Is snprintf() ALWAYS null terminating?

by

As the other answers establish: It should:

snprintf … Writes the results to a character string buffer. (…)
will be terminated with a null character, unless buf_size is zero.

So all you have to take care is that you don’t pass an zero-size buffer to it, because (obviously) it cannot write a zero to “nowhere”.

However, beware that Microsoft’s library does not have a function called snprintf but instead historically only had a function called _snprintf (note leading underscore) which does not append a terminating null. Here’s the docs (VS 2012, ~~ VS 2013):

http://msdn.microsoft.com/en-us/library/2ts7cx93%28v=vs.110%29.aspx

Return Value

Let len be the length of the formatted data string (not including the
terminating null). len and count are in bytes for _snprintf, wide
characters for _snwprintf.

  • If len < count, then len characters are stored in buffer, a
    null-terminator is appended, and len is returned.

  • If len = count, then len characters are stored in buffer, no
    null-terminator is appended, and len is returned.

  • If len > count, then count characters are stored in buffer, no
    null-terminator is appended, and a negative value is returned.

(…)

Visual Studio 2015 (VC14) apparently introduced the conforming snprintf function, but the legacy one with the leading underscore and the non null-terminating behavior is still there:

The snprintf function truncates the output when len is greater
than or equal to count, by placing a null-terminator at
buffer[count-1]. (…)

For all functions other than snprintf, if len = count, len
characters are stored in buffer, no null-terminator is appended,
(…)

More Related Contents:

  1. How do I make this Linux program compile? [closed]
  2. C read file line by line
  3. What is the difference between sigaction and signal?
  4. Return value range of the main function
  5. When should I use mmap for file access?
  6. Connecting n commands with pipes in a shell?
  7. Atomicity of `write(2)` to a local filesystem
  8. GCC with -std=c99 complains about not knowing struct timespec
  9. Compiling without libc
  10. What does “#define _GNU_SOURCE” imply?
  11. Kill Thread in Pthread Library
  12. Checking if a file is a directory or just a file [duplicate]
  13. Are there any platforms where using structure copy on an fd_set (for select() or pselect()) causes problems?
  14. How can I convert a file pointer ( FILE* fp ) to a file descriptor (int fd)?
  15. Is there a way to flush a POSIX socket?
  16. How to get the current time in milliseconds from C in Linux?
  17. What is the rationale for fread/fwrite taking size and count as arguments?
  18. Recursive mkdir() system call on Unix
  19. Create statically-linked binary that uses getaddrinfo?
  20. How to make parent wait for all child processes to finish?
  21. Redirect stdout to a file
  22. Linux/POSIX equivalent for Win32’s CreateEvent, SetEvent, WaitForSingleObject
  23. How to use nanosleep() in C? What are `tim.tv_sec` and `tim.tv_nsec`?
  24. How do I find the current machine’s full hostname in C (hostname and domain information)?
  25. How do I reimplement (or wrap) a syscall function on Linux?
  26. Why does wait() set status to 255 instead of the -1 exit status of the forked process?
  27. How to convert from UTC to local time in C?
  28. Difference between C standard library and C POSIX library
  29. Linux function to get mount points
  30. How to know if a Linux system call is restartable or not?

Leave a Comment