Struggling to build a portable code to generate a file in any operating system using C

Question

I'm trying to build a C portable code (for Windows, MacOS and Linux) that creates an output .txt file to receive the results of a numerical simulation.

Summarizing, the code takes the name of the file and the extension and checks if the file already exists in the directory. If so, it creates another file with the same name, but with a number between parenthesis (#) in the end to distinguish the old from the new one.

The problem is: it is working properly on the mac environment, however when I compile and run it on windows, the file is not created in the end of execution. I could not find what I'm doing wrong.

Also, I'm using Intel C/C++ Classic compiler. If I use another compiler, for example, the Intel® oneAPI DPC++/C++ Compiler for windows, it complains about the usage of sizeof(src) when I call the strncat(...) function.

So far, this is the version of the code:

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>

bool file_exists(char *filename);
FILE *create_output_file(char *name, char *ext, bool (*file_exists)(char *));

#define funcname "test"

int main(void) {
    
    // Create output files to store results
    char *output_filename = malloc(200 * sizeof(*output_filename));
    sprintf(output_filename, "%s_out", funcname);
    printf("output_filename = %s\n", output_filename);
    char *ext = ".txt";
    FILE *output_file = create_output_file(output_filename, ext, file_exists);
}

bool file_exists(char *filename) {
    // Try to open file with same name as filename
    FILE *testfile = fopen(filename, "r");
    // Bool variable to check the existence of file
    bool exists = false; 
    // Check the existence of a file called filename
    if (testfile != NULL) {
        // Returns true if the file exists
        exists = true; 
    }
    // Close the file
    fclose(testfile); 
    // Returns the existence of a file (1) = does exist, (0) = does not exist
    return exists;
}

FILE *create_output_file(char *name, char *ext, bool (*file_exists)(char *)) {
    // Form the full filename
    name = strncat(name, ext, sizeof(ext));
    printf("fullfilename = %s\n", name);
    // Check if a file with the filename exists in the directory
    if (file_exists(name)) {
        // If exists, assign the same name with "(number)" to differentiate the new version
        int j = 1;
        char *numb = malloc(10 * sizeof(*numb));
        sprintf(numb, "(%i)", j);
        // Remove the extension from the name string
        name[strlen(name) - strlen(ext)] = '\0';
        // Add (number) to the name and then add the file extension again
        name = strncat(name, numb, sizeof(numb));
        name = strncat(name, ext, sizeof(ext));
        // Check if the name with numbers exists until it doesn't
        int limit = 1e1;
        while (file_exists(name)) {
            j++;
            sprintf(numb, "(%i)", j);            
            if (j == limit) {
                name[strlen(name) - strlen(numb) + 1 - strlen(ext)] = '\0';
                limit = limit*10;    
            } else {
                name[strlen(name) - strlen(numb) - strlen(ext)] = '\0';
            }
            name = strncat(name, numb, sizeof(numb));
            name = strncat(name, ext, sizeof(ext));
        }
        // Free allocated memory
        free(numb); 
    }
    // After assign the proper name, create the output file
    FILE *output_file = fopen(name, "w");
    // Returns the file
    return output_file;
}

What am I missing here?

Answer 1

There are multiple problems:

• In file_exists you call fclose(testfile) even if fopen failed This has undefined behavior.

• in create_output_file , your usage of sizeof is incorrect: in strncat(name, ext, sizeof(ext)); sizeof is applied to a pointer, hence it evaluates to the size of a pointer, not the length of the string it points to. You could write

  strncat(name, ext, strlen(ext));
  

  but it would be exactly equivalent to strcat(name, ext);

  The function strncat is defined as

    char *strncat(char *dest, const char *src, size_t n);
  

  it copies at most n characters plus a null terminator from the string pointed to by src at the end of the string pointed to by dest.

• The code is too complicated, you have multiple memory leaks and you do not check for buffer overflow when composing the filename.

Here is a simplified version:

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

#define funcname "test"

bool file_exists(const char *filename) {
    // Try to open file with same name as filename
    FILE *testfile = fopen(filename, "r");
    if (testfile != NULL) {
        fclose(testfile);
        return true;
    } else {
        return false;
    }
}

#define MAX_FILE_NUM  10000

FILE *create_output_file(char *name, size_t size,
                         const char *ext,
                         bool (*file_exists)(const char *))
{
    size_t len = strlen(name);
    int i = 0;

    snprintf(name + len, size - len, "%s", ext);
    while (file_exists(name)) {
        if (++i > MAX_FILE_NUM) {
            // all file versions tried.
            return NULL;
        }
        snprintf(name + len, size - len, "(%d)%s", i, ext);
    }
    return fopen(name, "w");
}

int main() {
    // Create output files to store results
    char output_filename[200];
    snprintf(output_filename, sizeof output_filename, "%s_out", funcname);
    printf("output_filename = %s\n", output_filename);
    const char *ext = ".txt";
    FILE *output_file = create_output_file(output_filename,
                            sizeof output_filename, ext, file_exists);
    if (output_file == NULL) {
        printf("could not open output file, last try: %s\n", output_filename);
    } else {
        printf("actual output_filename = %s\n", output_filename);
        fclose(output_file);
    }
    return 0;
}