The only acceptable shape for a system call

Answer

int fd = open("data.txt", O_RDONLY); if (fd == -1) { perror("open"); return 1; }

Test for -1, name the call in the perror label so the message is findable, then decide. Skipping this is the single most common way a C program on UNIX fails silently.

S&K 3e ch18 §18.3–18.4, §18.7.4; system-call return convention and errno per POSIX.1-2024 §2.3