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Portability is a concern when using the fread() and fwrite() functions across multiple, heterogeneous systems. In particular, it is never guaranteed that reading or writing of scalar data types such as integers, let alone aggregate types such as arrays or structures, will preserve the representation or value of the data. Implementations may differ in structure padding, floating-point model, number of bits per byte, endianness, and other attributes that cause binary data formats to be incompatible.

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This noncompliant code example reads data from a file stream into a data structure.:

struct myData {
  char c;
  long l;
};

/* ... */

FILE *file;
struct myData data;

/* initializeInitialize file */

if (fread(&data, sizeof(struct myData), 1, file) < sizeof(struct myData)) {
  /* Handle error */
}

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The best solution is to use either a text representation or a special library that will ensures data integrity.:

struct myData {
  char c;
  long l;
};

/* ... */

FILE *file;
struct myData data;
char buf[25];
char *end_ptr;

/* initializeInitialize file */

if (fgets(buf, 1, file) == NULL) {
  /* Handle error */
}

data.c = buf[0];

if (fgets(buf, sizeof(buf), file) == NULL) {
  /* Handle Error */
}

data.l = strtol(buf, &end_ptr, 10);

if ((ERANGE == errno)
 || (end_ptr == buf)
 || ('\n' != *end_ptr && '\0' != *end_ptr)) {
    /* Handle Error */
}

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Reading binary data that has a different format than expected may result in unintended program behavior.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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Bibliography

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[Summit

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1995

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]

20.5

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on

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C-FAQ

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Image Added Image Added | http://c-faq.com/misc/binaryfiles.html]Image Removed      09. Input Output (FIO)      Image Modified