#include <sys/types.h>
#include <stdio.h>
#include <assert.h>
#include <errno.h>
char buf[5];

int main(int argc, char **argv)
{
    ssize_t len = 0;
    char *buffer=buf;
    //buffer = realloc(buffer,10);
    FILE *f = fopen(argv[1],"r");
    ssize_t a = getline(&buffer, &len, f);
    printf("%zd", a);
    fclose(f);
    return 0;
}

getline (lineptr, linelen, fp)
     char** lineptr;
     size_t* linelen;
     FILE* fp;
{
  return _IO_getdelim (lineptr, linelen, 'n', fp);
}

if (*lineptr == NULL || *n == 0)
{
    *n = 120;
    *lineptr = (char*) malloc(*n);

    if (*lineptr == NULL)
    {
        result = -1;
        goto unlock_return;
    }
}

char buf[5];

int main(int argc, char **argv)
{
    ssize_t len = 5;   // !!!!!!!!!!!!!!!!!
    char *buffer=buf;
    FILE *f = fopen(argv[1],"r");
    ssize_t a = getline(&buffer, &len, f);
    printf("%zd", a);
    fclose(f);
    return 0;
}

ssize_t len = 0;
char *buffer= NULL;
ssize_t a = getline(&buffer, &len, f);

ssize_t len = k;  // Сами решите, чему оно равно
char *buffer= malloc(k);
ssize_t a = getline(&buffer, &len, f);