fopen + fgets does not work

[realmonster]

Sample program.

#include <stdio.h>

int main()
{
    FILE *f = fopen("mysave.txt","w");
    fprintf(f, "my info");
    fclose(f);
    f = fopen("mysave.txt", "r");
    char buff[1000];
    char *res = fgets(buff, sizeof(buff) - 1, f);
    fclose(f);
    printf("%p\n%s\n", res, buff);
    return 0;
}

Testing:

$ gcc testfgets.c -o testfgets
$ ./testfgets
0x7ffd5aaa6270
my info
$ libTAS ./testfgets
Attempt 1: Connected.
[libTAS f:0] Thread 22322 (main) fopen call with filename mysave.txt and mode w
[libTAS f:0] Thread 22322 (main)   savefile detected
[libTAS f:0] Thread 22322 (main) fclose call.
[libTAS f:0] Thread 22322 (main) fopen call with filename mysave.txt and mode r
[libTAS f:0] Thread 22322 (main)   savefile detected
[libTAS f:0] Thread 22322 (main) fclose call.
(nil)
��u(
[libTAS f:0] Thread 22322 (main) close call
[libTAS f:0] Thread 22322 (main) Unknown file descriptor 4

Expected result: to see non null pointer, and "my info" text.

[CasualPokePlayer]

Seems that fgets works perfectly fine. The issue is that the first call to fopen for the "save file" using "w". This marks the "file" as write-only. The later fopen ends up not re-opening the file (due to the logic of save files), resulting in fgets rejecting the file as it sees it as write-only. If the first fopen call is changed to open the file as "w+" it ends up working out.

[realmonster]

thanks to @CasualPokePlayer for the info which helped me to find out one of possible solutions.

to reproduce bug without libTAS, here is a code:

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>

int main()
{
    int fd = memfd_create("mysave.txt", 0);
    FILE *f = fdopen(fd, "w");
    fwrite("my info", 1, 7, f);
    fseek(f, 0, SEEK_SET);
    char buff[1000];
    memset(buff, 0, sizeof(buff));
    printf("%p\n", f);
    char *res = fgets(buff, sizeof(buff) - 1, f);
    printf("fclose %d\n", fclose(f));
    printf("%p\n%s\n", res, buff);
    return 0;
}

Because libTAS doesn't close files, this still "w", and glibc doesn't allow fgets for "w" files. If we set "w+" we will be able to read it. and it will print actual data.

But we want to have new FILE* with appropriate mode. This is possible using dup. Here is an example:

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>

int main()
{
    int fd = memfd_create("mysave.txt", 0);
    int fdcopy = dup(fd);
    FILE *f = fdopen(fd, "w");
    printf("f=%p\n", f);
    fwrite("my znfz", 1, 7, f);
    fseek(f, 0, SEEK_SET);
    char buff[1000];
    memset(buff, 0, sizeof(buff));
    printf("read=%d\n", fread(buff, 1, 1, f));
    fclose(f);
    FILE *f1 = fdopen(fdcopy, "r");
    printf("f1=%p", f1);
    char *res = fgets(buff, sizeof(buff) - 1, f1);
    printf("%p\n%s\n", res, buff);
    printf("write=%d\n", fwrite(buff, 1, 1, f1)); 
    printf("fclose %d\n", fclose(f1));
    return 0;
}