CERT C: Rec. ERR01-C

Use ferror() rather than errno to check for FILE stream errors

Since R2026a

Description

Use ferror() rather than errno to check for FILE stream errors¹

Polyspace Implementation

The rule checker checks for Use of errno for checking FILE stream error.

Examples

Use of errno for checking FILE stream error

Issue

This issue occurs if your code reads errno immediately following a function that manipulates a FILE stream. Functions that manipulate a FILE stream include: fread, fgetc, fgets, fgetpos, fsetpos, fflush, puts, fputs, gets, getc, getchar, putchar, putc, fputc, fputwc, and printf.

Risk

Using errno for error checking can be error-prone and confusing. For example, if you check errno after a chain of calls to FILE stream functions, it is not clear which function sets the errno value. In fact, unless you explicitly set errno to zero beforehand, it is possible that the value of errno is not set by any of the relevant functions. If you call more than one function that modifies the value of errno to indicate an error, the errno value must be checked following each call. Otherwise, the value of errno set by a function is overwritten by subsequent calls.

Fix

Use ferror() to check for errors on a file stream. This function checks a specific FILE stream for errors and returns a nonzero value if the error indicator for the stream is set.

Example

This code manually checks errno after an fgets() function call. Polyspace^® reports a violation.

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main() {
    errno = 0;  // Reset errno before the operation

    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        if (errno != 0) {
            fprintf(stderr, "Error opening file: %s\n", strerror(errno));
        }
        return 1;
    }

    char buffer[256];
    errno = 0;  // Reset errno before reading

    if (fgets(buffer, sizeof(buffer), file) == NULL) {
        if (errno != 0) { //Noncompliant
            fprintf(stderr, "Error reading file: %s\n", strerror(errno));
        } else if (feof(file)) {
            printf("End of file reached.\n");
        }
    } else {
        printf("Read line: %s", buffer);
    }

    errno = 0;  // Reset errno before closing

    if (fclose(file) != 0) {
        if (errno != 0) {
            fprintf(stderr, "Error closing file: %s\n", strerror(errno));
        }
    }

    return 0;
}

Correction

To fix this violation, replace errno checking by a call to ferror().

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main() {
    errno = 0;  // Reset errno before the operation

    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        if (errno != 0) {
            fprintf(stderr, "Error opening file: %s\n", strerror(errno));
        }
        return 1;
    }

    char buffer[256];

    if (fgets(buffer, sizeof(buffer), file) == NULL) {
        if (ferror(file)) { //Compliant
            fprintf(stderr, "Error reading file: %s\n", strerror(errno));
        } else if (feof(file)) {
            printf("End of file reached.\n");
        }
    } else {
        printf("Read line: %s", buffer);
    }

    if (fclose(file) != 0) {
        if (errno != 0) {
            fprintf(stderr, "Error closing file: %s\n", strerror(errno));
        }
    }

    return 0;
}

Check Information

Group: Rec. 12. Error Handling (ERR)

PQL Name: std.cert.ERR01_C

Version History

Introduced in R2026a