Synopsis

Do not alternately input and output from a stream without an intervening flush or positioning call.

Enabled by default

Yes

Severity/Certainty

Low/High

Full description

Receiving input from a stream directly following an output to that stream, or outputting to a stream after receiving input from that stream, without an intervening call to fflush(), fseek(), fsetpos(), or rewind() if the file is not at end-of-file is undefined behaviour. Consequently, a call to fseek(), fflush(), or fsetpos() is necessary between input and output to the same stream.

Coding standards

Code examples

The following code example fails the check and will give a warning:

#include <stdio.h>

enum { BUFFERSIZE = 32 };

extern void initialize_data(char *data, size_t size);

void func(const char *file_name) {
    char data[BUFFERSIZE];
    char append_data[BUFFERSIZE];
    FILE *file;

    file = fopen(file_name, "a+");
    if (file == NULL) {
        /* Handle error */
    }

    initialize_data(append_data, BUFFERSIZE);

    if (fwrite(append_data, 1, BUFFERSIZE, file) != BUFFERSIZE) {
        /* Handle error */
    }
    if (fread(data, 1, BUFFERSIZE, file) < BUFFERSIZE) {
        /* Handle there not being data */
    }

    if (fclose(file) == EOF) {
        /* Handle error */
    }
}

The following code example passes the check and will not give a warning about this issue:

#include <stdio.h>

enum { BUFFERSIZE = 32 };
extern void initialize_data(char *data, size_t size);

void func(const char *file_name) {
    char data[BUFFERSIZE];
    char append_data[BUFFERSIZE];
    FILE *file;

    file = fopen(file_name, "a+");
    if (file == NULL) {
        /* Handle error */
    }

    initialize_data(append_data, BUFFERSIZE);
    if (fwrite(append_data, BUFFERSIZE, 1, file) != BUFFERSIZE) {
        /* Handle error */
    }

    if (fseek(file, 0L, SEEK_SET) != 0) {
        /* Handle error */
    }

    if (fread(data, BUFFERSIZE, 1, file) != 0) {
        /* Handle there not being data */
    }

    if (fclose(file) == EOF) {
        /* Handle error */
    }
}