C copy substring from text file - c

Question

Say I have the following text file -
name:asdfg
address:zcvxz
,
name:qwerwer
address:zxcvzxcvxz
,
And I wanna copy the name (without "name:") to a certain string variable, the address to another and so on.
How do I do so without corrupting memory?
Tried using (example) -
char buf[50];
while (fgets(buf, 50, file) != NULL) {
if (!strncmp(buf, "name", 4))
strncpy(somestring, buf + 5, 20)
//do the same for address, continue looping
but the text lines differ in length, so it seems to copy all sorts of crap from the buffer, as the strings arent null terminated so it copies "asdfgcrapcrapcrap".

Answer 1

You are to be commended for using fgets to handle your file I/O as it provides a much more flexible and robust way to read, validate and prepare to parse the lines of data you read. It is generally the recommended way to do line-oriented input (either from a file or from the user). However, this is one of those circumstances where treating multiple records as formatted input does have some advantages.
Let's start with an example reading your data file and capturing the name:.... and address:... data in a simple data structure to hold both the name and address data values in a 20-char array for each. Each line is read, the length is validated, the trailing '\n' is removed and then strchr is used to locate the ':' in the line. (we don't care about lines without ':'). The label before ':' is copied to tmp and then compare against "name" or "address" to determine which value to read. Once the address data is read, both name and addr values are printed to stdout,
#include <stdio.h>
#include <string.h>
enum { MAXC = 20, MAXS = 256 };
typedef struct {
char name[MAXC],
addr[MAXC];
} data;
int main (int argc, char **argv) {
char buf[MAXS] = "",
*name = "name", /* name/address literals for comparison */
*addr = "address";
data mydata = { .name = "" };
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!fp) { /* validate file open for reading */
fprintf (stderr, "error: file open failed '%s'.\n", argv[1]);
return 1;
}
while (fgets (buf, MAXS, fp)) { /* read each line */
char *p = buf, /* pointer to use with strchr */
tmp[MAXC] = ""; /* storage for labels */
size_t len = strlen (buf); /* get buf len */
if (len && buf[len - 1] == '\n') /* validate last char is '\n' */
buf[--len] = 0; /* overwrite with nul-character */
else if (len + 1 == MAXS) { /* handle string too long */
fprintf (stderr, "error: line too long or no '\n'\n");
return 1;
}
if ((p = strchr (buf, ':'))) { /* find ':' in buf */
size_t labellen = p - buf, /* get length of label */
datalen = strlen (p + 1); /* get length of data */
if (labellen + 1 > MAXC) { /* validate both lengths */
fprintf (stderr, "error: label exceeds '%d' chars.\n", MAXC);
return 1;
}
if (datalen + 1 > MAXC) {
fprintf (stderr, "error: data exceeds '%d' chars.\n", MAXC);
return 1;
}
strncpy (tmp, buf, labellen); /* copy label to temp */
tmp[labellen] = 0; /* nul-terminate */
if (strcmp (name, tmp) == 0) /* is the label "name" ? */
strcpy (mydata.name, p + 1);
else if (strcmp (addr, tmp) == 0) { /* is the label "address" ? */
strcpy (mydata.addr, p + 1);
/* record complete -- output results */
printf ("\nname : %s\naddr : %s\n", mydata.name, mydata.addr);
}
}
}
if (fp != stdin) fclose (fp); /* close file if not stdin */
return 0;
}
(note: there are many ways to structure this logic. The example above just represents a semi-standard method)
Example Use/Output
$./bin/nameaddr <dat/nameaddr.txt
name : asdfg
addr : zcvxz
name : qwerwer
addr : zxcvzxcvxz
Here is where I will have a tough time convincing you that fgets was the way to go for this problem. Why? Here we are essentially reading formatted input that is comprised of 3-lines of data. The format string for fscanf doesn't care how many lines are involved, and can easily be constructed to skip '\n' within the formatted input. This can provide (a more fragile), but attractive alternative for the right input files.
For example, the code above can be reduced to the following using fscanf for a formatted read:
#include <stdio.h>
#define MAXC 20
typedef struct {
char name[MAXC],
addr[MAXC];
} data;
int main (int argc, char **argv) {
data mydata = { .name = "" };
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!fp) { /* validate file open for reading */
fprintf (stderr, "error: file open failed '%s'.\n", argv[1]);
return 1;
}
/* read 3-lines at a time separating name and address at once */
while (fscanf (fp, " name:%19s address:%19s ,",
mydata.name, mydata.addr) == 2)
printf ("\nname : %s\naddr : %s\n", mydata.name, mydata.addr);
if (fp != stdin) fclose (fp); /* close file if not stdin */
return 0;
}
(the output is the same)
In the rare case, for the correct data file, fscanf can provide a viable alternative to a line-oriented read with fgets. However, your first choice should remain a line-oriented approach using either fgets or POSIX getline.
Look both over and let me know if you have further questions.

Answer 2

If the name is 20 characters or longer, strncpy() won't copy the null terminator to the destination string, so you need to add it yourself.
strncpy(somestring, buf + 5, 19);
somestring[19] = '\0';