return the number of bytes the datatype will use in memory, not the length of the string. E.g. sizeof(int) returns 4 bytes on a 32-bit system (well, depending on the implementation I guess). Since you use a constant in your array, you can happily pass that to the printf.