`fread(3)` returns 0 for success. why?

prog.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

int main() {    
    char cmdline[256];
    char proc_file[256];
    snprintf(proc_file, sizeof(proc_file), "/proc/%d/cmdline", getpid());
    FILE *f = fopen(proc_file, "r");
    if (f == NULL) {
        fprintf(stderr, "Failed to open %s: %s\n", proc_file, strerror(errno));
        exit(EXIT_FAILURE);
    }
    size_t nread = fread(cmdline, sizeof(cmdline), 1, f);
    
    printf("nread=%lu\n", nread);
    printf("cmdline=%s\n", cmdline);
    
    return EXIT_SUCCESS;
}