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Alternatively, input character data as a null-terminated byte string and convert to an integer value using strtol() or a related function. (see INT06See ERR34-C. Use strtol() or a related function to convert a string token to an integer).Detect errors when converting a string to a number.)

Noncompliant Code Example

This noncompliant code example uses the scanf() function to read a string from stdin and convert it to a long. The scanf() and fscanf() functions have undefined behavior if the value of the result of this operation cannot be represented as an integer.

long slnum_long;

if (scanf("%ld", &slnum_long) != 1) {
  /* handleHandle error */
}

In general, do not use scanf() to parse integers or floating-point numbers from input strings , because the input could contain numbers not representable by the argument type.

Compliant Solution (Linux)

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This compliant example uses the Linux {{scanf()}} implementation's built -in error handling to validate input. On Linux platforms, {{scanf()}} sets {{errno}} to {{ERANGE}} if the result of integer conversion cannot be represented within the size specified by the format string \[ [Linux 08|AA. C References#Linux 08]\]. Note that this is a platform dependent solution. Therefore, this should only be used where portability is not a 2008]. Note that this solution is platform dependent, so it should be used only where portability is not a concern.

Code Block
bgColor #ccccff lang c
long slnum_long; errno = 0; if (scanf("%ld", &slnum_long) != 1) { /* handleHandle error */ } else if (ERANGE == errno) { if (puts("number out of range\n") == EOF) { /* Handle error */ } }

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This compliant example uses fgets() to input a string and strtol() to convert the string to an integer. Error checking is provided to make sure that the value is a valid integer in the range of long.

Code Block
bgColor #ccccff lang c
char buff[25]; char *end_ptr; long slnum_long; if (fgets(buff, sizeof(buff), stdin) == NULL) { if (puts("EOF or read error\n") == EOF) { /* Handle error */ } } else { errno = 0; slnum_long = strtol(buff, &end_ptr, 10); if (ERANGE == errno) { if (puts("number out of range\n") == EOF) { /* Handle error */ } } else if (end_ptr == buff) { if (puts("not valid numeric input\n") == EOF) { /* Handle error */ } } else if ('\n' != *end_ptr && '\0' != *end_ptr) { if (puts("extra characters on input line\n") == EOF) { /* Handle error */ } } }

Note that this solution treats any trailing characters, including white-space whitespace characters, as an error condition.

Risk Assessment

While it Although it is relatively rare for a violation of this recommendation to result in a security vulnerability, it can easily result in lost or misinterpreted data.

Automated Detection

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Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Other Languages

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Related Guidelines

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References

\[[Klein 02|AA. C References#Klein 02]\]
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.20.1.4, "The strtol, strtoll, strtoul, and strtoull functions," and Section 7.19.6, "Formatted input/output functions"
\[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID 192|http://cwe.mitre.org/data/definitions/192.html], "Integer Coercion Error"; and [CWE ID 197|http://cwe.mitre.org/data/definitions/197.html], "Numeric Truncation Error"
\[[Linux 08|AA. C References#Linux 08]\] [scanf(3)|http://www.kernel.org/doc/man-pages/online/pages/man3/scanf.3.html]

Bibliography

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Image Added Image Added Image AddedINT04-C. Enforce limits on integer values originating from untrusted sources      04. Integers (INT)      INT06-C. Use strtol() or a related function to convert a string token to an integer