Q: I'm reading lines from a file into an array, with this code:
char linebuf[80];
char *lines[100];
int i;
for(i = 0; i < 100; i++) {
char *p = fgets(linebuf, 80, fp);
if(p == NULL) break;
lines[i] = p;
}
Why do all the lines end up containing copies of the last line?
A: You have only allocated memory for one line, linebuf. Each time you call fgets, the previous line is overwritten. fgets doesn't do any memory allocation: unless it reaches EOF (or encounters an error), the pointer it returns is the same pointer you handed it as its first argument (in this case, a pointer to your single linebuf array).
To make code like this work, you'll need to allocate memory for each line. See question 20.2 for an example.
References:
K&R1 Sec. 7.8 p. 155
K&R2 Sec. 7.7 pp. 164-5
ISO Sec. 7.9.7.2
H&S Sec. 15.7 p. 356
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