I am contacting you because I have a problem with my program. For my studies, I am currently programming in C a function to read a file, I am only allowed to use the following functions/system calls: malloc, free, exit, (f)open, (f)close, (f)read, (f)write, getline, ioctl, usleep, sigaction, signal, stat, lstat, fstat

The problem is that when calling my function my_filetostr(), it blocks the program at the read() system call and nothing happens.

here's my code:

char *my_filetostr(const char *filepath)
{
    int fd = 0;
    struct stat s;
    char *buff = NULL;
    if (fd = open(filepath, O_RDONLY) > 0) {
        stat(filepath, &s);
        buff = malloc(sizeof(char) * s.st_size   1);
        my_printf("fd : %d, size : %d\n", fd, s.st_size);
        if (read(fd, buff, s.st_size) == s.st_size) {
            buff[s.st_size] = '\0';
        }
        close(fd);
    }
    return buff;
}

I specify that my file exists (my error handling works).

I tried to put some my_printf (we had to recode printf) in order to see where the program stops and to display some useful information:

• So I know that the problem comes from read()
• st_size returns the right size and filedescriptor has a correct value.

The program is just supposed to move on.

CodePudding user response：

You are assigning to fd inside the if condition.

This usually causes bugs and should be avoided

Your issue here is specifically because > has higher precedence compared to =, so fd becomes 1 which is stdout.

I have modified your code to highlight the issue https://godbolt.org/z/M4PbcPfev

fd : 1, size : 140728905723182

CodePudding user response：

Thanks you @stark , @user3840170 , @tejas

My code now works :

char *my_filetostr(const char *filepath)
{
    int fd = -1;
    struct stat s;
    char *buff = NULL;
    if ((fd = open(filepath, O_RDONLY)) != -1) {
        fstat(fd, &s);
        buff = malloc(sizeof(char) * s.st_size   1);
        if (read(fd, buff, s.st_size) == s.st_size) {
            buff[s.st_size] = '\0';
        }
        close(fd);
    }
    return buff;
}

I have performed the following actions:

• Replace stat with fstat (but that didn't solve the problem)
• Replace int fd = 0 by int fd = -1
• Replace fd = open(filepath, O_RDONLY) > 0 par (fd = open(filepath, O_RDONLY)) != -1

Thank you for your answers.