Using snprintf to avoid buffer overruns

c, printf

Solution

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.

Problem

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: - GCC 4.4.1 - C99

Original source