diff --git a/libraries/AP_Filesystem/AP_Filesystem_posix.cpp b/libraries/AP_Filesystem/AP_Filesystem_posix.cpp
index 80c42675a8..031241cc91 100644
--- a/libraries/AP_Filesystem/AP_Filesystem_posix.cpp
+++ b/libraries/AP_Filesystem/AP_Filesystem_posix.cpp
@@ -60,12 +60,12 @@ int AP_Filesystem_Posix::close(int fd)
     return ::close(fd);
 }
 
-ssize_t AP_Filesystem_Posix::read(int fd, void *buf, size_t count)
+int32_t AP_Filesystem_Posix::read(int fd, void *buf, uint32_t count)
 {
     return ::read(fd, buf, count);
 }
 
-ssize_t AP_Filesystem_Posix::write(int fd, const void *buf, size_t count)
+int32_t AP_Filesystem_Posix::write(int fd, const void *buf, uint32_t count)
 {
     return ::write(fd, buf, count);
 }
diff --git a/libraries/AP_Filesystem/AP_Filesystem_posix.h b/libraries/AP_Filesystem/AP_Filesystem_posix.h
index dce6aa728f..42fdad3ec6 100644
--- a/libraries/AP_Filesystem/AP_Filesystem_posix.h
+++ b/libraries/AP_Filesystem/AP_Filesystem_posix.h
@@ -32,7 +32,7 @@ public:
     int open(const char *fname, int flags) override;
     int close(int fd) override;
     int32_t read(int fd, void *buf, uint32_t count) override;
-    ssize_t write(int fd, const void *buf, uint32_t count) override;
+    int32_t write(int fd, const void *buf, uint32_t count) override;
     int fsync(int fd) override;
     int32_t lseek(int fd, int32_t offset, int whence) override;
     int stat(const char *pathname, struct stat *stbuf) override;