Objective C stringWithCString "Method cache corrupted" - objective-c

Question

I am creating a method for executing shell commands. It looks like this:
NSString *cShellStr(NSString *command, int maxBufferSize) {
if (maxBufferSize<1) {
maxBufferSize = INT_MAX;
}
NSString *newCommand = [NSString stringWithFormat:#"%# 2>&1", command];
const char *cStrCommand = [newCommand cStringUsingEncoding:NSUTF8StringEncoding];
FILE *fp;
fp = popen(cStrCommand, "r");
if (fp == NULL) {
NSLog(#"Failed to open process");
return nil;
}
char *buffer = NULL;
buffer = (char*)malloc(4);
if (buffer == NULL) {
NSLog(#"Failed to allocate memory");
return nil;
}
while (!feof(fp)) {
buffer = realloc(buffer, sizeof(buffer)+1);
sprintf(buffer, "%s%c", buffer, fgetc(fp));
}
NSString *ret = [NSString stringWithCString:buffer encoding:NSASCIIStringEncoding];
fclose(fp);
free(buffer);
return ret;
}
However, it usually (strangely not always) gets this error at [NSString stringWithCString:buffer encoding:NSASCIIStringEncoding]:
objc[33674]: Method cache corrupted. This may be a message to an invalid object, or a memory error somewhere else.
objc[33674]: receiver 0x7fff7d62aec0, SEL 0x7fff99ac69f5, isa 0x7fff7d62aee8, cache 0x7fff7d62aef8, buckets 0x100200780, mask 0x3, occupied 0x2, wrap bucket 0x100200780
objc[33674]: receiver 0 bytes, buckets 64 bytes
objc[33674]: selector 'class'
objc[33674]: isa 'NSString'
objc[33674]: Method cache corrupted.
objc[33674]: Method cache corrupted. This may be a message to an invalid object, or a memory error somewhere else.
objc[33674]: receiver 0x7fff7d62aec0, SEL 0x7fff99aea2d4, isa 0x7fff7d62aee8, cache 0x7fff7d62aef8, buckets 0x100200780, mask 0x3, occupied 0x2, wrap bucket 0x100200780
objc[33674]: receiver 0 bytes, buckets 64 bytes
objc[33674]: selector 'stringWithCString:encoding:'
objc[33674]: isa 'NSString'
objc[33674]: Method cache corrupted.
I have stepped through it with lldb and confirmed that there are no other errors and that buffer contains exactly the text I would expect. Am I incorrectly using stringWithCString?
Extra info:
Xcode version:
Laptop info:
Clang version:
Apple LLVM version
6.0 (clang-600.0.51) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin13.3.0
Thread model: posix

Answer 1

while (!feof(fp)) {
buffer = realloc(buffer, sizeof(buffer)+1);
sprintf(buffer, "%s%c", buffer, fgetc(fp));
}
This loop should be rewritten as:
size_t len = 0, cap = 4;
buffer = malloc(cap); // TODO check NULL
int c;
while (EOF != (c = fgetc(fp))) {
if (len >= cap) {
cap += cap;
buffer = realloc(buffer, cap); // TODO check NULL
}
buffer[len++] = c;
}
// check feof/ferror

Answer 2

sprintf(buffer, "%s%c", buffer, fgetc(fp));
This is undefined behaviour because you are using buffer twice. Anything could go wrong, and lucky for you, it does. I'm saying "lucky for you" because with a bit of bad luck it would only go wrong in the hand of customers.
I didn't watch out; the issue that user3125367 found is ten times worse.