Why does the system call "read()" block my program? - c

Question

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.

Answer 1

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

Answer 2

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.