Search and replace within a file using PCRE in C - c

Question

I want to parse a shell style key-value config file with C and replace values as needed.
An example file could look like
FOO="test"
SOME_KEY="some value here"
ANOTHER_KEY="here.we.go"
SOMETHING="0"
FOO_BAR_BAZ="2"
To find the value, I want to use regular expressions. I'm a beginner with the PCRE library so I created some code to test around. This application takes two arguments: the first one is the key to search for. The second one is the value to fill into the double quotes.
#include <pcre.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#define OVECCOUNT 30
int main(int argc, char **argv){
const char *error;
int erroffset;
pcre *re;
int rc;
int i;
int ovector[OVECCOUNT];
char regex[64];
sprintf(regex,"(?<=^%s=\\\").+(?<!\\\")", argv[1]);
char *str;
FILE *conf;
conf = fopen("test.conf", "rw");
fseek(conf, 0, SEEK_END);
int confSize = ftell(conf)+1;
rewind(conf);
str = malloc(confSize);
fread(str, 1, confSize, conf);
fclose(conf);
str[confSize-1] = '\n';
re = pcre_compile (
regex, /* the pattern */
PCRE_CASELESS | PCRE_MULTILINE, /* default options */
&error, /* for error message */
&erroffset, /* for error offset */
0); /* use default character tables */
if (!re) {
printf("pcre_compile failed (offset: %d), %s\n", erroffset, error);
return -1;
}
rc = pcre_exec (
re, /* the compiled pattern */
0, /* no extra data - pattern was not studied */
str, /* the string to match */
confSize, /* the length of the string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* output vector for substring information */
OVECCOUNT); /* number of elements in the output vector */
if (rc < 0) {
switch (rc) {
case PCRE_ERROR_NOMATCH:
printf("String didn't match");
break;
default:
printf("Error while matching: %d\n", rc);
break;
}
free(re);
return -1;
}
for (i = 0; i < rc; i++) {
printf("========\nlength of vector: %d\nvector[0..1]: %d %d\nchars at start/end: %c %c\n", ovector[2*i+1] - ovector[2*i], ovector[0], ovector[1], str[ovector[0]], str[ovector[1]]);
printf("file content length is %d\n========\n", strlen(str));
}
int newContentLen = strlen(argv[2])+1;
char *newContent = calloc(newContentLen,1);
memcpy(newContent, argv[2], newContentLen);
char *before = malloc(ovector[0]);
memcpy(before, str, ovector[0]);
int afterLen = confSize-ovector[1];
char *after = malloc(afterLen);
memcpy(after, str+ovector[1],afterLen);
int newFileLen = newContentLen+ovector[0]+afterLen;
char *newFile = calloc(newFileLen,1);
sprintf(newFile,"%s%s%s", before,newContent, after);
printf("%s\n", newFile);
return 0;
}
This code is working in some cases but if I want to replace FOO or ANOTHER_KEY theres something fishy.
$ ./search_replace.out FOO baz
========
length of vector: 5
vector[0..1]: 5 10
chars at start/end: b "
file content length is 94
========
FOO="9#baz"
SOME_KEY="some value here"
ANOTHER_KEY="here.we.go"
SOMETHING="0"
FOO_BAR_BAZ="2"
$ ./search_replace.out ANOTHER_KEY insert
========
length of vector: 10
vector[0..1]: 52 62
chars at start/end: h "
file content length is 94
========
FOO="baaar"
SOME_KEY="some value here"
ANOTHER_KEY=")insert"
SOMETHING="0"
FOO_BAR_BAZ="2"
Now if I change the format of the input file slightly to
TEST="new inserted"
FOO="test"
SOME_KEY="some value here"
ANOTHER_KEY="here.we.go"
SOMETHING="0"
FOO_BAR_BAZ="2"
the code is working fine.
I don't get it why the code is behaves differently here.

Answer 1

The extra characters before the substituted text come from not properly null-terminating your before string. (Just as you hadn't null-terminated the whole buffer str, as Paul R has pointed out.) So:
char *before = malloc(ovector[0] + 1);
memcpy(before, str, ovector[0]);
before[ovector[0]] = '\0';
Anyway, the business of allocating substrings and copying the contents seems needlessly complicated and prone to errors. For example, do the somethingLen variables count the terminating null character or not? Sometimes they do, sometimes they don't. I'd recommend to pick one representation and use it consistently. (And you should really free all allocated buffers after no longer using them and probably also clean up the compiled regex.)
You could do the replacement with just one allocation for the target buffer by using the precision field of the %s format specifier on the "before" part:
int cutLen = ovector[1] - ovector[0];
int newFileLen = confSize + strlen(argv[2]) - cutLen;
char *newFile = malloc(newFileLen + 1);
snprintf(newFile, newFileLen + 1, "%.*s%s%s",
ovector[0], str, argv[2], str + ovector[1]);
Or you could just use fprintf to ther target file if you don't need the temporary buffer.

Answer 2

You forgot to terminate str, so subsequently calling strlen(str) will give unpredictable results. Either change:
str = malloc(confSize);
fread(str, 1, confSize, conf);
to:
str = malloc(confSize + 1); // note: extra char for '\0' terminator
fread(str, 1, confSize, conf);
str[confSize] = '\0'; // terminate string!
and/or pass confSize instead of strlen(str) to pcre_exec.

Answer 3

Your string is allocated confSize bytes of memory. Let's say that confSize is 10 as an example.
str = malloc(confSize);
So valid indexes for your string are 0-9. But this line assigns '\n' to the 10th index, which is the 11th byte:
str[confSize] = '\n';
If you're wanting the last character to be '\n', it should be:
str[confSize - 1] = '\n';