I am using snprintf like this to avoid a buffer overrun:
char err_msg[32] = {0};
snprintf(err_msg, sizeof(err_msg) - 1, "[ ST_ENGINE_FAILED ]");
I added the -1 to reserve space for the null terminator in case the string is more than 32 bytes long.
Am I correct in my thinking?
Platform:
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) {
int e = errno;
size_t needed = snprintf(NULL, 0, "%s: %s (%d)", msg, strerror(e), e);
char *buffer = malloc(needed+1);
if (buffer) sprintf(buffer, "%s: %s (%d)", msg, strerror(e), e);
return buffer;
}
Combine this feature with va_copy and you can create very safe formatted string operations.
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.