Rule 07. Characters and Strings (STR)

Created by Robert C. Seacord, last modified by David Svoboda on Aug 06, 2025

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STR30-C. Do not attempt to modify string literals
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STR31-C. Guarantee that storage for strings has sufficient space for character data and the null terminator
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STR32-C. Do not pass a non-null-terminated character sequence to a library function that expects a string
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STR34-C. Cast characters to unsigned char before converting to larger integer sizes
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STR37-C. Arguments to character-handling functions must be representable as an unsigned char
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STR38-C. Do not confuse narrow and wide character strings and functions

Information for Editors
In order to have a new guideline automatically listed above be sure to label it str and rule.

Risk Assessment Summary

Rule	Severity	Likelihood	Detectable	Repairable	Priority	Level
STR30-C	Low	Likely	No	Yes	P6	L2
STR31-C	High	Likely	No	No	P9	L2
STR32-C	High	Probable	No	Yes	P12	L1
STR34-C	Medium	Probable	Yes	No	P8	L2
STR37-C	Low	Unlikely	Yes	Yes	P3	L3
STR38-C	High	Likely	Yes	No	P18	L1

Related Rules and Recommendations

FIO20-C. Avoid unintentional truncation when using fgets() or fgetws()

FIO30-C. Exclude user input from format strings

FIO40-C. Reset strings on fgets() or fgetws() failure

FIO47-C. Use valid format strings

INT07-C. Use only explicitly signed or unsigned char type for numeric values

POS30-C. Use the readlink() function properly

STR37-C. Arguments to character-handling functions must be representable as an unsigned char

1 Comment

David Keaton
This document uses the terminology of the C standard, which is that "null" is an English adjective and "NULL" is a C noun. The term "NUL" is from ASCII and does not appear in the C standard. For that, this document says "null character."
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- Mar 14, 2008